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The speed of light is torturously slow, and these 3 simple animations by a scientist at NASA prove it

  • The speed of light in a vacuum is about 186,282 miles per second (299,792 kilometers per second).
  • A scientist at NASA animated how long it takes light to travel around Earth, as well as between the planet, its moon, and Mars.
  • The physics animations show just how fast (and slow) the speed limit of the universe can be.

A series of new animations by a NASA scientist show just how zippy — and also how torturously slow — the speed of light can be.

Light speed is the fastest that any material object can travel through space. That is, of course, barring the existence of theoretical shortcuts in the fabric of space called wormholes (and the ability to go through them without being destroyed).

In a perfectly empty vacuum, a particle of light, which is called a photon, can travel 186,282 miles per second (299,792 kilometers per second), or about 670.6 million mph (1.079 billion kilometers per hour).

This is incredibly fast. However, light speed can be frustratingly slow if you're trying to communicate with or reach other planets, especially any worlds beyond our solar system.

Read more : Astronomers found a 'cold super-Earth' less than 6 light-years away — and it may be the first rocky planet we'll photograph beyond the solar system

To depict the speed limit of the cosmos in a way anyone could understand, James O'Donoghue , a planetary scientist at NASA's Goddard Space Flight Center, took it upon himself to animate it.

"My animations were made to show as instantly as possible the whole context of what I'm trying to convey," O'Donoghue told Business Insider via Twitter . "When I revised for my exams, I used to draw complex concepts out by hand just to truly understand, so that's what I'm doing here."

O'Donoghue said he only recently learned how to create these animations — his first were for a NASA news release about Saturn's vanishing rings . After that, he moved on to animating other difficult-to-grasp space concepts, including a video illustrating the rotation speeds and sizes of the planets. He said that one "garnered millions of views" when he posted it on Twitter .

O'Donoghue's latest effort looks at three different light-speed scenarios to convey how fast (and how painfully slow) photons can be.

How fast light travels relative to Earth

One of O'Donoghue's first animations shows how fast light moves in relation to Earth.

Earth is 24,901 miles around at its center. If our world had no atmosphere (air refracts and slows down light a little bit), a photon skimming along its surface could lap the equator nearly 7.5 times every second.

In this depiction , the speed of light seems pretty fast — though the movie also shows how finite it is.

How fast light travels between Earth and the moon

A second animation by O'Donoghue takes a big step back from Earth to include the moon.

On average, there is about 238,855 miles (384,400 kilometers) of distance between our planet and its large natural satellite.

This means all moonlight we see is 1.255 seconds old, and a round-trip between the Earth and moon at light speed takes about 2.51 seconds.

This timing is growing every day, however, as the moon is drifting farther from Earth at a rate of about 1.5 inches (3.8 centimeters) per year. (The moon is constantly sapping Earth's rotational energy via ocean tides , boosting its orbit to a greater and greater distance.)

How fast light travels between Earth and Mars

O'Donoghue's third speed-of-light animation illustrates the challenge that many planetary scientists deal with on a daily basis.

When NASA tries to talk to or download data from a spacecraft, such as the InSight probe on Mars , it can do so only at the speed of light. This is much too slow to operate a spacecraft in "live mode" as you would a remote-controlled car. So, commands must be carefully thought out, prepackaged, and aimed at the precise location in space at the precise time so that they don't miss their target.

Read more : NASA can hear the 'haunting' sound of dust devils tearing across Mars with its new $830 million lander

The fastest a conversation could ever happen between Earth and Mars is when the planets are at their nearest point to one another, an event called closest approach that happens once roughly every two years. On average, that best-case-scenario distance is about 33.9 million miles (54.6 million kilometers).

As that 60-second clip of O'Donoghue's full movie on YouTube shows, light takes 3 minutes 2 seconds to travel between Earth and Mars at closest approach. That's six minutes and four seconds for a light-speed round-trip.

But on average, Mars is about 158 million miles from Earth — so the average round-trip communication takes about 28 minutes and 12 seconds.

The speed of light gets more depressing the farther you go

The hurdle of light's finite speed gets even more challenging for spacecraft such as New Horizons, which is now more than 4 billion miles from Earth , and the Voyager 1 and 2 spacecraft, each of which have reached the space between stars .

The situation gets downright depressing when you start looking outside the solar system. The closest-known exoplanet , called Proxima b, is about 4.2 light-years away from us (a distance of about 24.7 trillion miles or 39.7 trillion kilometers).

However, the fastest any spacecraft has ever gone is NASA's Parker Solar Probe at about 213,200 mph ; at that speed, it'd take 13,211 years to reach Proxima b.

A Russian-American billionaire's Breakthrough Starshot project envisions a way to address this speed problem. The multidecade plan is to build and fly tiny "nanocraft" past such exoplanets via ultrapowerful laser blasts , ideally at a planned cruise velocity of 20% of the speed of light. Yet the entire concept is still theoretical, may end up not working, and would operate at a fraction of light-speed.

Space is impossibly vast. Although the universe is about 13.77 billion years old, its edge is about 45.34 billion light-years away in any direction and is increasing due to expansion .

That's far too big to illustrate in a simple animation. One illustration comes close, though: this image created by musician Pablo Carlos Budassi , which combines logarithmic maps of the universe from Princeton and images from NASA to capture it all in one picture.

This story has been updated.

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Watch: What humans will look like on Mars

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The elapsed time taken by a signal travelling from the Earth to a spacecraft or other celestial body, then immediately transmitted or reflected back to the original transmission point. This is about equal to twice the one-way light time, but it varies because the motions of the Earth and space objects create different travelling times each way. RTLT to the Sun is about 17 minutes, and it was about 23 hours to the Voyager 1 probe in October 2001.

From:   round-trip light time   in  A Dictionary of Space Exploration »

Subjects: Science and technology — Astronomy and Cosmology

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Physics from the Moon

Figure caption

The Apollo 11 mission is remembered primarily as a technological feat—epitomized by a first footprint in lunar soil—but it was also a scientific achievement. When Neil Armstrong and Buzz Aldrin stepped out of the Eagle Lander on July 20, 1969, they installed several small measuring devices including a seismometer, a solar wind particle collector, and an array of reflectors meant to bounce laser pulses back to Earth. These instruments—and others brought by subsequent Apollo missions—remained on the Moon long after the humans left. They provided a number of key results, such as observing the first moonquakes and measuring the makeup of the lunar interior. “The Apollo scientific legacy has been enormous and underpins much of our understanding of the terrestrial planets,” says planetary scientist Ian Crawford from the University of London.

For the physics community, the biggest payoff from the Apollo science program has come from the reflector arrays, which are part of the lunar laser ranging (LLR) experiment. For 50 years and counting, scientists have been pinging laser pulses off these reflectors to measure the Moon-Earth distance to high precision. By treating our rocky satellite as a giant test mass, the LLR research program has tested several predictions from gravity theory at unprecedented levels. Thanks to these successes, an updated LLR reflector is now scheduled to fly back to the Moon on a commercially built lander. Other physics-related projects, such as radio telescopes on the far-side of the Moon, may be able to piggyback on future crewed missions. As in the Apollo years, the exploration of our planet’s cosmic companion continues to offer new opportunities for physics research.

Lunar Laser Ranging Turns 50

The first LLR array was deployed during the Apollo 11 mission, but other arrays were installed during Apollo 14 and 15 and as part of the Soviet Union’s robotic Luna program. An array consists of between 14 and 300 retroreflectors, each of which is a piece of glass shaped like a corner sliced off of a solid glass cube. Light entering a retroreflector reflects off of all three mutually perpendicular back surfaces and exits in the direction it came from. Researchers on Earth measure the round-trip travel time—roughly 2.5 seconds—of laser pulses reflecting off these retroreflectors and returning back to detectors on Earth. The first laser reflection to be recorded was at Lick Observatory in California on August 1, 1969, but since then several other facilities in the US, the Soviet Union, France, and elsewhere have used the LLR arrays.

As the retroreflector arrays require no power, they remain the only astronaut-installed equipment still in operation on the Moon. “The LLR experiment has been surprisingly long-lived, and it has given us an impressive record of results,” says Tom Murphy from the University of California, San Diego, who is the lead investigator of the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO)—which has been running since 2005. From a mountain top in southern New Mexico, Murphy and his colleagues fire 100-picosecond laser pulses at one of the five LLR arrays on the Moon and capture the return light. The efficiency is miniscule: only about one out of 1 0 1 8 emitted photons makes the complete return trip to the detector. But even with just a few detected photons, researchers can measure the travel time and from that determine the Moon-Earth distance to roughly the nearest millimeter.

Figure caption

Like other LLR researchers, Murphy’s team tracks the orbit of the Moon over time, and with those data they can perform several tests of gravitational theory, such as confirming Einstein’s equivalence principle, verifying the constancy of Newton’s gravitational constant, and placing limits on deviations from the expected 1 ∕ r 2 law of gravity. In many cases, LLR provides the most precise tests of any experiment (see 16 November 2017 Synopsis ).

“We are fortunate to have the Moon,” Murphy says. The Moon is far enough from Earth that it is essentially in orbit around the Sun (the Moon’s path around the Sun barely differs from the Earth’s path ). This surprising fact is important because the tests of gravity theory look for perturbations in the Moon’s solar orbit caused by the Earth. If the Moon were closer, then Earth’s gravity would dominate the Moon’s motion, and the current testing strategy wouldn’t work. If the Moon were farther, not enough light would bounce back. “If we were living on Mars, the same gravity tests with one of its moons—Phobos or Deimos—wouldn’t be as precise,” Murphy says.

LLR Due for Facelift

The LLR experiments were not expected to last this long, says Doug Currie from the University of Maryland, College Park, a developer of the Apollo retroreflectors. One of the early concerns was that lunar dust kicked up by meteorites would cover the retroreflectors and thus limit their operational time to just a few years. “Dust has been a problem,” Currie says, but not as severe as originally predicted. “Laser [technology] has grown faster than the dust has accumulated.”

In fact, the precision of LLR experiments has continuously increased, from 150 millimeters in the 1970s to a few millimeters now. To reach even higher precision, scientists will have to deal with lunar “libration,” which is a wobbling in the Moon’s orbital position that causes the reflector arrays to tilt back and forth slightly with respect to observers on Earth. “We don’t know if an observed photon came from the near or the far corner of the array,” Currie explains. This uncertainty over the photon’s exact path translates into an error in the Moon-Earth distance estimate.

To overcome the effects of libration, Currie is now working on a new project: the Next Generation Lunar Retroreflector (NGLR). This updated reflector is designed to stand alone, rather than in an array, which means that there’s no uncertainty about which reflector a photon is coming from. To compensate for not having the collective effect of multiple reflectors, Currie’s team made NGLR larger than earlier versions, giving it a diameter of 100 mm, as compared with 38 mm for the Apollo reflectors. Using NGLR, the Moon-Earth distance could be measured at submillimeter precision, which would allow researchers to look for deviations in the Moon’s orbit predicted by certain alternatives to the general theory of relativity.

