When the Sky Goes Dark: a Deep Dive into Eclipses

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Eclipses are awesome! That’s it, that’s the post!

Okay, that’s very clearly not the whole post, because there’s a whole lot more to say about eclipses than just that they’re awesome. But they are, so we should get that out of the way first. They occur a couple of times a year, but often in configurations that make it hard for any particular person in any particular spot to see them. Eclipse season is coming up, so I figured now was a good time to bring up these amazing celestial alignments.

It’s All About Geometry

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An eclipse happens when one astronomical body passes through the shadow of another. Here on Earth there are two ways this can happen. One way is when the Earth gets between the Moon and the Sun and its shadow falls on the Moon. We call this a lunar eclipse. The other way is if the Moon gets between the Earth and the Sun, and its shadow falls on the Earth. This is a solar eclipse.

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Since the Moon goes around the Earth roughly once a month, you might wonder why we don’t get these eclipses as frequently. The simple fact is that the Moon’s orbit isn’t perfectly flat (or perfectly circular…we’ll come back to that later). It’s tilted by just over five degrees from the ecliptic, the plane of the solar system marked by the Sun’s equator. That doesn’t sound like much, but it’s enough that most of the time when the Moon is at the point in its orbit between the Sun and Earth (the phase known as New Moon) its shadow falls just above or just below the Earth, not on it. Similarly most of the time when Earth is between the Moon and Sun (the Moon phase called Full Moon), its shadow falls just above or below the Moon. That means most of the time we don’t get an eclipse.

But twice a year or so the Moon’s orbit brings it across the ecliptic right around either Full Moon or New Moon and that’s when you have all the right geometry in place to get an eclipse.

The Lunar Type 

Let’s take a closer look at a lunar eclipse. As stated before, these will only happen when the Moon is in the Full Moon phase, since this is when it’s on the opposite side of the Earth from the Sun. The Earth is large enough, with a big enough shadow, compared to the Moon that it’s possible for the Earth’s shadow to fully cover the entire Moon. This is a total lunar eclipse.

You might assume during such an event that the Moon would just go dark. After all, it’s only lit by reflected sunlight and its view of the Sun was just cut off by the great big hunk of rock in its way. But that doesn’t account for Earth’s atmosphere, which scatters the light that passes through it.

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If you were on the Moon looking towards Earth during a lunar eclipse, you would be able to see a little sunlight. Specifically you’d be able to see the light through the atmosphere just around the edges of the Earth, where a person on Earth would be experiencing sunrise or sunset. Think of the last good sunrise or sunlight you saw. The sky turns red because that’s the wavelengths of sunlight that make it through the atmosphere best when the Sun is near the horizon. That means that’s also the wavelengths of light making it around the edges of the Earth, through the atmosphere, during a lunar eclipse.

So during a total lunar eclipse the Moon is receiving sunlight, but just a little and just the red wavelengths. This means the Moon turns a deep red during totality, the point where the Moon is fully in the Earth’s shadow. It’s very gothic and also epic and has earned this special alignment of celestial objects the fanciful nickname of “Blood Moon”.  

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Sometimes the Moon doesn’t pass all the way into Earth’s shadow—it might just clip it. In this case, only part of the Moon will be shadowed, and this is called a partial lunar eclipse. There is also a very special case called a penumbral eclipse. This has to do with the fact that Earth’s shadow has two parts—the deep dark umbra in the middle and the fainter penumbra surrounding it. When the Moon goes into the shadow, it passes through the penumbra first and passes out of it last. Very occasionally the geometry can be just right that the Moon will pass fully into the penumbra but not actually make it into the umbra. This is a penumbral eclipse, and it’s quite a faint effect.

Since the Earth’s shadow is large relative to the Moon it can take our satellite hours to pass all the way through it, depending on whether the Moon is going through the wider middle part of the shadow or just skimming through the narrower top or bottom. And it can be seen by anyone on the nighttime side of the Earth while the eclipse is in progress, which means it’s a lot easier to see a lunar eclipse than a solar one—a whole half of the Earth can spot it as it happens.

The next lunar eclipse Earth will get is going to be a partial one on October 28^th. Europe, Asia, and Africa will have great views of it. North and South America, not so much. We’ll be getting a penumbral eclipse in March though.

