Thursday, July 24, 2014

Here's why you'll be able to see a "blood moon" Monday night

A 2008 "blood moon," caused by a lunar eclipse. Abdallah

If you're up at around 2:00 am Eastern time on Monday night, you might want to look up. For the first time since 2011, residents of 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. So 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.

2000px-geometry_of_a_lunar_eclipse

Wikimedia Commons

What's going to happen during the eclipse?

At the start of the eclipse (around 1:20 am Eastern time), the moon will 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 1:58 am, 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.

At 3:07 am, things will get really cool, as the entire moon will appear darkened but 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.

It'll stay there until 4:25 am, when the same sequence of events will occur again, but in reverse. The dark portion of the moon will gradually shrink, disappearing entirely at 5:33 am. After that, there will just be subtly dimmed parts of the moon, and if you've stayed up through the whole thing, it's probably time to go to bed.

Eclipse_lune

A composite photo shows each of the stages of a total lunar eclipse. Luc Viatour

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.

There is a lot of dust in our atmosphere, so some of this light reflects off the dust 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.)

2000px-em_spectrum

Shorter wavelengths of light are filtered out by the earth's atmosphere, causing the longest wavelengths (red light) to disproportionately reach the moon. Wikimedia commons


A nice way to think of it, suggested by NASA, is to put yourself on the moon, observing the eclipse. At 3:07 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 South America — will be able to see the eclipse. For most of the rest of the planet, the timing won't be right — the eclipse will be occurring when the moon isn't visible, during daytime.Screen_shot_2014-04-14_at_1

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 — it'll start at 1:20 am Eastern time, 12:20 am Central, 11:20 pm Mountain and 10:20 pm Pacific.

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 first eclipse in a new tetrad, and will be followed by eclipses on October 8, 2014, 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 second and third eclipses 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.

What's this I hear about a biblical prophecy?

Pastor John Hagee, an evangelist and founder of Texas' Cornerstone Church, has garnered publicity for his belief that this new set of eclipses fulfills a biblical prophecy, and could be evidence that the apocalypse is nigh. He cites the line "the sun shall be turned into darkness and the moon into blood before the coming of the great and awesome day of the Lord," from Acts 2:19-20.

The thing is, the total lunar eclipses that produce red moons aren't particularly rare in terms of astronomical events, so if a red moon signaled the end of days, we'd have been apocalypse'd many times over during the past 2,000 years.

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