Here’s all you need to know to help us measure the size of Earth’s shadow during the upcoming eclipse.
Amateur astronomers often plan how they'll take photographs, image sequences, or even time-lapse videos of a lunar eclipse. But don't overlook the scientifically useful projects that are just begging to be carried out. You don't need anything but clear skies and some very simple equipment.
Total lunar eclipses come in a great variety of brightnesses and hues. In February 1860, Irish amateur Mary Ward likened the Moon to "a red-hot penny" in the sky. But the famously dark eclipse of December 1963 was so dim that some skywatchers could not find the Moon when they stepped outdoors near mid-totality!
To help in comparing reports from various observers, even years and cultures apart, French astronomer André Danjon devised a five-point scale that is still used today. As shown below, the Danjon values range from 0 (Moon almost invisible at mid-totality) to 4 (very bright copper-red or orange Moon). To learn how to give this eclipse a Danjon L rating, go here.
For many years Brazilian astronomer Helio C. Vital has led a very active group of observers in monitoring the brightness of the eclipsed Moon, not only as it moves across the shadow but also from one eclipse to the next. For example, he's found that a total eclipse is fairly bright if it takes place when Earth's atmosphere is nearly free of aerosols. But within a few years after a major volcanic eruption, eclipses are often much darker. Some darkening was even detected after the October 6, 2006, eruption of Mount Rabaul in Papua New Guinea.
So how do you make an estimate? If you wear thick glasses you can try taking them off so the Moon and bright planets or stars look like equal-size blobs. A better technique is to look at the Moon through the wrong end of binoculars, and to compare its brightness seen that way with various bright stars seen without optical aid. Do this at regular intervals as the Moon crosses the umbra, and in your report be sure to mention the power of the binoculars and which comparison stars you used.
Crater Timings — Measuring the Size of the Umbra
Timings of celestial events offered early mariners a way to find their longitude far from home. This method was used by Christopher Columbus, who timed the start and end of a lunar eclipse in 1504 during his fourth trip to the New World.
When astronomers tried to refine this method, however, they quickly found that the dark center of the Earth's shadow, called the umbra, was larger than pure geometry indicated by about 2%, because our atmosphere adds to Earth's effective diameter.
To time when the Moon's edge enters or leaves the shadow is often iffy. Instead, it's more accurate to time when individual spots and craters cross the shadow's edge. For example, from 697 crater timings sent in by Sky & Telescope readers, I derived an enlargement of 2.1% for the July 1982 eclipse. But for a similar event only six months later, 298 timings gave 1.7% enlargement. In each case the probable error was less than 0.1%. So the enlargement definitely varies slightly from eclipse to eclipse, for reasons not yet understood.
The Moon photo above has prominent features labeled; click the image for a larger version to print out and take with you to the telescope. Sky & Telescope predictions of these features' entrance and exit times in the umbra during the upcoming eclipse are given in the table below; you can also download a PDF version.
S&T ’s Eclipse-Timing Predictions for the Total Lunar Eclipse - May 26, 2021
|Entry (UT)||Exit (UT)|
Roger Sinnott / Sky & Telescope
Before making your timings, set a watch to accurate Internet time or use a time app that displays seconds on your phone. Write down the time (to the nearest 5 seconds) when the edge of the umbra crosses the center of the crater or other feature.
It's as simple as that! The shadow edge is a little fuzzy, so try to judge the part of the shadow-edge where the light falls off most rapidly, and adopt that for your timings.
It's best to use a scope with an aperture of at least 2.4 inches (60 mm), but not more than 8 inches (200 mm). The whole point is to make timings the same way they've been done for 300 years, so results can be compared.
If you carry out any of these simple projects at the next eclipse of the Moon, please e-mail the results to me. I'm collecting them for later analysis.
But no matter what you do, set aside a little time to sit back and enjoy the eclipse, too!