Friday, March 02, 2007

Partial Lunar Eclipse Tomorrow (But Not For Me)

If you're east of the Rocky Mountains tomorrow (Saturday), you'll get to see a partial lunar eclipse in the evening. Get the details (and an explanation of lunar eclipses) at this article on Sky & Telescope's website:

"This Weekend's Lunar Eclipse"

I won't be able to see it from California--the moon will have already grazed the Earth's shadow just before moonrise. I guess I'll just have to content myself with the sunny, 72 degree weather we have here instead. : P

Photo: a partial lunar eclipse in progress. If you didn't know any better, you might just think this is a crescent moon. But any familiarity with the curvature of the moon's usual shadow will inform you that something is amiss here--that's not a plain old crescent moon, it's a full moon passing through the shadow of the Earth! (And note that tomorrow night, the moon will be a full moon--so if you see anything otherwise, you can be sure it's the Earth's shadow involved.)

It is interesting to contemplate what one would see during a lunar eclipse if one were standing on the surface of the Moon: from the lunar perspective, it would in fact be a solar eclipse!

What would that look like, given that the Earth would appear much larger in the sky than the Sun? Well, you would see a ring of "sunlight streams" encircling the outside edge of the Earth's sphere--in other words, the sum of all sunsets happening at that moment on the Earth. It would undoubtedly look like a ring of reddish-orange. Two graphic artists have illustrated how that might appear: here is a high-quality composite illustration, imagined at the point immediately before or after the Sun is fully blocked (an effect known as "Bailey's Beads"), and here is an more instructive (though lower quality) animation of the process--to see it you'll have to scroll down to the second image on the page.
ADDENDUM: Be sure to note the shape of the Earth's shadow, as Aristotle and other ancient Greeks did. Prior to the Age of Discovery (e.g. Ferdinand Magellan's circumnavigation of the globe), this was one of a few phenomena that the Greeks had access to which had allowed them to infer, with certainty, the sphericity of the Earth.

No comments: