What Is a Blood Moon? | Difference Between Solar and Lunar Eclipses

(Last Updated On: May 15, 2022)

If you’ve been looking at the moon recently, you might notice the moon looks a little different. You also may have seen headlines about a Blood Moon, which did happen halfway through May 2022. Which sounds… pretty intimidating, but at least it’s not going to be turning you into an eldritch abomination or something. We hope, no promises. What you might not have heard is that the Blood Moon is actually a lunar eclipse, which might have you thinking: what’s the difference between a solar and lunar eclipse? Also, what is a Blood Moon?

Okay, what’s an eclipse, though?

You’re probably kind of familiar with what an eclipse is, at least in the context of both the solar and lunar ones. Eclipses are a thing outside the scope of just the Sun and Moon, though. An eclipse, in the context of astronomy, just means an object has been obscured by one of two means. The first is passing through the shadow of something else. The other way is for something to pass between the obscured object and the viewer. 

Fun trivia fact, if three things get themselves in a line it’s called a syzygy. This happens more often than you might think–since every solar and lunar eclipse is actually a syzygy. 

The word “eclipse” also has a pretty foreboding history. It’s derived from the Latin eclipsis or Greek ekleipsis. The Greek word literally means “an abandonment” or “a forsaking.” It was commonly also used in the context of the Sun and Moon.

Eclipses kind of have an anatomy. When it comes to observing them, you’re probably concerned with the umbra, antumbra, and penumbra.

The umbra is the position in which the light source is completely eclipsed–the object has covered it completely. The umbra is a kind of “cone” behind the middle-object, you can observe this if you put a light in front of something. You’ll see a cone-like shadow behind it. Just after the tip of that cone is the antumbra. If you’re observing an eclipse from the antumbra, it’s because the object covering the light source is too small to totally cover it from your position. Finally we have the penumbra, which just means the object hasn’t totally covered the light source. 

What’s the difference between a solar and lunar eclipse, then?

So we’ve established that a solar or lunar eclipse is a syzygy. That means the Sun, Moon, and Earth are in a straight line together–but not necessarily in that order. A solar eclipse occurs when the Moon is between the Earth and Sun. On the other hand, a lunar eclipse is just when the Earth is between the Moon and Sun. That means during a solar eclipse, the Moon casts a shadow on the Earth by covering the Sun. During a lunar eclipse, the Earth is the one casting the shadow–this time on the Moon. 

Solar eclipses only occur around once every 18 months, while a lunar one will probably come around once every 6. 

What’s up with the Blood Moon?

If you haven’t guessed already, a Blood Moon is when the Moon appears red during a total lunar eclipse. That’s kind of it, the name is exactly what you thought it was.

However, to understand why a Blood Moon is red, we’re going to need a bit of extra context. The Moon doesn’t produce its own light, when you see the Moon shining in the sky it’s actually just reflecting light from the Sun. When the Earth gets between the Sun and Moon most of the Sun’s light isn’t able to get on the Moon. However, the Sun is pretty big and some light still gets on the Moon after it passes through the Earth’s atmosphere. 

When the light does go through the atmosphere, some wavelengths get filtered out before they make it to the Moon. Generally, these wavelengths are on the shorter side, which means blues will be filtered out. Eventually, the filtered light makes it to the Moon–but all that’s left is the reds on the color spectrum for the Moon to reflect. 

By the way, this is actually the same reason for why the sky is blue

Speaking of eclipses, only a few countries get to observe them when they roll around. See if you know which ones here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.