Figure caption

The Race to Go Back

Currie’s retroreflectors should be going to the Moon in the next few years, as the project was selected on 1 July for NASA’s Commercial Lunar Payload Services (CLPS) program. The CLPS program is a partnership between NASA and private companies that are developing lunar landers. NASA has already commissioned three commercial Moon landing service providers to deliver scientific and technological instruments, such as a fast-moving rover and two sample-acquisition devices. In the big picture, the CLPS program is laying the groundwork for NASA’s Artemis program, which aims to land astronauts on the lunar surface by 2024 and to create a sustainable human presence by 2028. In parallel, both China and India have recently sent orbiters and landers to the Moon.

Figure caption

What might physicists expect to gain from this new space race? Several researchers have begun arguing for radio antennas on the far side of the Moon. “That’s where we want to go in the next decade,” says astrophysicist Jack Burns from the University of Colorado, Boulder. The Moon’s far side, which always faces away from Earth, is one of the most radio-quiet places in the solar system because it is shielded from the electromagnetic noise of Earth’s communication network. The Moon also lacks the Earth’s thick ionosphere, so low-frequency radio waves (less than 30 MHz) can reach the lunar surface.

Several efforts are being made to test the feasibility of radio astronomy from the Moon. The Chinese space agency installed a small radio spectrometer on its Chang’e 4 spacecraft, which landed on the far side of the Moon in January of this year. Similarly, Burns and colleagues—led by Stuart Bale at the University of California, Berkeley—have developed the Lunar Surface Electromagnetics Experiment (LuSEE), which has a low-frequency radio antenna for picking up solar flare activity, as well as radio emissions from Jupiter and other planets. Like NGLR, LuSEE will soon hitch a ride to the Moon on one of the CLPS commercial landers.

The long-term goal is to use radio telescopes to study the “dark ages,” the time before the first stars formed about 400 million years after the big bang (see 2 July 2018 Viewpoint ). Understanding that early epoch could reveal new information about dark matter and allow tests of cosmic inflation, the theory that the Universe expanded rapidly immediately after the big bang, before expanding more slowly. To probe the dark ages, Burns and others—like Joe Silk from the Institute of Astrophysics in Paris—advocate setting up an array of antennas on the Moon’s far side. At a recent cosmology conference in Spain, Silk said such an array would not be too hard to build, and it would add a broader scientific purpose to the next round of crewed missions to the Moon.

As demonstrated with Apollo, scientific research and human exploration can have a mutually beneficial relationship: the science helps to legitimize the cost of crewed missions, while at the same time the large collective effort of an astronaut program provides a boost to research. Although robotic missions are often less expensive, they are generally less efficient in the amount of science they accomplish, Crawford argues [ 1 ]. “The argument that much of Apollo science could have been done robotically is a red herring—some of it could have been, some of it couldn't, but the point is most of it would not have been done otherwise,” he says.

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

  • I. A. Crawford, “Dispelling the myth of robotic efficiency,” Astron. Geophys. 53 , 2.22 (2012) .

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What is a Round Trip Flight? (Everything You Should Know)

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A round-trip flight is when you fly from somewhere and then return to that original location.

When you book a round-trip flight, you receive a single ticket that covers both the outbound and inbound legs of the journey.

Round-trip flights are usually cheaper than booking multiple one-way flights, especially for international destinations. But you sacrifice flexibility and have to pay more upfront for a round-trip ticket.

Rescheduling round-trip flights can also be both difficult and expensive. 

Round-trip flights can include layovers and connecting flights . But as long as the start and end points are the same, it’s a round-trip flight. 

Table of Contents

  • 1 How Round Trip Flights Work
  • 2 Round Trip Flights Are Cheaper than One Way Flights
  • 3 You Don’t Have to Fly Both Legs
  • 4 How Long You Can Stay on a Round Trip Ticket
  • 5.1 1. Lower Costs
  • 5.2 2. Lower Taxes
  • 5.3 3. Fewer Cancellation Fees
  • 5.4 4. Vouchers
  • 6.1 1. Changing the date can be more expensive
  • 6.2 2. Expensive domestic flights
  • 6.3 3. High Upfront Costs
  • 6.4 4. Automatic Itinerary Cancellations
  • 6.5 5. Decreased Flexibility
  • 7 One-Way and Open-Jaw Flights

How Round Trip Flights Work

A round-trip flight includes a flight from your original location, let’s call ‘A,’ to your destination, let’s call ‘B,’ as well as a flight from B back to A. 

With a round-trip ticket, you get a flight to B, where you’ll stay for as long as you like before going back to A. 

Round-trip flights are the most popular flights for travelers who are flying for a vacation.

Round Trip Flights Are Cheaper than One Way Flights

Round trips are almost always cheaper than one-way flights when booking with the same airline, with most airlines incentivizing round trips, especially for leisure travel, and especially to international destinations. 

For example:

  • A round trip from London to New York may cost $1,000.
  • A one-way trip to New York City from London might cost $600, and a one-way trip from London to New York City may also cost $600.
  • In this example, you’d save $200 by choosing a round trip with the same airline rather than booking two separate flights. 

But it could be cheaper for you to fly with two different airlines.

Let’s say you find a British Airways flight from London to New York City for $600. But you also find a Delta Air Lines flight from New York City to London for only $300. 

In that case, you’d save $100 by buying two one-way flights from different airlines. 

You Don’t Have to Fly Both Legs

You’re not technically obligated to fly both legs of a round-trip flight.

If you fly the first leg from your location to the destination, you could stay there longer and miss the return flight if you wanted to.

But airlines dislike this behavior and may penalize your flying privileges if you do this repeatedly. 

If you miss the first leg of your round-trip flight, the airline will most likely automatically cancel the return flight, too. 

How Long You Can Stay on a Round Trip Ticket

You can stay on a round-trip flight for as little as one day to as long as a year.

The exact duration you can stay depends on the airline’s booking policies and flight availability. 

Pros of Round Trip Flights

1. lower costs.

Round trips from the same airlines are almost always cheaper than booking two one-way flights.

Round trips for international flights are especially cheaper than purchasing two one-way tickets from the same airline. 

2. Lower Taxes

You only have to pay sales tax once with a round-trip flight.

That’s because you only have to pay for one ticket, which includes both your flights. 

3. Fewer Cancellation Fees

You’ll only be charged a single cancellation fee if you cancel a round-trip flight.

But if you cancel two one-way flights, you’ll pay cancellation fees for both flights. 

4. Vouchers

You can save a lot of money on round-trip flights if you have a voucher, like a companion voucher from a credit card.

You’ll save more when booking round-trip flights, since an individual round-trip flight costs more than an individual one-way flight.

And vouchers can only be used once and are usually a percentage discount. 

Cons of Round Trip Flights

1. changing the date can be more expensive.

Changing the dates for a round-trip flight costs between $0 to $400.

In some cases, it could be so expensive that you’d be better off missing a flight and booking a new one-way flight instead. 

2. Expensive domestic flights

Round-trip domestic flights with the same airline could be more expensive than booking multiple one-way domestic flights with different airlines.

Most airlines are more price competitive for domestic routes rather than international ones. 

3. High Upfront Costs

You have to pay more up-front for a round-trip flight than when booking a one way flight.

4. Automatic Itinerary Cancellations

If you miss the first leg of your round trip, the airline may also cancel your return trip, and you will lose your money.

5. Decreased Flexibility

You have to meet the scheduled flight date and times for a round trip flight.

Whereas with multiple one-way trips, you could just book your return flight whenever you want while at your destination. 

You can technically change your return flight for a round-trip, but it’s often expensive and difficult. 

One-Way and Open-Jaw Flights

A one-way flight is a flight from one destination to another destination that doesn’t include a return flight.

An open-jaw flight i s when you travel from your location to a first destination.

Then you travel to a different subsequent destination(s) and return to your original location from one of the subsequent destinations. 

For example, you could fly from New York to Paris. Then drive from Paris to Berlin and fly from Berlin back to New York.

In conclusion:

  • A round-trip flight is when you fly from your location to another destination, and then fly back to your original location.
  • With a round-trip flight, you purchase one ticket, which gives you an inbound and outbound flight.
  • Round-trip flights are most popular among tourists.
  • Booking a round-trip flight has many advantages over booking multiple one-way flights.
  • Most importantly, round-trip flights are almost always cheaper than multiple one-way flights, especially for international destinations.
  • Round trip flights are also more convenient for people who are only traveling a short time, such as for a vacation.
  • But round trip flights are less flexible and more expensive to alter. 

See Also: A Complete Guide to Airline Operations

Ella Dunham

Ella Dunham, a Freelance Travel Journalist and Marketing Manager, boasts an impressive career spanning eight years in the travel and tourism sectors.

Honored as one of "30 Under 30" by TTG Media (the world’s very first weekly travel trade newspaper), a "Tour Operator Travel Guru" and "Legend Award" winner, Ella is also a Fellow of the Institute of Travel, a Member of the Association of Women Travel Executives, has completed over 250 travel modules, and hosts travel-focused segments on national radio shows where she provides insights on travel regulations and destinations.

Ella has visited over 40 countries (with 10 more planned this year).

Related Posts:

What is a round-trip flight?

Meghna Maharishi

Editor's Note

It's one of the first decisions we make when booking a flight: Should I book a round-trip or a one-way flight?

It's typically something you'll think about before you ever click "search" to find flights and airfare.

At face value, it seems like a pretty straightforward choice. Book a one-way flight if you're only flying in one direction, and book a round-trip flight if you'll be returning home ... right?

Unfortunately, it's not quite that simple. Thanks to airline pricing trends and our own personal scheduling quirks, there's a bit more to consider.

There may be cases where you wonder if you're better off booking two one-way flights to save money. You may have uncertain plans that make it difficult to commit to a return flight. In some cases, when visiting a few different cities, you may be better off with something different entirely: a multicity itinerary.

There are also plenty of additional considerations if you're booking an award flight using frequent flyer miles or flying internationally.

Here, we break down the basics of what you need to know about booking round-trip flights.

round trip time moon light

A round-trip flight is an itinerary from one destination to another, with a flight back to the original destination.

In most cases, this is what you probably book when going on vacation or visiting a family member for a holiday weekend.

Let's say I live in Charlotte and want to fly to Arizona for a spring break trip. I book an itinerary with an outbound flight to Phoenix Sky Harbor International Airport (PHX) and a return trip to Charlotte Douglas International Airport (CLT) aboard American Airlines.

round trip time moon light

The two flights, booked together on a single itinerary, constitute a round trip.

What is the difference between a round-trip flight and a one-way flight?

When you book a round-trip flight, your itinerary includes an outbound flight and a return trip.

A one-way flight only takes you one direction — say, from Charlotte to Phoenix — with no return flight scheduled.

Is a round-trip flight different from 2 one-way flights?

Yes, in terms of how you book your trip. No, in terms of your travel plans themselves.

Again, a round-trip itinerary includes both an outbound flight and a return trip to the city of origin. A one-way flight is a single trip from one airport to another, with no return booked.

Booking 2 one-way flights

However, if you book two one-way flights, you can, in essence, create your own version of a round trip. This could be on the same airline or on two entirely different airlines.

For the purposes of your travel experience, it's effectively a round trip.

But, know that in the airline computer system(s), you'd technically be traveling on two separate reservations. So, you'd receive different trip confirmation numbers for the outbound and return flights.

