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Do you live in North America, South America, Australia, or eastern Asia? Then you get to see a lunar eclipse on the night of April 14–15! And while North America is the best place to watch—we’ll get to see the whole event—the real action doesn’t begin until 05:58 UTC on the April 15, which is just before 02:00 EDT, so it’s a bit late. You might just want to stay up for it, though.
A lunar eclipse is when the Moon slips into the shadow of the Earth and gets dark. Unlike a solar eclipse (where the Moon blocks the Sun) a lunar eclipse lasts for hours and is perfectly safe to observe without protection. In fact, I find using binoculars is best!
How does this work? The Sun lights up the Earth (big duh there), and anything that’s illuminated casts a shadow. Normally the Earth’s shadow just goes off into space, but sometimes the geometry works out that the Moon passes into it. The Moon has to be opposite the Sun in the sky for that to happen, so lunar eclipses only happen when the Moon is full.
The Earth actually casts two shadows; a wide, fuzzy one called the penumbra and a narrower, darker one called the umbra nested inside the penumbra. If the Sun were a point source in the sky (a little dot) there would only be one dark shadow, but because the Sun has a finite extent (that is, we see it as a disk) the geometry is a little more complicated. If you could see the shadows in the sky, the penumbra would be a big circle about five times wider than the Moon, and the umbra would be a circle inside it about half that size.
It helps to think of it from the Moon’s viewpoint. If you were standing there, looking back at the Earth and Sun, you’d see the Earth (barely; you’re seeing it’s night side) sliding slowly over the face of the Sun. At the moment the edge of the dark Earth starts to block the sun, you’re entering the penumbra. It’s getting darker, but most of the Sun is still unblocked, so it’s not getting very much darker. About an hour later the Earth completely blocks the Sun, and you’ve entered the umbra. The Earth is much bigger than the Sun from your point of view (about four times larger) so the Sun stays blocked for a while. Finally, the Sun peeks out the other side of the Earth; you’ve left the umbra and are in the penumbra again, and things start getting brighter.
What does this mean for us here on Earth? We’ll see the Moon enter the penumbra at 04:53 UTC April 15, or 00:53 EDT (53 minutes after midnight). Again, it’s no big deal, and you’d hardly notice. But the Moon’s edge enters the darker umbra at 05:58 UTC (01:58 EDT) and over the course of a few minutes you’ll see that part of the Moon get dark. Over the next hour or so more of the Moon will fall into the Earth’s darker shadow, and at 07:06 UTC (03:06 EDT) the entire Moon will be dark. It’ll stay that way for the next hour and 18 minutes, until it starts to move out of the umbra at 08:24 UTC (04:24 EDT), and will start to be illuminated by the Sun again. The umbral eclipse ends at 09:33 UTC (05:33 EDT).
Here’s a diagram that may help:
The Moon moves from right to left in the diagram. The positions are labeled. P1 is when it moves into the penumbra, U1 is when it moves into the umbra, U2 is when it’s fully immersed, U3 is when it starts to leave the umbra, U4 is when it’s out of the umbra, and P4 when the Moon leaves the penumbra, and the eclipse ends. The times are listed in the lower right in UTC. Subtract four hours for Eastern U.S. time, and so on.
Sometimes when the Moon is fully immersed in the Earth’s shadow it can turn an eerie blood red due to the way the Earth’s atmosphere scatters light—it’s the same reason the Sun can look redder at sunrise and sunset.
Want to hear something poetic? If you were standing on the Moon during the deepest times of the eclipse, from your view you’re seeing all the sunrises and sunsets on earth at that moment.
When someone tells you science is cold and emotionless, tell them that.
The only problem with this eclipse is the timing; it happens late Monday night/early Tuesday morning for most of the U.S. But don’t let that stop you! If you have clear skies you really should go out and look. And if you have a camera, please take some pictures! With a little planning you can get some amazing shots like the ones I’ve scattered through this post (see Related Posts below for many more). Check out this incredible time-lapse animation made by Jeffrey Sullivan of a lunar eclipse in 2011:
Observing the Moon with a telescope or binoculars during an eclipse is a wonderful thing, but if you only have your eyes, that’s fine too. It’s fun to go out every few minutes between U1 and U2 and watch the Moon get eaten by the Earth’s arcing shadow.
I hope you have clear skies and good viewing for this event! And if you don’t, never fear: There’s another one in October, then a third in April 2015, and a fourth in September 2015 too. You’ll have plenty of chances to see this lovely astronomical bit of geometrical alignment over the next year and a half.