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ECLIPSES by Roger Sinnott
Useful Projects for a Lunar Eclipse
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. 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. Looking through the wrong end of binoculars also helps.
CRATER TIMINGS: 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. And click here for our predictions of these features' entrance and exit times in the umbra.
Before making your timings, set a watch to accurate radio time signals. 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. Use a 2.4-inch or larger scope.
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!
Posted by Roger Sinnott, February 19, 2008