Is booking 2 one-ways cheaper than a round-trip flight?

In the U.S., splitting a round trip up into two one-way flights on the same airline and travel dates typically makes no difference in terms of price.

However, on a small number of routes, airlines do charge a premium for one-way bookings compared to the price they charge for a round trip. This is more common internationally, where round-trip flights can be a better value than two one-way trips. Booking two separate one-way flights tends to be more expensive for international travel.

Also, budget carriers frequently offer one-way fares at the same price as a round-trip ticket.

That means if you booked separate one-way flights, you'd most likely end up paying the same as, or even more than, a round-trip fare, depending on the route.

Booking 2 one-way flights on different airlines

On the other hand, there are cases where, thanks to a tool like Google Flights , you might discover that you can save money by booking an outbound, one-way flight on one airline and a one-way return flight on a different airline.

For example, last year, TPG contributor Sean Cudahy needed to travel to North Texas for the weekend. Round-trip flights on a single airline from the Washington, D.C., region to Dallas Fort Worth International Airport (DFW) were coming in at more than $600 that particular weekend.

However, he saved a couple hundred dollars by mixing and matching: He booked a one-way, outbound flight to DFW aboard Delta Air Lines and a separate, one-way return flight on American Airlines.

Just keep in mind this can be risky. If your flight on one airline gets significantly delayed — to the point that you miss your return flight — your second airline won't automatically rebook you. The airline staff may not have much sympathy for your situation since your troubles happened aboard a different carrier.

Can I book a round-trip flight to 1 city and then return home from another?

Yes. These flights are known as open-jaw or multicity itineraries. Many airlines offer this booking option.

Let's say I want to fly from Newark Liberty International Airport (EWR) to Orlando International Airport (MCO). I'm going to visit Walt Disney World for a few days. Then, I'm going to take a Brightline train to South Florida and spend a few days at the beach before flying back to New York.

Since these are airports heavily served by JetBlue, I'll use that carrier as an example. On JetBlue's website, I'll select "Multi-city" instead of searching "Roundtrip" or "One-way" flights.

round trip time moon light

I'll need to separately enter each leg of the trip. Let's do a Saturday departure from Newark to Orlando, and then a Thursday return from Fort Lauderdale/Hollywood International Airport (FLL) to Newark.

round trip time moon light

You'll end up booked on a single itinerary, with the outbound and return flights linked, but with the different city combinations.

Can I buy a round-trip flight with an open return?

No, not exactly. When you book a round-trip flight, you'll generally have to specify a return leg and date.

If your plans are likely to change, though, what you'll want to do instead is pick a date that's far enough out. You'll need to book with an airline or in a cabin class that doesn't charge change fees. Then, reschedule your return trip once your plans are set. Alternatively, you can book a "flexible" fare, which is more expensive but generally allows easier changes.

Make sure you're familiar with an airline's change-fee policy before booking an open-return round-trip flight. For example, most airlines won't let you cancel or change basic economy tickets.

round trip time moon light

Should I book mileage or award tickets as 2 one-ways or a round trip?

It largely depends on the route. In some cases, you'll get better award availability if you book two one-way flights. In others, the taxes for two one-way award flights could end up being higher than what you'd pay for a round-trip itinerary.

However, in most cases nowadays, award tickets for two one-way flights and a round-trip flight tend to add up to the same number of miles. Just be sure to check both on an airline's website to ensure you're getting the best possible award availability .

Related reading:

  • When is the best time to book flights for the cheapest airfare?
  • The best airline credit cards
  • What exactly are airline miles, anyway?
  • 6 real-life strategies you can use when your flight is canceled or delayed
  • Maximize your airfare: The best credit cards for booking flights
  • The best credit cards to reach elite status
  • What are points and miles worth? TPG's monthly valuations


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How Do You Solve a Moon Mystery? Fire a Laser at It

Researchers have used reflective prisms left on the moon’s surface for decades, but had increasingly seen problems with their effectiveness.

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By Katherine Kornei

The moon is drifting away. Every year, it gets about an inch and a half farther from us. Hundreds of millions of years from now, our companion in the sky will be distant enough that there will be no more total solar eclipses .

For decades, scientists have measured the moon’s retreat by firing a laser at light-reflecting panels, known as retroreflectors, that were left on the lunar surface, and then timing the light’s round trip. But the moon’s five retroreflectors are old, and they’re now much less efficient at flinging back light. To determine whether a layer of moon dust might be the culprit, researchers devised an audacious plan: They bounced laser light off a much smaller but newer retroreflector mounted aboard a NASA spacecraft that was skimming over the moon’s surface at thousands of miles per hour. And it worked.

These results were published this month in the journal Earth, Planets and Space.

Of all the stuff humans have left on the moon , the five retroreflectors, which were delivered by Apollo astronauts and two Soviet robotic rovers, are among the most scientifically important. They’re akin to really long yardsticks: By precisely timing how long it takes laser light to travel to the moon, bounce off a retroreflector and return to Earth (roughly 2.5 seconds, give or take), scientists can calculate the distance between the moon and Earth.

Arrays of glass corner-cube prisms make this cosmic ricochet possible. These optical devices reflect incoming light back to exactly where it came from, ensuring that retroreflectors send photons on a tight, neat flip turn.

Making repeated measurements over time allows researchers to piece together a better picture of the moon’s orbit, its precise orientation in space and even its interior structure .

But the moon’s suitcase-size retroreflectors, delivered from 1969 through 1973, are now showing their age. In some instances, they’re only about one-tenth as efficient as expected, said Tom Murphy, a physicist at the University of California, San Diego, who was not involved in the research. “The returns are severely depressed.”

One obvious culprit is lunar dust that has built up on the retroreflectors. Dust can be kicked up by meteorites striking the moon’s surface. It coated the astronauts’ moon suits during their visits, and it is expected to be a significant problem if humans ever colonize the moon.

While it has been nearly 50 years since a retroreflector was placed on the moon’s surface, a NASA spacecraft launched in 2009 carries a retroreflector roughly the size of a paperback book. That spacecraft, the Lunar Reconnaissance Orbiter, circles the moon once every two hours, and it has beamed home millions of high-resolution images of the lunar surface .

The Lunar Reconnaissance Orbiter “provides a pristine target,” said Erwan Mazarico, a planetary scientist at NASA Goddard Space Flight Center who, along with his colleagues, tested the hypothesis that lunar dust might be affecting the moon’s retroreflectors.

But it’s also a moving target. The orbiter skims over the moon’s surface at 3,600 m.p.h. “It’s hard enough to hit a stationary target,” said Dr. Murphy, who leads the Apache Point Observatory Lunar Laser-ranging Operation , or APOLLO, a project that uses the retroreflectors on the moon’s surface. “We’re going to give you a smaller array and make it move on you.”

In 2017, Dr. Mazarico and his collaborators began firing an infrared laser from a station near Grasse, France — about a half-hour drive from Cannes — toward the orbiter’s retroreflector. At roughly 3 a.m. on Sept. 4, 2018, they recorded their first success: a detection of 25 photons that made the round trip.

The researchers notched three more successes by the fall of 2019. After accounting for the smaller size of the orbiter’s retroreflector, Dr. Mazarico and his colleagues found that it often returned photons more efficiently than the Apollo retroreflectors.

There isn’t enough evidence yet to categorically blame the dust for the poorer performance of the moon’s retroreflectors, said Dr. Mazarico, and more observations are being collected. But Dr. Murphy and other scientists said the new findings were helping build the case.

“For me, the dusty reflector idea is more supported than refuted by these results,” he said.

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The Basics of Lunar Ranging

How does it work.

The basic idea is that we want to test our understanding of gravity to unprecedented precision. The earth-moon system is an ideal laboratory for performing such a measurement. The earth and moon are attracted to each other and to the sun by gravity, so that studying the dynamics of this system is a way to explore the precise behavior of gravity. For instance, we can ask the question: "do the earth and moon fall toward the sun with the same acceleration?" To do this, we need to measure the exact shape of the lunar orbit. We want to measure the earth-moon distance at various points in the lunar cycle (crescent, quarter moon, gibbous, and full phases). Each measurement will be precise to one millimeter —about the thickness of a paperclip. We do this by measuring the round-trip travel time of a light pulse bouncing off lunar reflectors. Multiplying this time by the speed of light gives us the distance. In addition to characterizing the shape of the lunar orbit, we we will be able to follow the slow recession of the lunar orbit by 3.8 cm (1.5 inches) per year due to tidal friction.

One way to state why millimeter-precision measurements help us understand gravity is: any theory of gravity will predict certain specific paths/orbits of the bodies under its influence, given initial conditions. Is general relativity predicting exactly the correct path for the moon? If not, we've got trouble. General relativity (GR) predicts deviations from Newtonian gravity at the several-meter level in the lunar orbit. So millimeter-level measurement precision puts GR to a hard test.

What's the point?

When Einstein came up with his theory of general relativity, he claimed that this was a better description of gravity than Newton's time-tested and very successful model. Many reacted with healthy skepticism—nobody was complaining about deficiencies in Newtonian gravity (at least not many people were). But today, test after test has shown that Newtonian gravity doesn't cut it in ultra-precise tests. It's still good enough to plan interplanetary probe trajectories, but not good enough to describe everything we see. And surprisingly, understanding general relativity is absolutely vital in getting the global positioning system (GPS) to work. This system would utterly fail in an hour's time if we didn't anticipate that time runs more slowly in a gravitational field—a consequence predicted by general relativity.

Today, there is a growing sentiment in the physics and astrophysics communities that general relativity is likely not the last word on gravity. First, GR is not compatible with quantum mechanics. Maybe we live in the kind of universe where we can have two incompatible descriptions of the fundamental rules of how matter behaves and interacts. But past reductions/unifications (e.g., electricity and magnetism; the weak nuclear force and electromagnetism) suggest otherwise. Who would you bet on? Aspects of quantum mechanics have been tested to phenomenal precisions: part-in-a-trillion levels. But GR has been tested only to part-in-100,000 levels so far. It's more likely to be incomplete than is quantum mechanics. And perhaps more convincingly, we now see that the expansion of the universe is accelerating ! This is a total surprise, and not consistent with the predictions of general relativity. It is highly likely that the solution to this puzzle will involve a modification to or replacement of general relativity.

Lunar laser ranging has been performed for the past 35 years, now reaching a precision of 2 centimeters. APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) will improve this performance by at least a factor of ten. While this factor of ten (or better) may not be enough to expose a shortcoming in the theory of general relativity, there is a chance that it will . We can't afford not to look. At the very least, APOLLO will place more stringent constraints on existing and future alternatives to Einstein's theory of gravity. Many new phenomena are discovered in science by looking closely at things we think we understand perfectly, only to find that our knowledge or understanding is incomplete.

Ultra-short bursts of light

How do we measure the distance to the moon to such phenomenal precision? We "ping" the moon with ultra-short pulses of light. To do this, we have a laser that generates intense bursts of light only 100 picoseconds long—that's one tenth of a billionth of a second! Light, which travels 7 earth circumferences every second, only travels about an inch in this time. So these pulses are like little "bullets" of light.

Better than a stopwatch...

