Do-It-Yourself Guide to Measuring the Moon’s Distance

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When the distance from the Earth to the Moon comes up, the common figure thrown around is 402,336 km (or 250,000 miles). But have you every wondered how astronomers got that figure? And how exact it really is? There are a couple of ways you can measure the distance of the Moon that don’t require lasers or any instruments. All you need are your eyes, a clear sky, and someone else willing to stand outside all night with you. 

There are two ways to measure the distance from the Earth to the Moon on your own: using a Lunar eclipse and using parallax. Let’s look at eclipses first.

The phases of a Lunar eclipse. Photo credit: Keith Burns for NASA/JPL

The Ancient Greeks used Lunar eclipses – the phenomena of the Earth passing directly between the sun and the Moon – to determine the distance from the Earth to its satellite. It’s a simple matter of tracking and timing how long it takes the Earth’s shadow to cross over the Moon.

Start with the few knowns. We know, as did the Ancient Greeks, that the Moon travels around the Earth at a constant speed – about 29 days per revolution. The diameter of the Earth is also known to be about 12,875 km or 8,000 miles.By tracking the movement of the Earth’s shadow across the Moon, Greek astronomers found that the Earth’s shadow was roughly 2.5 times the apparent size of the Moon and lasted roughly three hours from the first to last signs of the shadow.

From these measurements, it was simple geometry that allowed Aristarchus (c. 270 BC) to determined that the Moon was round 60 Earth radii away (about 386,243 km or 240,000 miles). This is quite close to the currently accepted figure of 60.3 radii.

You can follow Aristarchus’ method in your own backyard if you have a clear view of a Lunar eclipse. Track the movement of the Earth’s shadow on the Moon by drawing the changes and time the eclipse. Use your measurements to determine the Moon’s distance.

Lunar parallax: the moon as observed from Italy and China at the same time during a lunar eclipse. Photo credit: measurethemoon.org/wordpress

For the second method, you’ll need a friend to help out. The Ancient Greeks also knew about parallax, an object’s apparent change in position when seen from two different viewpoints. You can experience parallax by holding a pen out at arm’s length and looking at it with one eye at a time. As you switch between your left and right eye, the pen will appear to move back and forth.

The same thing can be seen on a giant scale. Two observers in different parts of the world (at least 3,200 km or 2,000 miles apart) will see the Moon’s position as different from where calculations say it should be in the night sky.

To find the distance of the Moon from the Earth, you and a friend stand 3,200 km apart and each take a picture of the Moon at exactly the same time. Then, compare your images. The Moon will be in a different spot, but the background stars will be in the same place. What your images have given you is a triangle. You know the base (the distance between you and your friend), and you can find the angle at the top (the point of the Moon in this triangle). Simple geometry will give you a value for the distance of the Moon.

It might be a little more labour intensive than searching the internet, but determining the Moon’s distance yourself is sure to be more fun! If you really want to get involved, check out International Measure the Moon Night on Dec. 10, 2011. Join participants around the world who register their own events and share their images and observations!

A graph showing which parts of the world have the best chance of measuring the moon's distance using these two methods. Regions in red can see full eclipses while regions covered in red bars are best suited to measurements using parallax. Photo credit: measurethemoon.org/wordpress

Historic Photos Commemorate First and Last Shuttle Crews

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In an historic photo shoot earlier this month, NASA commemorated the space shuttle’s retirement, personifying the thirty-year program with the first and last astronaut crews to fly the vehicle.

The shuttle program has certainly come a long way from STS-1 to STS-135. 

Young and Crippen. The STS-1 crew's official portrait, 1981. Image credit: NASA.

John Young and Robert Crippen launched on STS-1 in the shuttle Columbia on April 12, 1981, twenty years after Yuri Gagarin became the first man to orbit the Earth. It was a shakedown cruise, with the two astronauts spending only two days in orbit. They checked out the spacecraft’s systems, the vehicle’s overall flight worthiness, and made the first runway landing from orbit. The only payload the crew carried was a Development Flight Instrumentation (DFI) package. It contained sensors to measure and record Columbia’s performance in orbit and the stresses it felt during launch, ascent, orbital flight, descent and landing.

Thirty years and two months later, the crew of STS-135 had a much busier mission on their hands. Launched on July 8, 2011 in the Atlantis orbiter, the crew’s primary mission objective was to transfer thousands of pounds of supplies into the International Space Station and take thousands more pounds of unneeded cargo back down to Earth.

Atlantis stayed docked to the ISS for eight of its twelve days in orbit. The crew, along with the Expedition 28 crew that spent close to four months aboard the station, played a real life and oversized version of Tetris to get all the supplies squared away in the ISS’ multi-purpose module.

The crews of STS-135 and Expedition 28 pose with the flag flown in STS-1. Credit: NASA

With the cargo transfer complete, Atlantis undocked from the station on July 19. The crew spent the last two days of the final mission in orbit, deploying experiments and readying the spacecraft for landing. Atlantis touched down on the runway at the Kennedy Spaceflight Centre on July 21.

NASA’s complete image gallery, which includes images of the STS-135 post flight wrap up as well as pictures with the STS-1 crew, highlights the personal strain that runs through manned spaceflight. And it doesn’t stop there. During STS-135’s mission, commander Chris Ferguson presented the ISS’s crew the U.S. flag John Young and Robert Crippen carried into space on STS-1. The flag will remain on display on the station until the next crew that launches from the U.S. retrieves it. After returning to Earth, the flag will be launched again with the first crew to embark on a journey beyond Earth orbit.

The Crews of STS-1 and STS-135. John Young, STS-1 commander, Robert Crippen, STS-1 pilot, with the STS-135 crew of commander Chris Ferguson, pilot Doug Hurley and mission specialists Sandy Magnus and Rex Walheim. Photo credit: NASA Photo/Houston Chronicle, Smiley N. Pool