March 2024: This post has been updated and republished from Sept 2017.

Show of hands — who’s heard about the lunar eclipse coming up on March 14?

As I always do for eclipses, both solar and lunar, I’ve spruced up and republished this post for the occasion. Don’t worry — we’ll be back to our regularly scheduled programming on cosmology next up!

The lunar eclipse is often known as the blood moon, named for its red appearance. Sometimes it’s even mistaken for Mars, as in the case of the “Mars hoax” back in 2002. It was claimed then that Mars would look as large as the full moon on August 27.

In truth, Mars will never appear as large as the full moon to the naked eye (a fancy way of saying that you’re not looking through a telescope or binoculars). What really happened was that the moon passed through the Earth’s shadow.

Wait a second. The Earth has a shadow? And it’s red?

Believe it or not, the Earth’s shadow is actually something you can see yourself.

You’re looking at the bluish haze close to the horizon in this image. This is a particularly pronounced Earth’s shadow. It doesn’t always look this obvious, or this amazing, but you can always see it if you know where to look. You’ll always find it on the horizon opposite the sunset.

What you’re seeing isn’t really that surprising, if you think about it. Every object standing within reach of a light source casts a shadow. The Earth’s shadow, as seen from our surface, is just the dark region of space where our atmosphere isn’t lit up by the sun.

But shadows are simply an absence of light — which means they normally appear black.

So why the heck is the Earth’s shadow blue?

Because, to put it simply, the Earth’s shadow isn’t a “perfect” shadow. If it managed to block every single ray of the sun’s light, then it would cast a black shadow.

Instead, the Earth’s atmosphere scatters the sun’s light.

Here, you can see the geometry involved. Notice that the “night” side of the Earth is just the side of the Earth that the sun’s light can’t reach.

In essence, nighttime is a shadow — the shadow cast by the Earth itself.

You can think of the Earth’s shadow as the outer edge of the night’s darkness slipping around the curve of the horizon to meet you. Before night falls, you can see the Earth’s shadow on the horizon, rising higher and higher until it blends into the night.

Close to the edge of the shadow, where night meets day, some light from the day side is scattered by the atmosphere and sneaks around to the night side.

Of all the colors of the rainbow, blue light scatters best…

That’s why the daylight sky is blue. So, the light you see at the very edge of the shadow is blue.

This wouldn’t be possible on an airless world. Crazy, huh?

Okay, so that explains the blue Earth’s shadow. But how come when it falls over the moon, it looks red?

Well, it’s for the same reason — or, more precisely, the same basic scientific mechanism: Just as blue light scatters best, red light penetrates best.

That’s why the sunset is red. When the sun appears low on the horizon from any one observer’s vantage point, it has to pass through more atmosphere to reach that observer’s eyes. Only red light is able to make the journey.

That is why the lunar eclipse appears red.

At night, we can’t see the Earth’s shadow because there’s nothing for it to be cast on. But it would be red, because the sun’s light has had to penetrate a lot of atmosphere to get to the dark side of the planet.

But when the moon passes through the Earth’s shadow, there’s something for all that red light to fall on. It’s the difference between shining a flashlight straight up into the sky, and onto the ground. And we see the moon bathed in red.

But here’s the million-dollar question…

Why doesn’t this happen every month?

It’s a good question. If the moon’s orbit were actually flat with ours, then that’s what would happen. Every full moon, the moon would pass through the Earth’s shadow and we’d get a lunar eclipse.

But that’s not what happens…

The moon’s orbit is tilted a bit. Normally, when full moon happens, the moon is somewhere above or below the Earth’s shadow — and it doesn’t get bathed in red.

But the moon’s orbit wobbles a bit. Try dropping a frisbee — or anything light enough to bounce a little bit — and notice the way it wobbles almost like a top before settling flat on ground.

The plane of the moon’s orbit does a similar motion. And once or twice every year, it lines up with the Earth’s shadow.

When the moon passes through our planet’s shadow, we get an eclipse.

It’s not always a total eclipse, though. The orbits don’t always line up perfectly. Sometimes, the moon crosses into the shadow but is never completely covered, and get a partial eclipse like this:

Notice that this looks a lot like one of the lunar phases, in that it looks like we’re seeing the nighttime and daytime sides of the moon. But lunar eclipses can only happen during full moon.

For reference, here are the actual phases of the moon.

The darkness in the previous photo of the moon isn’t its nighttime side, it’s the curve of the Earth’s shadow crossing over its daylight side.

Here’s what happens during a partial lunar eclipse:

This diagram refers to the “umbra” and “penumbra” of the Earth’s shadow. The umbra is simply the darker inner region. The penumbra is technically part of the shadow, but it’s so much brighter that it doesn’t really look like a shadow.

If the moon only crosses through the penumbra, we technically get an eclipse: a penumbral eclipse. But there’s not much of a visual difference. You can’t tell there’s an eclipse happening.

It’s why the eclipse diagramed here won’t be a total one. The only part of the Earth’s shadow that matters for a total eclipse is the umbra.

When we get a total lunar eclipse, the moon passes through the umbra entirely. It might not pass through the dead center, but the entire moon will spend some time in the umbra. What happens then is something more like this:

Take it from me — lunar eclipses are one of the coolest sights you’ll ever see, not least of all because they’re easy to see with inexpensive equipment.

Seriously — you can see it with the unaided eye. For a closer look, all you really need is a pair of binoculars.

Personally, I usually view them with a 5-inch refractor telescope. In fact, I’ve taken a photo of one using that telescope, and I’ve used it as the logo for this blog ever since!

In case you haven’t noticed it before, here it is:

It’s…certainly not a very impressive shot. Way too blurry for my taste.

But I bet I can do better…in just under a week!

This coming eclipse, I’m looking to bag another shot to use as my new blog logo. This time, in focus…

Anyway.

Anyone in the Americas will be able to view this eclipse at least partially; you can find out how it will look from your location over here on TimeAndDate.com. Anyone in most of North America and about half of South America will be able to see a total eclipse.

If you can catch it for yourself…I hope you enjoy!

Next up, we’ll be back to our deep-dive into dark matter.