In essence, we measure the time it takes for the pulse of light to travel to the moon and back. This can take anywhere from 2.34 to 2.71 seconds, depending on how far away the moon is at the time (the earth-moon distance ranges from 351,000 km to 406,000 km). We can time the round trip to few-picosecond precision, or a few trillionths of a second.

But what do we measure to ?

We measure to the retroreflector arrays left on the moon by the Apollo astronauts, and by an unmanned Soviet rover carrying a French-built reflector. These define very specific points of reference on the lunar surface. This is far better than measuring to the rough-and-tumble surface. We would never have any hope of measuring the lunar distance to millimeter precision without these well-defined reflectors. We aim at one reflector at a time when performing the measurement.

Where on earth do we measure from ?

The telescope used for APOLLO has a 3.5 meter diameter mirror, and is located at the Apache Point Observatory in southern New Mexico. We use the telescope as a gigantic (3.5 meter wide) laser pointer and also as a signal receiver. We reference our measurements to the center of the telescope mount, where the azimuth axis and elevation axis intersect each other. As the telescope swings around to point at different parts of the sky, this point stays fixed—almost.... The position of the telescope relative to the center of the earth isn't as stationary as you might imagine. The continental plate drifts, the tides from the moon and sun make the site swell by about a foot twice a day, weather systems can push the local crust down, etc. We have to be aware of all of these influences and take them into account in order to extract the scientifically useful center-to-center distance between the earth and moon.

The observing technique

We will typically measure the distance to each of the four available reflectors in turn over a half-hour period. Then we'll do the same thing a few nights later. By doing this over months and years, we will characterize the shape of the lunar orbit to high enough precision to be able to say something about the workings of gravity.

The pointing challenge

To concentrate as much laser power as possible onto the reflector array, we must ensure that the beam leaving the telescope is as collimated (parallel, non-diverging) as possible. We use a laser both because we can get ultra-short pulses of light from a laser, and also because the light from a laser is extraordinarily directional—not diverging the way a flashlight, or even searchlight, would. Even so, the turbulent atmosphere distorts the beam, imparting a divergence of about one arcsecond (sometimes more). One arcsecond is 1/3600th of a degree, or the angular size of a quarter about five kilometers (about 3 miles) away. At the distance of the moon, this angle translates to 1.8 kilometers (just over a mile). Though this is large compared to the size of the reflector (most of the light is wasted—never hitting the reflector), it is still a challenge to point and maintain the laser beam on this tiny patch of the moon.

A cartoon of the geometry

As the above schematic illustrates, the beam we send to the moon diverges (much exaggerated) due to the earth's atmosphere. Only about one part in 30 million of the light we send to the moon is lucky enough to actually strike the targeted reflector. But the reflector is composed of small corner cubes, and for reasons related to the uncertainty principle in quantum mechanics, the light returning from each of these small apertures is forced to have a divergence (called diffraction). In the case of the Apollo reflectors, this divergence is in the neighborhood of 8 arcseconds. This means that the beam returning to the earth has a roughly 15 kilometer (10 mile) footprint when it returns to the earth. We scrape up as much of this as our telescope will allow, but a 3.5 meter aperture will only get about one in 30 million of the returning photons—coincidentally the same odds of hitting the reflector in the first place.

Staggering Odds

It always comes down to statistics.

We can't achieve millimeter-range precision from a single photon. Our laser pulse is broader than this (2.5 cm), and we don't know if a particular photon was in the leading or trailing edge of the beam, or right in the middle. Likewise, the slight tilt of the reflector array introduces a similar uncertainty. In all, we have about 30 to 50 millimeters of uncertainty per photon. But if we collect many photons, the average round-trip time for the ensemble will have a higher precision. The statistical rule for this is that the reduction in error one gets by obtaining N independent measurements is the square-root of N . So to get a reduction to one millimeter from 30–50 mm, we need 900–2500 photons, depending mostly on the degree of tilt of the reflector array for that observation. At, say, one photon per pulse, we will get 1200 photons per minute, and should have adequate numbers for a millimeter measurement in a matter of minutes.

What's so hard about it all?

Is it safe.

It's all fun and games until someone shoots an eye out! Working with a powerful laser demands some attention to safety. We follow strict safety guidelines when working around the laser, wearing protective glasses that only admit one ten-millionth of any laser light hitting them to pass through. But once we have expanded the beam to fill the 3.5 meter telescope aperture, it is far less dangerous—almost eye-safe, in fact (far too weak to cause damage to anything but eyes or sensitive detectors). Nonetheless, we are diligent about not hitting aircraft, which, more than creating an eye-hazard would potentially startle pilots. Some have reacted in horror when we tell them that we are shooting a laser at the moon. "Why would you want to destroy the moon?" Rest assured that 2.3 watts of laser power spread over a 2 kilometer patch on the moon is nothing compared to the sun's 1380 watts per square meter. Not even enough to tickle.

So now we know what we want to do, and how to do it. But that's the easy part. The first hard part is under our belts: we have constructed the apparatus and established initial operation (July 2005). In the fall of 2005, we acquired our first lunar ranges with the system ( see full story ). We are now working (winter, 2006) to establish a routine operational state ( track our progress ). In the spring of 2006, the long effort of data collection will begin, and the next hard part begins: making sense of all these measurements!

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If daylight saving time seems tricky, try figuring out the time on the moon

Geoff Brumfiel, photographed for NPR, 17 January 2019, in Washington DC.

Geoff Brumfiel

Carmen Molina Acosta

round trip time moon light

As more robots and people travel to the moon in coming years, some researchers believe it's time to set a lunar time standard. NASA Johnson hide caption

As more robots and people travel to the moon in coming years, some researchers believe it's time to set a lunar time standard.

As U.S. clocks shift forward this weekend, many earthlings will find themselves momentarily confused about what time it is. But scientists say a far larger temporal problem is looming on the horizon: With multiple missions to the moon in the planning phase, it's time to set a Lunar time standard.

"We need to define a time on the moon," says Javier Ventura-Traveset of the European Space Agency (ESA). Without it, Ventura-Traveset warns, docking spacecraft could tumble into each other, astronauts might get lost on the lunar surface, and of course, nobody will know when they can take their lunch break.

When humans first traveled to the moon in the 1960s and 1970s, they didn't worry too much about what time to use, according to Andrew Chaikin , an author and space historian. "They set their watches to Houston time because that's where mission control was," he says. More formally, he adds, the Apollo missions used something called "mission elapsed time," which was a clock that started the second the rocket lifted off from Earth.

But in the decades since those missions, time has taken on a new importance here on Earth.

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While nations set their times according to their position on our rotating ball of rock, all time zones globally are defined against single Coordinated Universal Time (UTC). UTC is set by the International Bureau of Weights and Measures in Paris, France. It's created using an ensemble of atomic clocks all over the globe: they feed their times into the Parisian central laboratory, thereby ensuring that every nation ticks in synchrony to within a tiny fraction of a second.

In our modern world, that timing is essential to keeping computer networks humming and markets trading. Perhaps most importantly, time is the cornerstone of the world's global navigation systems.

Satellites – like those used for GPS – send time signals down to Earth. Because those signals arrive a fraction of a second later than the current time on the ground, the time difference can be used to determine a person's position on the planet's surface to astonishing accuracy.

Ventura-Traveset is part of a European effort to create a GPS-like system for the moon. Known as " Moonlight ," the system would use a small number of satellites to create a communications and navigation network around a crewed landing site on the lunar surface.

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But for it to work, both the landing site and the satellites need to know what time it is.

Currently, lunar missions use UTC like everyone else, and that might seem like the simplest solution, Ventura-Traveset says. But in fact, he believes it'll be tricky to use UTC on the moon. Part of the problem is the sheer distance between Earth and the moon, which means a timing signal from Earth would take over a second to get from earth to the moon – an eternity by the standards of today's atomic clocks.

But there's an even more fundamental problem: Einstein's theory of relativity states that time actually ticks differently in different parts of space . Specifically, the moon's lower gravity and its motion relative to Earth cause time to pass around 56 microseconds faster each earth day.

This 56 microseconds is not some abstract concept: Every day, astronauts living on the moon will age 56 microseconds more quickly than they will on Earth. That's far too small a time gap to make a difference in a human lifespan, but for navigation, that kind of time difference is enormous, Ventura-Traveset says.

To get accurate positions, "you need the level of nanoseconds," he says. In other words, the different rate of time flow on the moon means that clocks on the lunar surface cannot simply be run from Earth. Instead, he thinks a set of lunar clocks will need to be created that keep a special "moon" time.

Given the moon's proximity to Earth, Ventura-Traveset thinks that it may make sense to use some additional calculations to keep lunar time in sync with UTC, to give the illusion of continuity between the Earth and the moon. But as humans explore further into the solar system, he believes it will be necessary to create completely separate timescales. "If you are on Mars, or you are even farther away, probably you have your own time," he says.

ESA is working with NASA to figure out how to create standards for communications and navigation on the moon. In a statement, NASA told NPR that "subject matter experts throughout the international community are discussing an approach to provide recommendations to the International Astronomical Union for lunar reference frame and time systems."

Ventura-Traveset hopes ESA's proposal for a separate lunar time will ultimately be accepted by other space agencies around the world. "It's fascinating times," he muses.

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How long does it take to get to the moon?

Here we explore how long it takes to get to the moon and the factors that affect the journey to our rocky companion.

how long does it take to travel to the moon? Artist's illustration of an astronaut sitting on the moon looking back at Earth and taking a photo of our home planet with a phone.

  • Traveling at the speed of light
  • Fastest spacecraft
  • Driving to the moon

Q&A with an expert

  • Calculating travel times

Moon mission travel times

Additional resources, bibliography.

If you wanted to go to the moon, how long would it take? 

Well, the answer depends on a number of factors ranging from the positions of Earth and the moon , to whether you want to land on the surface or just zip past, and especially to the technology used to propel you there.

The average travel time to the moon (providing the moon is your intended destination), using current rocket propulsion is approximately three days. The fastest flight to the moon without stopping was achieved by NASA's New Horizons probe when it passed the moon in just 8 hours 35 minutes while en route to Pluto . 

Currently, the fastest crewed flight to the moon was Apollo 8. The spacecraft entered lunar orbit just 69 hours and 8 minutes after launch according to NASA .

Here we take a look at how long a trip to the moon would take using available technology and explore the travel times of previous missions to our lunar companion. 

Related: Missions to the moon: Past, present and future

How far away is the moon?

To find out how long it takes to get to the moon, we first must know how far away it is. 

The average distance between Earth and the moon is about 238,855 miles (384,400 kilometers), according to NASA. But because the moon does not orbit Earth in a perfect circle, its distance from Earth is not constant. At its closest point to Earth — known as perigee — the moon is about 226,000 miles (363,300 km) away and at its farthest — known as apogee — it's about 251,000 miles (405,500 km) away.

How long would it take to travel to the moon at the speed of light?

Light travels at approximately 186,282 miles per second (299,792 km per second). Therefore, a light shining from the moon would take the following amount of time to reach Earth (or vice versa):  

  • Closest point: 1.2 seconds 
  • Farthest point: 1.4 seconds 
  • Average distance:  1.3 seconds 

How long would it take to travel to the moon on the fastest spacecraft so far?

