A total lunar eclipse — sometimes called a "blood moon" — is coming to the West Coast early Saturday morning, April 4.
Today, the moon will slowly wander into the Earth's shadow. Eventually, the shadow will completely envelop the moon and block nearly all sunlight, creating a total eclipse and causing the moon to glow red from small amounts of light passing through our atmosphere:
The total eclipse will only be visible to residents in the western half of the United States (as well as people throughout the Pacific, East Asia, and Oceania). It starts right at 4:58 am Pacific and will last just five minutes.
On the East Coast, the sun will have already risen by that time, making the blood moon invisible. However, Easterners can wake up before sunrise and get a glimpse of the partial eclipse, starting around 6:16 am Eastern.
Unlike a solar eclipse, a lunar eclipse is totally safe to watch without eye protection, and lasts for a few hours.
1) 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.
When only part of the moon enters the core part of the Earth's shadow (the umbra), just part of it darkens. This is a partial eclipse, and it's what US residents on the East Coast will be able to see if they wake up early.
When the entire moon enters the umbra, nearly all the sunlight is blocked from hitting it. This is called totality, and it's during this stage that the darkened moon will glow red — which is why lunar eclipses are sometimes called blood moons.
2) How can I watch the lunar eclipse?
The great thing about lunar eclipses is that unlike solar eclipses, they don't require any sort of eye protection to watch — 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.
People across most of the US will be able to see the early partial stages of the eclipse, which will begin at 6:16 am ET. However, sunrise will interrupt it for people east of the Mississippi.
Most residents of the western US, though, will be able to see a total eclipse right at 4:58 am PT (or 5:58 am MT). It'll only last for four minutes and 43 seconds, because the moon is just briefly skimming the outer edge of the umbra.
Sunrise will end the latter stages of the partial eclipse for West Coast residents, too, but the entire thing will be visible for people throughout the Pacific and Oceania.
If it's cloudy in your area or you're otherwise unable to see the eclipse, you can watch a live stream from the Slooh Community Observatory.
3) What will the eclipse look like?
At the very start of the eclipse, when parts of the moon begin to pass through the penumbra, the eclipse will look pretty subtle — basically, the moon will just be slightly dimmer than usual.
At 6:16 am ET, parts of moon will enter 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.
At 4:58 am PT, things will get really cool, as the entire moon will begin glowing an eerie red 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.
The moon will only stay totally eclipsed until 5:03 am PT, then will return to being a white sliver, with the dark portion gradually shrinking as the moon moves out of the umbra. The darkened portion will disappear entirely at 6:45 am PT, as parts of the moon escape the penumbra and it once again becomes just a slightly dimmed version of its normal self.
4) 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 — slight amounts of it will pass through Earth's atmosphere and then hit the moon.
As the light passes through the atmosphere, some of it 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.)
A nice way to think of it, suggested by NASA, is to put yourself on the moon, observing the eclipse. At 4:58 am PT, 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.
5) 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 third eclipse in a tetrad — the previous one was during October 2014 — and it'll be followed by another eclipse on 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.)
As it happens, the 21st century as a whole is going to feature eight tetrads (that's 32 eclipses), an unusually high number. So in terms of lunar eclipses, this is a pretty good time to be alive.