How to Measure the Moon this Weekend

The people of Byzantium viewed a lunar eclipse as a bad omen, but today it’s just another time to do science

A lunar eclipse turns the moon reddish brown
A lunar eclipse turns the moon reddish brown Sky & Telescope / Akira Fujii

On the night of May 22, 1453, the people of Byzantium could see an eerie red shadow cross the Moon. It was a partial eclipse–the Earth had gotten in between the Sun and Moon–and the Byzantines took it as a bad omen. And perhaps they were right–the city of Constantinople fell before the month’s end.

A full lunar eclipse will take place this weekend, visible from Asia, Australia and western North America. But people today don’t view this astronomical event as a worrying sign. Instead, it’s time for science! And you can participate.

The Classroom Astronomer magazine has set up a website,, to coordinate observations of the position of the moon in the sky as it passes through our planet’s shadow. And if you’re in the right place, you can measure the distance from the Earth to the Moon.

There are two ways to do this. The first is called the Shadow Method, and it’s the way that the ancient Greeks first measured the distance between the Earth and Moon thousands of years ago. Amy Shira Teitel explains in Universe Today:

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 kilometers, 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 (circa 270 B.C.) to determined that the Moon was around 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.

The second method, the Lunar Parallax Method, was familiar to the ancient Greeks but they lacked the ability to communicate over the far distances that is necessary to carry this out. Telephones and the Internet make this easily possible now. Two observers at least 2,000 miles apart will have to snap a picture of the Moon at the exact same moment. Because the angle at which the Moon and the stars behind it will be different for each person, the images they snap will be slightly different, particularly the stars in the background. “What your images have given you is a triangle,” Teitel explains. “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.”

If the people behind get enough participants, they’ll be able to compare all the various calculations, determine which method is more accurate and figure out how close two people have to be to get an accurate calculation with the Lunar Parallax Method.

If you’re not up for calculations, there are a few other lunar eclipse science projects you might want to participate in:

  • Roger Sinnott of Sky & Telescope is collecting telescopic timings of the the passage of Earth’s shadow across lunar craters (find instructions here) as part of a long-term project to track the unpredictability of the diameter of the shadow.
  • John Westfall of the Association of Lunar and Planetary Observers is collecting timings of when the phases of the lunar eclipse begin and end, made with the unaided eye, to calibrate similar observations made in the past when mariners used the Moon to determine longitude.
  • Richard Keen of the University of Chicago will collect reports of the Moon’s brightness from amateur astronomers for use in volcano-climate studies.

After reading all this and seeing the picture above, you may be wondering why the Moon in a lunar eclipse turns red, not black. “That red light on the Moon during a lunar eclipse comes from all the sunrises and sunsets around the Earth at the time,” says Robert Naeye, editor in chief of Sky & Telescope. “If you were an astronaut standing on the Moon and looking up, the whole picture would be clear. The Sun would be covered up by a dark Earth that was ringed all around with a thin, brilliant band of sunset- and sunrise-colored light, bright enough to dimly light the lunar landscape around you.”

If, like me, you’ll miss out on this chance to see a lunar eclipse, your next opportunity will come in April 2014.

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