The fastest spacecraft is NASA's Parker Solar Probe , which keeps breaking its own speed records as it moves closer to the sun. On Nov. 21, 2021, the Parker Solar Probe clocked a top speed of 101 miles (163 kilometers) per second during its 10th close flyby of our star, which translates to a blistering 364,621 mph (586,000 kph). According to a NASA statement , when the Parker Solar Probe comes within 4 million miles (6.2 million kilometers) of the solar surface in December 2024, the spacecraft's speed will top 430,000 miles per hour (692,000 km/h)!

So if you were theoretically able to hitch a ride on the Parker Solar Probe and take it on a detour from its sun-focused mission to travel in a straight line from Earth to the moon, traveling at the speeds the probe reaches during its 10th flyby (101 miles per second), the time it would take you to get to the moon would be:

  • Closest point: 37.2 minutes
  • Farthest point: 41.4 minutes
  • Average distance: 39.4 minutes

How long would it take to drive to the moon?

Let's say you decided to drive to the moon (and that it was actually possible). At an average distance of 238,855 miles (384,400 km) and driving at a constant speed of 60 mph (96 km/h), it would take about 166 days.  

We asked Michael Khan, ESA Senior Mission Analyst some frequently asked questions about travel times to the moon. 

Michael Khan is a Senior Mission Analyst for the European Space Agency (ESA). His work involves studying the orbital mechanics for journeys to planetary bodies including Mars.

And what affects the travel time?

The time it takes to get from one celestial body to another depends largely on the energy that one is willing to expend. Here  "energy" refers  to the effort put in by the launch vehicle and the sum of the manoeuvres of the rocket motors aboard the spacecraft, and the amount of propellant that is used. In space travel, everything boils down to energy. Spaceflight is the clever management of energy.

Some common solutions for transfers to the moon are 1) the Hohmann-like transfer and 2) the Free Return Transfer. The Hohmann Transfer is often referred to as the one that requires the lowest energy, but that is true only if you want the transfer to last only a few days and, in addition, if some constraints on the launch apply. Things get very complicated from there on, so I won't go into details.

The transfer duration for the Hohmann-like transfer is around 5 days. There is some variation in this duration because the moon orbit is eccentric, so its distance from the Earth varies quite a bit with time, and with it, the characteristics of the transfer orbit.

The Free Return transfer is a popular transfer for manned spacecraft. It requires more energy than the Hohmann-like transfer, but it is a lot safer, because its design is such that if the rocket engine fails at the moment you are trying to insert into the orbit around the Moon, the gravity of the Moon  will deflect the orbit exactly such that it returns to the Earth. So even with a defective propulsion system, you can still get the people back safely. The Apollo missions flew on Free Return transfers. They take around 3 days to reach the moon.

Why are journey times a lot slower for spacecraft intending to orbit or land on the target body e.g. Mars compared to those that are just going to fly by?

If you want your spacecraft to enter Mars orbit or to land on the surface, you add a lot of constraints to the design problem. For an orbiter, you have to consider the significant amount of propellant required for orbit insertion, while for a lander, you have to design and build a heat shield that can withstand the loads of atmospheric entry. Usually, this will mean that the arrival velocity of Mars cannot exceed a certain boundary. Adding this constraint to the trajectory optimisation problem will limit the range of solutions you obtain to transfers that are Hohmann-like. This usually leads to an increase in transfer duration.

Calculating travel times to the moon — it's not that straightforward

A problem with the previous calculations is that they measure the distance between Earth and the moon in a straight line and assume the two bodies remain at a constant distance; that is, assuming that when a probe is launched from Earth, the moon would remain the same distance away by the time the probe arrives. 

In reality, however, the distance between Earth and the moon is not constant due to the moon's elliptical orbit, so engineers must calculate the ideal orbits for sending a spacecraft from Earth to the moon. Like throwing a dart at a moving target from a moving vehicle, they must calculate where the moon will be when the spacecraft arrives, not where it is when it leaves Earth. 

Another factor engineers need to take into account when calculating travel times to the moon is whether the mission has the intention of landing on the surface or entering lunar orbit. In these cases, traveling there as fast as possible is not feasible as the spacecraft needs to arrive slowly enough to perform orbit insertion maneuvers. 

More than 140 missions have been launched to the moon, each with a different objective, route and travel time. 

Perhaps the most famous — the crewed Apollo 11 mission — took four days, six hours and 45 minutes to reach the moon. Apollo 10 still holds the record for the fastest speed any humans have ever traveled when it clocked a top speed of while the crew of Apollo 10 traveled 24,791 mph (39,897 kph) relative to Earth as they rocketed back to our planet on May 26, 1969.

The first uncrewed flight test of NASA's Orion spacecraft and space launch system rocket — Artemis 1 — reached the moon on flight day six of its journey and swooped down to just 80 miles (130 km) above the lunar surface to gain a gravitational boost to enter a so-called "distant retrograde orbit." 

Read more about how space navigation works with accurate timekeeping with these resources from NASA . Learn more about how before the days of GPS engineers were able to navigate from Earth to the moon with such precision with this article by Gwendolyn Vines Gettliffe published at the Massachusetts Institute of Technology (MIT) 'ask an engineer' feature. 

Hatfield, M. (2021). Space Dust Presents Opportunities, Challenges as Parker Solar Probe Speeds Back toward the Sun – Parker Solar Probe. [online] Available at: .

NASA (2011). Apollo 8. [online] NASA. Available at: . (n.d.). How many people have walked on the Moon? [online] Available at: . 

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Daisy Dobrijevic

Daisy Dobrijevic joined in February 2022 having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K.

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  • UFOareAngels The fact that we are still asking this question proves we never went to the moon and are never going back. Reply
  • Rathelor Those Parker Solar Probe travel times seems a little too high. Reply
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Map showing which part of the Earth the Moon is currently above the horizon at.

UTC time = Sunday, 28 January 2024, 13:26:00.

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On Sunday, 28 January 2024, 13:26:00 UTC the Moon is at its zenith at Latitude: 11° 08'  North , Longitude: 166° 59'  West

The ground speed is currently 442.75 meters/second, 1593.9 kilometres/hour, 990.4 miles/hour or 860.6 nautical miles/hour (knots). The table below shows position of the the Moon compared to the time and date above:

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Science in School

Science in School

To the moon and back: reflecting a radio signal to calculate the distance teach article.

Author(s): Richard Middelkoop

Using a simple calculation, measure the distance between Earth and the Moon with the help of a local amateur radio station.

Inspired by an earlier article in Science in School that used photography to measure the distance to the Moon ( Cenadelli et al., 2016 ), we set up an experiment with groups of scouts around the globe to do the same with radio signals. With help from a qualified radio user, the groups sent radio signals from their transmitter stations to the Moon. The signals bounce off the surface of the Moon and back to Earth, where they are detected by a receiver. This radio transmission technique, which is known as ‘moon bounce’ or ‘Earth-Moon-Earth’ communication, was used extensively for military communication in the days before satellites.

Since radio waves are a type of electromagnetic radiation, they travel at the speed of light. Due to the travel time between Earth and the Moon, the reflected radio signal is delayed typically by a few seconds. Using this time delay, the groups calculated the distance that the radio wave travelled and successfully measured the distance to the Moon.

round trip time moon light

Measuring the distance from Earth to the Moon

In this article, we describe how to conduct the activity at your school, beginning with contacting a radio amateur (a person licensed by the relevant authorities to transmit high-power radio signals) for help. We then explain how to transmit and measure the radio signal, and carry out the final calculation. The experiment, which must be carried out when the Moon is above the horizon w1 is suitable for students aged 11 and above and will take around 1.5–2 hours, including set-up time.

For the teacher

The use of an amateur radio station is essential to send the radio signal to the Moon, so you will need to ask your local or national radio amateur club w2 (most countries have one) for their assistance. People with an interest in radio transmission can take an examination to obtain a licence that allows them to transmit radio signals at amateur radio frequencies.

This requires:

  • An antenna capable of being pointed at the Moon to convert the signal into radio waves, and vice versa (figure 1)
  • A radio transmitter/receiver (figure 2) to transmit the radio waves and receive the bounced waves from the Moon
  • A dual-channel oscilloscope (figure 3) to show the time delay between transmitting and receiving the radio waves (figure 4)

Your radio amateur will be able to provide the equipment, if necessary with the help of a local amateur radio club. If the radio amateur sends the signals from the amateur radio station, the returning signals can be streamed via the internet to be viewed at your school (see ‘A louder alternative’ section).

round trip time moon light

For the radio amateur

  • Set up the transmitter/receiver and connect it to the antenna.. The antenna and radio transmitter should be within line of sight of the Moon, and the receiver should not be disturbed by interference signals, such as large electric installations nearby. You can find out where exactly the Moon is positioned in the sky, as seen from your location at the time of the experiment, by looking on the Sky Live website w3 .
  • Select an appropriate frequency in a VHF or UHF amateur radio band.
  • Point the antenna towards the Moon.
  • Connect the oscilloscope to the sound input of the transmitter so that it shows the signal being transmitted.
  • Connect the output of the transmitter/receiver to the second channel of the oscilloscope.
  • Transmit a signal in Morse code or as a series of pulses that easily show on the oscilloscope.
  • On the receiver, listen for the reflection of your signal and watch it on the oscilloscope.
  • Set the transmitter/receiver in the ‘break-in mode’ to quickly switch between transmitting and receiving.
  • Adjust the antenna direction if needed.
  • Align the two signals seen on the oscilloscope and read the time delay between them from the screen.

For the students

Using the time delay, calculate the distance d to the Moon using the following equation

d = ( c x t ) / 2

d = distance of Earth to Moon in metres

c = the speed of light, 3 x 10 8 metres per second

t =time delay in seconds

The radio signal covers the same distance twice (Earth to the Moon, and back), hence the need to divide by 2

For example, with a delay time of 2.56 seconds:

d = [(3 x 10 8 ) x 2.56] / 2

d = 348 000 000 m

Extension options

  • Radio-amatorul ar putea trimite şi recepţiona semnale multiple, astfel încât elevii să poată obţină mai multe măsurători ale timpului de întârziere, pentru a calcula o medie a valorilor şi respectiv o deviaţie standard, cu scopul de a creşte precizia rezultatului.
  • Înregistraţi pe suport digital semnalele transmise şi recepţionate, folosind un dispozitiv audio simplu, precum smart-phone-ul, pentru a le putea analiza ulterior. Aceasta ar permite elevilor să deruleze activitatea şi în absenţa radio-amatorului.

Why does the distance to the Moon vary slightly depending on the observation point on Earth?

Due to the curvature of Earth, the simple formula introduces a small error: the distance to the Moon is slightly different depending on where the observation point is on Earth – close to the equator or closer to one of the poles (see figure 5). This error is very small compared to the huge distance from Earth to the Moon, so it is ignored for this experiment

round trip time moon light

The experiment to measure the distance to the Moon and back was carried out by several scout groups during their annual event, called Jamboree-On-The-Airi w4 (JOTA) in October. The groups were scattered all over the globe, so the aspect angle between their observation points and the Moon were all different.

Why would the result vary if you repeated the experiment two weeks later?

The distance from Earth to the Moon is not completely fixed. The Moon’s orbit around Earth is not a perfect circle, so the distance varies slightly (figure 6). The experiment was carried out in the same weekend so the distance variation had little to no influence.

round trip time moon light

What other sources of small errors are there in your experiment?

  • Delays in streaming the signals over the internet introduces a small error in the calculated distance. This extra delay is typically an order of magnitude smaller than the delay caused by the signal travel time between Earth and the Moon and is therefore ignored in this experiment.
  • The accuracy of the oscilloscope, which depends on the time base (the number of seconds per screen division), can also introduce errors. Typically, the reading can be accurate up to one-tenth of the time base setting. The lower the time base is set, the higher the sweep frequency and the more accurate the result.
  • A weak signal (one that is only just visible above background noise) is more difficult to read on the oscilloscope screen. Identifying the time delay is open to errors, and variations of up to several hundreds of milliseconds can easily occur. Taking multiple measurements and using the average can reduce the error margin.
  • Objects that partially block the path of the radio wave can cause the signal to scatter. This is more likely to occur in urban areas than in open fields and can result in multiple echoes that are visible on the oscilloscope, which in some cases can be stronger than the directly reflected signal from the Moon. As a result, students may mistakenly use the wrong echo to read the time delay.

A louder alternative

If the signal is not strong enough to carry out the activity using the method described for the radio amateur, or you wish to stream the radio signals via the internet, you can use this alternative method instead.

To determine whether the signal will be strong enough, the radio amateur should check the equipment sensitivity and find out exactly where the Moon is positioned in the sky prior to the activity. If they can’t hear the reflected signal, or if the visual signal is lost amongst background noise on the oscilloscope, they can use a large astronomy radio telescope at the Dwingeloo Radio Observatory in the Netherlands as the receiver (figure 7). The radio telescope has been refurbished and is operated by a group of radio amateurs. It receives the radio signal and converts it into a visible signal, which is streamed online and is available for anyone to view w5 .

round trip time moon light

  • In preparation, use the C A Muller Radio Astronomie Station (CAMRAS) website w6 to check for planned activities at the Dwingeloo Radio Observatory. If the telescope is unavailable, you can find an alternative receiver listed on the WebSDR website w7 . Anyone – not just radio amateurs – can use the website at any time. Check that the Moon will be visible from the observatory at the time of your planned experiment w3 .
  • Follow steps 1–4 of the original procedure
  • On the CAMRAS webpage showing the WebSDR stream w5 , shift the yellow slider to the same frequency that will be used to transmit your signal to the Moon (see figure 8).
  • Transmit a signal in Morse code or as a series of pulses that easily show on an oscilloscope connected to your computer.
  • On the computer, listen for the audio signal of the reflected radio wave and watch it on the oscilloscope. Students could also view the signals on separate computers.

round trip time moon light

  • Cenadelli D et al (2016) Geometry can take you to the Moon . Science in School 35 .

Web References

  • w1 – To find out the positions and times that the Moon rises and sets, visit the Heavens Above website .
  • w2 ­– Find your radio amateur using the International Amateur Radio Union website. .
  • w3 – Find out exactly where the Moon is in the sky from your location at the time of your experiment using the Sky Live website .
  • w4 – Jamboree-On-The-Air (JOTA) is an international event of the World Organization of the Scout Movement w8 (WOSM), encouraging scouts around the world to communicate with one another using amateur radio and the internet.
  • w5 – Visit the CAMRAS WebSDR stream to hear radio signals received by the Dwingeloo Radio Observatory amateur telescope in The Netherlands.
  • w6 – Find out whether the Dwingeloo Radio Observatory telescope will be available at the time of your experiment by visiting the CAMRAS website .
  • w7 – To see a list of available radio receivers and to stream signals via the internet, visit the WebSDR .
  • w8 – The World Organization of the Scout Movement (WOSM) is an independent, non-political, non-governmental organisation that is made up of 164 National Scout Organisations (NSOs) from 224 countries and territories around the world. With over 40 million members, WOSM is one of the largest youth movements in the world.
  • Iscra A, Quaglini MT, Rossi G (2006) Introducing radio transmission with a simple experiment . Science in School 3: 39–42.
  • Pössel M (2017) Parallax: reaching the stars with geometry . Science in School 39: 40–44.

Richard Middelkoop has a bachelor’s degree in electrical engineering and a master’s degree in telecommunications from the Eindhoven University of Technology in The Netherlands. He volunteers at the World Organization of the Scout Movement w8 (WOSM) to lead a team that organises an annual get-togetheă w4 for 1 million young people around the globe, by means of radio and internet connections

This activity can provide students with a unique opportunity to take a peek into the world of the experts, observe them while working and understand the science behind the instruments used. It is a fantastic way to put some of the theory learned about radio waves into practice, by calculating the distance of the Moon from different locations, and to investigate the Moon’s orbital pattern. It would also be a great opportunity to collaborate with another school on the other side of the globe and share results and experiences.

Catherine Cutajar, physics teacher, St. Martin’s College Sixth Form, Malta

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The Photonics Spotlight > 2016-07-05

The Round-trip Phase Shift in an Optical Resonator

Posted on 2016-07-05 as part of the Photonics Spotlight (available as e-mail newsletter!)

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Author: Dr. Rüdiger Paschotta , RP Photonics AG , RP Photonics AG

Abstract: The physics of phase shifts in resonators is relevant in many situations, for example in laser physics and ultra-precise laser measurements. This article discusses a number of interesting aspects in that context and can thus help to improve the understanding of the physics of laser beams and resonators.

Dr. Rüdiger Paschotta

In many situations, one needs to consider or calculate the round-trip phase shift in an optical resonator , i.e., the total optical phase shift experienced in a complete resonator round trip. Here, one can encounter various interesting aspects, which I discuss in the following. You may have some fun thinking about these things and at the same time improve your understanding of light beams and optical resonators.

Is It a Well Defined Quantity?

Very often, one considers the round-trip phase shift along the resonator axis. In reality, however, we do not have a ray circulating in the resonator, but a light beam (e.g., a laser beam ) with a finite transverse extension. One may now ask whether the on-axis phase shift is really meaningful; after all, part of the light travels somewhat away from the axis, and one might expect that it experiences a different phase shift there; for example, a lens in the resonator would cause a maximum phase shift on the axis and lower phase shifts away from it. Also, the circulating light has some finite divergence , i.e., it covers some range of propagation angles, and one could imagine that these should have an impact on the round-trip phase shift.

It is instructive now to consider resonator modes . By definition, these are field configurations which fully reproduce themselves after a complete resonator round trip. For such a mode, the round-trip phase shift is necessarily an integer multiple of – just because by definition we have a self-reproducing field configuration. This holds for fundamental (axial) modes as well as for higher-order modes. Here, it is perfectly clear that the on-axis round-trip phase shift is representative for the phase shift experienced along any ray which is, for example, transversely shifted away from the resonator axis. This is true despite possible transverse variations of phase shifts e.g. at lenses or curved mirrors : after each complete round trip, the wavefronts must stay unchanged – otherwise, we would not be dealing with a resonator mode.

Now, different modes of a resonator can have different round-trip phase shifts – differing by integer multiples of . You may now wonder what is the round-trip phase shift of some optical field which is a superposition of different modes, having different round-trip phase shifts: is it perhaps some kind of weighted average, no longer restricted to integer multiples of ? No, it isn't: in general, the round-trip phase shift of such a superposition is not a defined quantity. After all, how should one define the phase shift of a beam with arbitrarily crumbled wavefronts? Note also that in general we are not dealing with a monochromatic field anymore: different modes generally have different mode frequencies. So the round-phase shift is not a well defined quantity for arbitrary beams, but it is defined for resonator modes.

One may actually try to excite a resonator mode with a mode-matched monochromatic beam coming from outside, hitting a partially transparent mirror of the resonator. In that case, the optical frequency is controlled from outside and is no longer restricted to discrete mode frequencies. But how about the round-trip phase shift in such a case? It turns out that it is still an integer multiple of ; the external field causes a phase change where it enters the resonator. In that way, you can have discrete values of the round-trip phase shift despite possible continuous variations of the optical frequency.

The Round-trip Group Delay

In many situations, the round-trip time of an optical resonator is relevant; for example, the inverse of that is the pulse repetition rate from a mode-locked laser in case that a single pulse is circulating in its resonator. Many would think that you can calculate this simply as the round-trip distance divided by the velocity of light – possibly taking into account its reduction in optical components, based on their refractive index . This is not exactly true, however. A useful and meaningful definition of round-trip time is the round-trip group delay ; it tells you how much time a short (but not too short) optical pulse requires for one round trip – looking at the pulse maximum. Generally, the group delay is the derivative of the round-trip phase shift with respect to the angular frequency . In the case of resonator modes, you cannot really calculate that derivative since we have only modes with discrete optical frequencies and phase shifts. You may, however, look at a pair of neighbored resonator modes, differing by in their round-trip phase shift and by in terms of optical frequency . From that, you can get the round-trip group delay as – it is just the inverse mode spacing in terms of optical frequency. Interestingly, the mode spacing is influenced not only by the geometrical length, but also by chromatic dispersion and in principle even by wavelength-dependent diffraction effects. (See the encyclopedia article on waveguide dispersion for more details in that direction.)

Taking different pairs of neighbored modes, you will generally obtain slightly different values of the round-trip group delay: it is not just a constant value for a given resonator. Here, you can see how chromatic dispersion can make it difficult to keep the circulating pulse together over many resonator round trips; one therefore often uses some kind of dispersion compensation for mitigating such effects. It must be said, however, that zero chromatic dispersion is not always the best to have, since we also often have some substantial nonlinear effects in mode-locked lasers, which can nicely be combined with some amount of chromatic dispersion. By the way, that also implies that resonator modes, not taking into account any nonlinear effects, are not telling you everything about light propagation in lasers.

Detecting Small Changes of Round-trip Phase Shifts

Imagine that you have some small temperature change affecting the refractive index of some crystal or glass piece within a resonator; alternatively, you could have some change of the round-trip length caused by thermal drifts of mechanical parts or by whatever else. If monochromatic light with a fixed optical frequency were circulating in the resonator, one would then obtain a somewhat changed round-trip phase shift. If the resonator is a laser resonator , the situation is different: the laser will continue to operate on a resonator mode (if not on multiple resonator modes simultaneously), and the resonator mode frequency (or frequencies) will automatically adapt such that the round-trip phase shift stays constant (for each mode).

That can actually be quite convenient for measuring such influences: one can easily detect tiny changes of optical frequencies and therefore tiny influences on the optical field circulating in a laser resonator. As an example, consider a small 1064-nm laser with a 10-GHz mode spacing of its linear resonator. If an end mirror of that resonator is displaced by only 1 nm in the beam direction, this will change the mode frequencies by 10 GHz · (2 · 1 nm / 1064 nm) = 18.8 MHz. If the laser's emission linewidth is not too large, one should be able to detect such a change of optical frequency. Note that single-frequency solid-state lasers often have a linewidth only of a few kHz.

Another thought: if you can get a laser to operate on two modes having different polarization directions, and you can detect a beat note of the corresponding optical frequencies, that beat frequency will extremely sensitively react to the slightest birefringence introduced into your intracavity laser beam.

