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Update: The eclipse happened, and here are the best photos of it from around the world.
There's a good reason to wake up before sunrise on Wednesday morning. Starting at 6:25 am Eastern time, people across the United States will be able to see a total eclipse of the moon.
Over the course of a few hours, our planet will pass between the moon and the sun, blocking the sun's light from illuminating the moon as it usually does at night. But small amounts of light will pass through our dust-filled atmosphere and hit the moon, causing it to appear bright red — the reason lunar eclipses are often called "blood moons."
Unlike a solar eclipse, a lunar eclipse is totally safe to watch without eye protection, and lasts for a few hours. For everything you might want to know about tonight's lunar eclipse and how to watch it, read on.
What is a lunar eclipse?
Normally, light from the sun directly hits the moon — which is why the moon is illuminated at night. Every so often, though, the sun, earth, and moon align. When this happens, our planet blocks the sun's light, causing an eclipse.
What's going to happen during the eclipse?
At the very start of the eclipse, the moon will begin to pass through the outer part of Earth's shadow, called the penumbra. From our perspective, this part of the eclipse will look pretty subtle — basically, the moon will just be slightly dimmer than usual.
At 5:15 am Eastern time, parts of moon will enter the core of earth's shadow, called the umbra. It'll look like a dark bite has been taken out of the otherwise dimmed moon, with the dark zone gradually growing over time until the white moon is just a sliver.
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A composite photo shows each of the stages of a total lunar eclipse. (Stan Honda/AFP/Getty Images)
At 6:25 am, things will get really cool, as the entire moon will begin glowing an eerie red color instead of its usual white. This is because the sun, earth, and moon will be aligned perfectly, with the entire moon in the earth's umbra. This is the total eclipse.
For most people in the Eastern time zone, the sun will rise and the moon will set just a few minutes after entering this phase, ending the visible eclipse (look up your local sunrise and moonset time here). In fact, the close timing of these events might end up masking the total eclipse entirely, especially for people living right on the East coast. But most people in Central, Mountain, and Pacific will be able to see the total eclipse for longer, and some will see the process play out in reverse as well.
The moon will stay totally eclipsed until 7:24 am Eastern time, then will return to being a white sliver, with the dark portion gradually shrinking as it moves through the penumbra. The fully darkened portion will disappear entirely at 8:34 am Eastern time, as parts of the moon escape the penumbra and it once again becomes just a slightly dimmed version of its normal self.
(Fred Espenak, EclipseWise.com)
Why will the moon turn red during the eclipse?
When the sun, earth, and moon are aligned perfectly, not all of the sun's light will be completely blocked out by earth, because slight amounts of it will pass through our atmosphere and then hit the moon.
As it passes through the atmosphere, some of this light reflects off nitrogen and oxygen molecules and bounces away. Longer wavelengths of light are more likely to make it through without being bounced away and arrive at the moon, and red is the longest wave, so the moon ends up looking reddish. (This mechanism is the same reason sunrises and sunsets look red — basically, the more light is filtered through our atmosphere, the redder it gets.)
Shorter wavelengths of light are filtered out by the earth's atmosphere, causing the longest wavelengths (red light) to disproportionately reach the moon. (Philip Ronan/Wikimedia commons)
A nice way to think of it, suggested by NASA, is to put yourself on the moon, observing the eclipse. At 6:25 am, the earth will appear to align perfectly with the sun, blocking it out almost entirely. But the outer rim of earth will appear to be a bright red, due to the trickle of sunlight making it through the atmosphere. "As you scan your eye around Earth's circumference, you're seeing every sunrise and every sunset in the world, all of them at once," NASA writes.
Will I be able to see the blood moon?
People living across nearly the entire United States — along with parts of Canada, Central America, and the Pacific — will be able to see the start of the total eclipse, but not the latter stages. The farther west you live, the more of it you'll be able to see. People on the West Coast and living in the Pacific will be able to see the whole thing, and residents of East Asia will be able to see parts of the latter stages, but not the beginning.
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(NASA)
The great thing is that, unlike a solar eclipse, you don't need any sort of eye protection — all you have to do is look to the sky to see an astronomical wonder. If you want a really good view of it, you can use binoculars or a telescope.
Like a live TV show, the eclipse will happen at the same time across the country, so you have to adjust for your local time zone. You can find charts for Central, Mountain, and Pacific time here. If it's cloudy in your area or you're otherwise unable to see the eclipse, you can watch a live stream on the website Slooh.com.
How common are these eclipses?
Because they depend on a precise alignment of the sun, earth, and moon, eclipses happen in sporadic yet predictable sets of four closely-timed eclipses, called tetrads.
This is the second eclipse in a new tetrad — the previous one was on April 15, 2014, and it'll be followed by eclipses on April 4, 2015 and September 28, 2015. This particular tetrad is pretty special, as all four of its eclipses are total eclipses. (During partial or penumbral eclipses, which are more common, only the earlier, less-cool stages of the eclipse occur — there is no total blockage of the moon, and no eerie red glow.) But the next eclipse won't be visible from most of the US, so if you live in America and miss this one, your next chance will be in September 2015.
As it happens, the 21st century as a whole is going to feature eight tetrads (that's 32 eclipses), an unusually high number. Between 1600 and 1900, for comparison, there were zero tetrads. So in terms of lunar eclipses, this is a pretty lucky time to be alive.
