What is a partial lunar eclipse?
A lunar eclipse occurs when the Moon passes into the Earth's shadow. During a lunar eclipse, as the Full Moon passes into the Earth's shadow, it begins to look like a part of it is missing--almost like a bite was taken out of it.
If the eclipse is total, the Moon gets completely covered by the Earth's shadow. Once it reaches the central part of the shadow (usually an hour and half into the eclipse), it will begin to glow red, for reasons having to do with sunlight bouncing off the Earth's atmosphere. But this eclipse is only partial, meaning that the Moon won't get completely covered by the shadow, so we won't see that this time around. Instead it will look like this:
What time should I look?
You're going to have to get up really early. The eclipse begins at 3:17 AM Pacific Time, and lasts until moonset, which for us tomorrow is around 5:12AM. 4:38 AM is the moment of greatest eclipse; at that point 54% of the Moon will be covered by Earth's shadow.
Is it worth getting up this early? What should I expect to see?
You should expect to see a portion of the Moon "missing". Since it's not a total lunar eclipse, you won't get to see the Moon enter the reddish glow of the Earth's inner shadow. Some people don't find partial lunar eclipses that interesting, because it does just look a little bit like a crescent moon, and you can see a crescent moon twice a month.
But if you enjoy astronomy, you will find this event interesting. Here are three reasons why:
1) This missing portion of the Moon isn't a lunar phase, like a crescent moon. We see a crescent moon twice each month because of the changing angle at which we're looking at the Moon as it goes around the Earth. During a lunar eclipse, the Moon's phase is at first full; we're seeing its entire face lit up by sunlight. So when a part of it suddenly goes missing in a matter of several minutes, you're actually getting to see the Earth's shadow! It's not just that you're looking at the Moon from a certain angle.
2) Lunar eclipses are a great opportunity to get to directly observe, with your own eyes, the fact that the Earth is a sphere (ball), not flat. How do we know? Simply notice the shape of the Earth's shadow as it is cast on the Moon: it's curved! In fact the ancient Greeks were the first to pay attention to this fact and realize that the Earth is a sphere.
3) For us here in western North America, Saturday's eclipse will occur when the Moon appears very large! This is because for us the eclipse will be occurring as the Moon is setting. You have probably noticed that when the Moon is rising or setting, it appears much larger than it does higher in the sky. (This is an optical illusion, by the way.)
Is it dangerous to look at?
No, not even slightly. Lunar eclipses are not dangerous. Solar eclipses--when the Moon passes in front of the Sun--require caution, but only because staring at the Sun is dangerous in general. (During the moments before and after a solar eclipse is the only time you are likely to accidentally stare directly at the Sun. This is why they make special "solar eclipse glasses," to protect your eyes.)
How often do lunar eclipses occur?
Lunar eclipses are much more common than solar eclipses. Lunar eclipses occur about once a year, though not everyone gets to see them, depending on the time the eclipse occurs. And as you can see with this eclipse, not every lunar eclipse is total. The last total lunar eclipse visible in North America was Feb. 21, 2008, and the next one will be this Dec. 21, 2010. You can find more information about when they will occur here.
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