Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, April 27, 1995 TAG: 9504270021
SECTION: EXTRA PAGE: 3 EDITION: METRO
SOURCE: JOEL ACHENBACH
DATELINE: LENGTH: Medium
A: The moon has an incredible number of tricks up its sleeve: Solar eclipses, lunar eclipses, full moons, half moons, crescents, the optical illusion of looking huge when it's low on the horizon, the peculiarity of having one side permanently facing away from the Earth, and so on. The moon is an odd ball. Nothing else in the sky has such conspicuous phases. You'd never see a crescent sun, except in an eclipse.
The next time you look at a crescent moon, look closer. You may be able to see the rest of the moon - the darkened part. That part of the moon is turned away from the sun. But if it's night on that part of the moon, why isn't the surface too dark to see?
The Greeks wondered about this in ancient times, and guessed that the moon was slightly transparent, that some light was leaking into the dark part from the far side. Another theory arose in the 16th century that the moon's surface was exhibiting phosphorescence. (Centuries later we learned that the moon does, indeed, phosphoresce, but that's not why we see the dark part.)
To find the right explanation, you just have to imagine yourself on the moon's surface, say, somewhere on the Sea of Tranquillity, where Apollo 11 landed. What would you see in the night sky?
Right: Earth.
A full Earth, almost. A big, bright, gibbous Earth. And the Earth is casting light on the moon. We can see the dark part of the moon because of Earthshine.
The sun shines on the Earth, the light is reflected to the moon, and then reflected back to Earth. You lose a lot of brilliance in all that bouncing around but there's still enough to make the dark part of the moon visible.
Richard Berendzen, professor of physics at American University, says that Earth is actually shinier than the moon. The moon's albedo is 7 percent - meaning that 93 percent of the light that hits it is absorbed, and 7 percent is reflected back into space. The Earth's albedo is 39 percent. Venus is brightest of all, with an albedo of 65 percent.
Clouds are a big factor. They're white. Venus is covered with clouds, which is why it's so reflective. Earthshine is greatest when Earth is all clouded up.
We're told that Earthshine makes it possible to sometimes see a ``new'' moon, the moon's most shadowy phase, but personally we've never seen one. (We always thought it should be called a ``new and improved moon.'')
- Washington Post Writers Group
Joel Achenbach writes for the Style section of The Washington Post. Richard Thompson is a regular contributor to the Post.
by CNB