A lunar eclipse in 2000. Credit: Conrad Jung
If you haven't heard yet, there's a total lunar eclipse on the way: the event when the Moon, at the peak of its Full phase, passes through the Earth's long shadow and performs an almost complete vanishing act. Now you see it, now ... or, an hour or so later ... you don't ...
When? Tuesday morning, August 28th, beginning shortly after 12:52 AM PDT, and reaching its apex of darkness at about 3:37 AM. Where? Wherever you can see the Moon from, clear of trees if you can help it, and free of clouds (though you can't help that one). If you want company, check out what's going on at Chabot Space and Science Center.
It won't happen all at once. It will begin just past 12:52 AM as the Earth's shadow just begins to touch the edge of the Moon's bright disk. This preliminary shadow, called the "penumbra," is only half-shadow; in the penumbra sunlight is only partially blocked. Still, it's very unusual to see that first little "bite" taken out of the Moon. It won't be until past 1:50 AM, when much of the Moon is in penumbral grayness, that the Earth's full shadow--the umbra--cuts into the dance. Over the next hour, the Moon will slowly become engulfed in this much higher-achieving shadow, becoming so dark that you will easily overlook the Moon's presence in the sky if you aren't looking for it.
And then, the truly beautiful feature of this celestial event is revealed: the Moon, so low-key and subdued at what seems its darkest hour, will change color. You may notice the shading begin to change prior to deepest, darkest eclipse, but eventually the Moon will turn orange or coppery red. Here is the reward for being up at almost four in the morning!
Why red? While the Earth's solid sphere is blocking all of the sunlight that hits it, our planet's thin skin of an atmosphere lets some of the sunlight slip through--the redder colors in sunlight in particular. You've seen this effect at sunrise or sunset, when the Sun becomes orange or even red. In fact, it is the red light from all of the sunrises and sunsets going on all around the rim of the Earth that turns the Moon these hues. Were you actually on the Moon looking back at Earth during this event, you would see a large dark disk (the Earth) surrounded by a pale halo of sunlight, and lined all around the rim by a ring or orange or red.