Some Other Articles

You might also be interested in some older articles on " The Role of Diffraction in Optical Resonators ", " The Resonator Mystery " and " Are Compact Resonators More Stable? ".

This article is a posting of the Photonics Spotlight , authored by Dr. Rüdiger Paschotta . You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia .

Note that you can also receive the articles in the form of a newsletter or with an RSS feed .

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Watch CBS News

Documents say Fulton County DA Fani Willis was booked on flights bought by prosecutor with whom she's accused of having affair

By Graham Kates, Jared Eggleston

January 19, 2024 / 4:22 PM EST / CBS News

Fulton County District Attorney Fani Willis was booked on at least two sets of round trip flights purchased by a special prosecutor with whom she's accused of having a romantic entanglement , records appear to show.

Jocelyn Wade, the estranged wife of special prosecutor Nathan Wade, filed an exhibit in the couple's divorce proceedings on Friday purportedly showing the spending history of a credit card used by Nathan Wade. The document shows Nathan Wade booking tickets for himself and Willis on flights to and from San Francisco and Miami.

The new filing came one day after an attorney for Willis accused Jocelyn Wade of trying to interfere with the district attorney's election interference case against former President Donald Trump and other defendants. Jocelyn Wade is seeking to question Willis in the Wades' ongoing divorce case, and filed the new exhibit in response to Willis' claim.

Until Friday's filing, no evidence of the alleged relationship had been made public.

Willis was first publicly accused of being romantically involved with Nathan Wade last week in a filing by Michael Roman, one of Trump's co-defendants. Roman alleged in a motion that Willis and Wade carried on an "improper, clandestine personal relationship" while Willis paid him more than $650,000 over several years to work on the case. He claimed that some of that money was used for Caribbean cruises they took together, as well as for trips to Florida and California's Napa Valley.

That same day, Willis was served a subpoena  in the Wades' divorce case. Her attorney called the subpoena "an attempt to harass and damage" Willis' reputation.

Willis' office has said it will respond to Jocelyn Wade's accusations in a filing due on Feb. 2. A hearing on the matter is set for Feb. 15.

A spokesperson for Willis did not immediately return a request for comment on Friday.

Many of the filings in the Wades' divorce proceedings are sealed. A coalition of news organizations, including CBS News, has filed a request to unseal those documents.

Willis defended the decision to hire Wade — who had not previously prosecuted a complex racketeering case — during a speech at an Atlanta church on Sunday. She called him a  "superstar" who has "impeccable credentials," noting that he has been a lawyer for two decades and a municipal judge for 10 years.

Trump and Roman have each pleaded not guilty to racketeering charges in a case that accuses them and others of plotting to illegally overturn Georgia's 2020 presidential election results.

It is unclear what, if any, bearing the accusations against Willis and Nathan Wade will have on the case. CBS News legal analyst Rikki Kleiman says the allegations could have consequences whether they're proven or not.

"I do not expect this case to be dismissed and go away, but it is not out of the question for a different prosecutor and a different prosecutor's office to take charge of the case, to simply remove the taint of the appearance of impropriety," she said. 

The controversy has caught the attention of Trump's attorney in the case, Steven Sadow, who posted about it on the social media network LinkedIn Friday.

"PROOF — look at pages 12-15: Travel and hotel records of Special Prosecutor Wade and DA Willis," Sadow posted , sharing a copy of Jocelyn Wade's filing.

Graham Kates is an investigative reporter covering criminal justice, privacy issues and information security for CBS News Digital. Contact Graham at [email protected] or [email protected]

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Milwaukee Journal-Sentinel

You can fly round trip to Florida out of Mitchell International Airport for $90 right now

L onging to escape the snow that's been falling on Milwaukee all week? Or perhaps you're looking to get away before the upcoming cold snap that's expected to bring statewide lows in the negative single digits next week?

If so, Hopper, a website that tracks airfare and other travel prices, has good news for you.

New data from Hopper found that January 2024 will have some of the year's best deals on domestic airfare in the U.S. Round-trip plane tickets are expected to average $253 in January. This is 6% cheaper than January 2023 and 11% cheaper than December 2023, the data revealed. So, now is the perfect time to take a break from the winter weather in a warmer destination.

Get daily updates on the Packers during the season.

Additional data shared with the Journal Sentinel by Hopper economists revealed which cities have the cheapest round-trip airfare from Milwaukee in January. Here's how to find the best deals.

When is the cheapest month to fly from Milwaukee in 2024?

Hopper predicted that January will be the cheapest month to fly domestically in the first half of 2024. We're expected to see the cost of domestic airfare remain below 2023 and pre-pandemic levels for the next six months, Hopper said.

Here are Hopper's airfare predictions for round-trip domestic flights nationwide:

  • January 2024: $253
  • February 2024: $276
  • March 2024: $296
  • April 2024: $294
  • May 2024: $302

"January will be the cheapest month of the year to book travel until the fall shoulder season in September and October, so a great time for travelers to take advantage of low prices and book 2024 trips," Hopper's lead economist Hayley Berg wrote. "Fares will rise into late spring as the spring break and summer travel period heat up."

Where is the cheapest place to fly from Milwaukee right now?

Hopper's data found that the cheapest round-trip flights from Milwaukee this month are to Orlando, Fla. ($90), Fort Myers, Fla. ($112), Las Vegas ($129) and New York City ($130).

Below are 11 more round-trip flights from Milwaukee under $200.

Where can you fly from Milwaukee for less than $200 round trip in January 2024?

Data from Hopper said round-trip airfare to these destinations from Milwaukee is cheap this month:

  • Orlando, Fla. ($90)
  • Fort Myers, Fla. ($112)
  • Las Vegas ($129)
  • New York City ($130)
  • Atlanta ($132)
  • Houston ($154)
  • Denver ($159)
  • Charlotte, N.C. ($159)
  • Dallas ($159)
  • Phoenix ($170)
  • Los Angeles ($172)
  • San Diego ($173)
  • Boston ($175)
  • Philadelphia ($178)
  • Newark, N.J. ($188)

One thing to note is that Hopper and many other travel booking sites do not list flights from Southwest, the largest carrier serving the Milwaukee Mitchell International Airport, the airport's director of public affairs and marketing Harold Mester said.

"The only place where Southwest’s fares can be found is at ," Mester wrote in an email. "It should also be noted that, unlike some of the ultra-low fare carriers, Southwest doesn’t charge passengers for checked luggage or carry-on bags."

A full list of nonstop routes from Milwaukee, including which airlines serve each route, is available online here .

This article originally appeared on Milwaukee Journal Sentinel: You can fly round trip to Florida out of Mitchell International Airport for $90 right now

A Southwest Airlines flight takes off Sunday, March 27, 2022 at Mitchell Airport in Milwaukee, Wis. The Dallas-based airline is the market share leader at the airport.

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Stock market today: Wall Street hits record high following a 2-year round trip scarred by inflation

The Fearless Girl statue stands outside of the New York Stock Exchange is shown on Friday, Jan. 19, 2024, in New York. Wall Street is rising Friday and may break past its all-time high set two years ago, before the highest inflation and interest rates in decades sent financial markets tanking worldwide. (AP Photo/Peter K. Afriyie)

A street sign at the intersection of Wall Street and Broadway is shown on Friday, Jan. 19, 2024, in New York. Wall Street is rising Friday and may break past its all-time high set two years ago, before the highest inflation and interest rates in decades sent financial markets tanking worldwide. (AP Photo/Peter K. Afriyie)

The exterior of the New York Stock Exchange is shown on Friday, Jan. 19, 2024, in New York. Wall Street is rising Friday and may break past its all-time high set two years ago, before the highest inflation and interest rates in decades sent financial markets tanking worldwide. (AP Photo/Peter K. Afriyie)

A person departs the Wall Street subway station on Friday, Jan. 19, 2024, in New York. Wall Street is rising Friday and may break past its all-time high set two years ago, before the highest inflation and interest rates in decades sent financial markets tanking worldwide. (AP Photo/Peter K. Afriyie)

The exterior of the New York Stock Exchange is shown on Friday, Jan. 19, 2024, in New York. (AP Photo/Peter K. Afriyie)

A security officer stands outside an entrance to the New York Stock Exchange on Friday, Jan. 19, 2024, in New York. Wall Street is rising Friday and may break past its all-time high set two years ago, before the highest inflation and interest rates in decades sent financial markets tanking worldwide. (AP Photo/Peter K. Afriyie)

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NEW YORK (AP) — Wall Street returned to record heights Friday to cap a punishing, two-year round trip dogged by high inflation and worries about a recession that seemed inevitable but hasn’t arrived.

The S&P 500, which is the centerpiece of many 401(k) accounts and the main measure that professional investors use to gauge Wall Street’s health, rallied 1.2% to 4,839.81. It erased the last of its losses since setting its prior record of 4,796.56 at the start of 2022. During that time, it dropped as much as 25% as inflation soared to levels unseen since Thelonious Monk and Ingrid Bergman were still alive in 1981.

Even more than high inflation itself, Wall Street’s fear was focused on the medicine the Federal Reserve traditionally uses to treat it. That’s high interest rates, which press the brakes on the economy by making borrowing more expensive and hurting prices for stocks and other investments. And the Fed rapidly hiked its main interest rate from virtually zero to its highest level since 2001, in a range between 5.25% and 5.50%.

Historically, the Fed has helped induce recessions through such increases to interest rates. Coming into last year, the widespread expectation on Wall Street was that it would happen again.

FILE - A Spirit Airlines 319 Airbus approaches Manchester Boston Regional Airport for a landing, Friday, June 2, 2023, in Manchester, N.H. Shares of Spirit Airlines jumped in morning trading Friday, Jan. 19, 2024, after the struggling discount carrier boosted its fourth-quarter guidance after a strong holiday travel season. Spirit said it expects to post revenue of $1.3 billion when it releases its results for the final quarter of 2023 early next month. (AP Photo/Charles Krupa)

But this time was different, or at least it has been so far. The economy is still growing, the unemployment rate remains remarkably low and optimism is on the upswing among U.S. households.

“I don’t think this cycle is normal at all,” said Niladri “Neel” Mukherjee, chief investment officer of TIAA’s Wealth Management team. “It’s unique, and the pandemic introduced that element of uniqueness.”

After shooting higher as snarled supply chains caused shortages because of COVID-19 shutdowns, inflation has been cooling since its peak two summers ago. It’s eased so much that Wall Street’s biggest question now is when the Federal Reserve will begin moving interest rates lower.

Such cuts to rates can act like steroids for financial markets, while releasing pressure that’s built up on the economy and the financial system.

Treasury yields have already relaxed significantly on expectations for rate cuts, and that helped the stock market’s rally accelerate sharply in November. The yield on the 10-year Treasury slipped Friday to 4.13%, and it’s down sharply from the 5% that it reached in October, which was its highest level since 2007.

Of course, some critics say Wall Street has gotten ahead of itself, again, in predicting how soon the Federal Reserve may begin cutting interest rates.

“The market is addicted to rate cuts,” said Rich Weiss, chief investment officer of multi-asset strategies at American Century Investments. “They just can’t get enough of it and are myopically focused on it.”

Repeatedly since the Fed began this rate-hiking campaign early in 2022, traders have been quick to forecast an approaching easing of rates, only to be disappointed as high inflation proved to be more stubborn than expected. If that happens again, the big moves higher for stocks and lower for bond yields may need to revert.

This time around, though, the Fed itself has hinted that rate cuts are coming, though some officials have indicated they may begin later than the market is hoping for. Traders are betting on a nearly coin flip’s chance that the Fed will start cutting in March, according to data from CME Group.

“The truth is likely somewhere between what the Fed is saying and what the market is expecting,” said Brian Jacobsen, chief economist at Annex Wealth Management. “That will continue to cause dips and rips” for financial markets “until the two reconcile with each other.”

Some encouraging data came Friday after a preliminary report from the University of Michigan suggested the mood among U.S. consumers is roaring higher. It said sentiment jumped to its highest level since July 2021. That’s important because spending by consumers is the main driver of the economy.

Perhaps more importantly for the Fed, expectations for upcoming inflation among households also seem to be anchored. A big worry has been that such expectations could take off and trigger a vicious cycle that keeps inflation high.

Friday’s lift for Wall Street came with a big boost from technology stocks, something that’s become typical in its run higher.

Several chip companies rose for a second straight day after heavyweight chipmaker Taiwan Semiconductor Manufacturing Co. delivered a better forecast for revenue this year than analysts expected. Broadcom rose 5.9%, and Texas Instruments climbed 4%.

All told, the S&P 500 rose 58.87 points to its record. The Dow Jones Industrial Average set its own record a month earlier, and it gained 395.19, or 1.1%, Friday to 37,863.80. The Nasdaq composite jumped 255.32, or 1.7%, to 15,310.97.

Last year, a select few Big Tech companies were responsible for the wide majority of the S&P 500’s gains. Seven of them accounted for 62% of the index’s total return, according to S&P Dow Jones Indices.

Many of those stocks — Microsoft, Apple, Alphabet, Nvidia, Amazon, Meta Platforms and Tesla — rode a furor in the market around technology related to artificial intelligence. The hope is AI will lead to a boom in profits, both for companies using it and for companies providing the hardware for it.

Investors may have wished they had stayed in just those stocks, which got the nickname of “the Magnificent 7.” But some of them remain below their record highs, such as Tesla. It’s still down 48% from its all-time high set in November 2021.

Friday’s return of the S&P 500 to a record serves as another example that investors who stay patient and spread their investments across the U.S. stock market end up making back all their losses. Sometimes it can take a long time, like the lost decade of 2000 through 2009 when the S&P 500 tumbled through the dot-com bubble bust and the global financial crisis. But the market has historically made investors whole again, given enough time.

Including dividends, investors with S&P 500 index funds already returned to break-even a month ago.

Of course, risks still remain for investors. Besides uncertainty about when the Fed will begin cutting interest rates, it’s also still not a sure thing that the economy will avoid a recession.

Hikes to interest rates take a notoriously long time to make their way fully through the system, and they can cause things to break in unexpected places within the financial system.

AP Writers Matt Ott and Zimo Zhong contributed.

round trip time moon light


  1. Why is the Moon so bright right now and how long will the full Moon last?

    round trip time moon light

  2. Round-trip time Meaning

    round trip time moon light

  3. Calculating round trip time

    round trip time moon light

  4. Round-trip time

    round trip time moon light

  5. Round Wall Light Moon Light Fixture Entry Light Modern Wall

    round trip time moon light

  6. Moon / How Does The Moon Glow Wsu Insider Washington State University

    round trip time moon light


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  1. How long does it take for moonlight to reach the earth?

    Answer Light travels through space at just over 186,000 miles per second. The moon is just under 250,000 miles from Earth, so light from the Moon's surface has to travel more than one second (about 1.3 seconds) to reach us. Among other things, the astronauts left reflecting mirrors on the moon.

  2. NASA Movies Show How Fast Light Travels From Earth to the ...

    This means all moonlight we see is 1.255 seconds old, and a round-trip between the Earth and moon at light speed takes about 2.51 seconds. This timing is growing every day, however, as...

  3. Round-trip light time

    Overview round-trip light time Quick Reference (RTLT) The elapsed time taken by a signal travelling from the Earth to a spacecraft or other celestial body, then immediately transmitted or reflected back to the original transmission point.

  4. Lunar Laser Ranging experiments

    The distance can be calculated from the round-trip time of laser light pulses travelling at the speed of light, which are reflected back to Earth by the Moon's surface or by one of several retroreflectors installed on the Moon.

  5. Physics

    Researchers on Earth measure the round-trip travel time—roughly 2.5 seconds—of laser pulses reflecting off these retroreflectors and returning back to detectors on Earth. The first laser reflection to be recorded was at Lick Observatory in California on August 1, 1969, but since then several other facilities in the US, the Soviet Union ...

  6. What is a Round Trip Flight? (Everything You Should Know)

    A round-trip flight is when you fly from your location to another destination, and then fly back to your original location. With a round-trip flight, you purchase one ticket, which gives you an inbound and outbound flight. Round-trip flights are most popular among tourists. Booking a round-trip flight has many advantages over booking multiple ...

  7. What Is Round Trip Time?

    The RTT is the time between sending a message from a source to a destination (start) and receiving the acknowledgment from the destination at the source point (end). We can also see RTT referred to as Round Trip Delay (RTD).

  8. Light As A Cosmic Time Machine by Andrew Fraknoi

    Those 2.6 seconds were exactly the round-trip travel time for radio waves between the Earth and the Moon. The Sun is 93 million miles away, so sunlight takes 8 and 1/3 minutes to get to...

  9. What is a round-trip flight?

    A round-trip flight is an itinerary from one destination to another, with a flight back to the original destination. In most cases, this is what you probably book when going on vacation or visiting a family member for a holiday weekend. Let's say I live in Charlotte and want to fly to Arizona for a spring break trip.

  10. How Do You Solve a Moon Mystery? Fire a Laser at It

    The orbiter skims over the moon's surface at 3,600 m.p.h. "It's hard enough to hit a stationary target," said Dr. Murphy, who leads the Apache Point Observatory Lunar Laser-ranging ...

  11. Lunar distance

    A lightyear is 24,611,700 Lunar distances. [17] [18] Geostationary Earth Orbit is 42,164 km (26,199 mi) from Earth center, or 1 9.117 LD = 0.109 68 LD (or 0.109 68 LDEO [19]) Distance between the Earth and Moon ( mean value) - sizes and distance to scale - with travel time at speed of light animated

  12. The Basics of Lunar Ranging

    In essence, we measure the time it takes for the pulse of light to travel to the moon and back. This can take anywhere from 2.34 to 2.71 seconds, depending on how far away the moon is at the time (the earth-moon distance ranges from 351,000 km to 406,000 km). We can time the round trip to few-picosecond precision, or a few trillionths of a second.

  13. A Lunar time zone? Here's why telling time in space is tricky : NPR

    But scientists say a far larger temporal problem is looming on the horizon: With multiple missions to the moon in the planning phase, it's time to set a Lunar time standard. "We need to define a ...

  14. Animation: Using Planets to Visualize the Speed of Light

    The Speed of Light Visualized Using Planets. We often come across the term "light-year" in the context of space travel. But what does it actually mean? A light-year is the distance light travels in one year. At a speed of 186,000 miles/sec (300,000km/sec), light travels 5.88 trillion miles (9.46 trillion km) in a year—a distance well ...

  15. What is the round-trip travel time for light between Earth a

    A car travels at 68 \frac {2} {3} 32 miles per hour for 2 \frac {1} {12} 2121 hours. Find the distance that the car has traveled correct to 3 significant digits. A rocket traveling at 0.50 \mathrm {c} 0.50c sets out for the nearest star, Alpha Centauri, which is 4.3 4.3 ly away from earth. It will return to earth immediately after reaching ...

  16. How long does it take to get to the moon?

    The spacecraft entered lunar orbit just 69 hours and 8 minutes after launch according to NASA. Here we take a look at how long a trip to the moon would take using available technology and...

  17. Round-trip light time

    round-trip light time: 1 n the elapsed time it takes for a signal to travel from Earth to a spacecraft (or other body) and back to the starting point Synonyms: RTLT Type of: elapsed time the time that elapses while some event is occurring

  18. How long does it take for moonlight to reach the earth?

    Since moonlight is reflected sunlight, it originates at the Sun's surface 93 million miles from the Earth/Moon system. It takes about 8 minutes for that trip, so the short, final hop from the Moon to the Earth is trivial by comparison. Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

  19. Moon Light World Map

    On Friday, January 26, 2024 at 09:24:00 UTC the Moon is at its zenith at Latitude: 21° 15' North, Longitude: 129° 14' West. The ground speed is currently 418.97 meters/second, 1508.3 kilometres/hour, 937.2 miles/hour or 814.4 nautical miles/hour (knots). The table below shows position of the the Moon compared to the time and date above:

  20. To the Moon and back: reflecting a radio signal to calculate the

    d = distance of Earth to Moon in metres. c = the speed of light, 3 x 10 8 metres per second. t =time delay in seconds. The radio signal covers the same distance twice (Earth to the Moon, and back), hence the need to divide by 2. For example, with a delay time of 2.56 seconds: d = [ (3 x 10 8) x 2.56] / 2. d = 348 000 000 m.

  21. The Round-trip Phase Shift in an Optical Resonator

    A useful and meaningful definition of round-trip time is the round-trip group delay; it tells you how much time a short (but not too short) optical pulse requires for one round trip - looking at the pulse maximum. Generally, the group delay is the derivative of the round-trip phase shift with respect to the angular frequency ω = 2 π ν.

  22. Documents say Fulton County DA Fani Willis was booked on flights bought

    Fani Willis faces hearing over misconduct claim made by Trump co-defendant 05:07. Fulton County District Attorney Fani Willis was booked on at least two sets of round trip flights purchased by a ...

  23. round-trip light time

    Define round-trip light time. round-trip light time synonyms, round-trip light time pronunciation, round-trip light time translation, English dictionary definition of round-trip light time. Noun 1. round-trip light time - the elapsed time it takes for a signal to travel from Earth to a spacecraft and back to the starting point RTLT elapsed time

  24. You can fly round trip to Florida out of Mitchell International ...

    You can fly round trip to Florida out of Mitchell International Airport for $90 right now. New data from Hopper showed January will be the best time to purchase domestic plane tickets.

  25. Wall Street hits record high after a 2-year round trip scarred by

    NEW YORK (AP) — Wall Street returned to record heights Friday to cap a punishing, two-year round trip dogged by high inflation and worries about a recession that seemed inevitable but hasn't arrived. The S&P 500, which is the centerpiece of many 401 (k) accounts and the main measure that professional investors use to gauge Wall Street's ...

  26. Time Out reveals world's best cities for 2024

    1: New York City, USA: Media company Time Out has named New York City as its best city for 2024.New York City's museum and theater scene were name-checked by Time Out as reasons why the US city ...

  27. Multi-Stop Route Planning and Optimization Tools

    Find the shortest routes between multiple stops and get times and distances for your work or a road trip. Easily enter stops on a map or by uploading a file. Save gas and time on your next trip. Route Planner. Optimize routes, save time ... Stops Let us re-order stops Round trip Optimize for Shortest time Shortest distance.