The Solar Type 

Solar eclipses get a lot more hype than lunar ones, partly because they’re harder to see. The Moon is a lot smaller than the Earth, which means its shadow is a lot smaller as well. When the Moon blocks the Sun, it only does so for a small strip of the Earth, which is known as the path of totality. Anyone outside the path might notice a little dimming, depending on how much of the Sun the Moon blocks from their perspective, but nothing like the drama of a total solar eclipse. 

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That’s another reason for the hype around solar eclipses—they’re incredible. I got to see one for the first time in August 2017, when the path of totality cut straight across the US. It was one of the most amazing sights I’ve ever seen anywhere ever. I was vibrating from sheer adrenaline for hours afterwards. Being in the Moon’s shadow means the sky darkens and the stars come out during the daytime. My lizard hindbrain knew something weird was happening as the light began to dim and the temperature dropped, and it didn’t care that my logical frontbrain knew exactly what was going on. I got goosebumps all over in the middle of a nice, warm summer day.

During a total solar eclipse the Moon covers the disc of the Sun, but not its corona. This is a thing we Earthlings don’t pay attention to because we normally can’t see it. It’s basically the outermost plasma layer of the Sun’s atmosphere and it can extend thousands of miles beyond the Sun’s surface. It’s all lit up, but nothing compared to the brightness of the surface itself, so we generally don’t see it. 

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But when the Moon blocks the light from the surface the corona becomes viscerally visible around it, glowing like a white fire around the black of the Moon’s disc. I imagined that if I were an ancient human with no knowledge of celestial mechanics I would have seen it as a great hole in the sky, leading somewhere potentially horrible but also quite possibly wondrous and absolutely not meant for puny mortal eyes.

Solar eclipses are far more fleeting than their lunar counterparts, lasting minutes at most for any particular observer, the steady motion of the Moon in its orbit quickly carrying it back out of the way of the Sun. The highly ephemeral nature of these experiences is part of its beauty, if you ask me.

Just as with the lunar type, a solar eclipse can be partial, when the Moon does not fully cover the Sun. This can happen either because the geometry isn’t right for the Moon to fully block our star or because you’re near but not in the path of totality for a total eclipse.

There’s also another funny bit of geometry that can lead to what is called an annular eclipse. It is a remarkable cosmic coincidence that we can get the total solar eclipses that we do. It has to do with the fact that our Sun and Moon appear about the same size in the sky. The Sun, of course, is actually a whole very lot larger than the Moon but it’s also a whole very lot farther away. Nowhere else in our solar system has this very lovely arrangement.

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But, as stated before, the Moon’s orbit isn’t a circle, it’s an ellipse. Not much of one, but it’s definitely not perfectly round. Sometimes the Moon is slightly farther away from the Earth than at other times. And when it’s slightly farther away it will appear slightly smaller. An annular eclipse 

happens when the Moon passes in front of the Sun while it’s at the farther end of its orbit, meaning the Moon appears slightly smaller than the Sun in the sky. The Sun does not totally disappear behind the Moon. Instead a ring of sunlight remains visible around the Moon’s disc. This is sometimes referred to as a “ring of fire” eclipse, and is one of the fun, special little quirks of our Moon.

There will be an annular solar eclipse visible in certain parts of the Western United States on October 14. The next total solar eclipse (in case you haven’t heard the hype) will occur on April 8, 2024 and cut beautifully across an arc of the US stretching from South Texas to Maine.

And Now for the Safety Stuff

Let me make one thing very, perfectly, completely clear: staring at the Sun without proper eye protection is bad. You know this, it was covered in, like, day 2 of kindergarten (day 1 being look both ways before crossing the street). It doesn’t matter if the Moon is partially or even mostly covering the Sun. If any part of the disc of the Sun is visible, do not look at it without appropriate eye protection. The only time it is safe to look at the Sun with your eyes is during totality in a total solar eclipse. You’ll note this means that partial or annular solar eclipses are not safe to look at without protection.

There will be a whole lot of folks peddling eclipse glasses in the coming months in preparation for the April event and not all of them will be reputable. Make sure any eclipse glasses you purchase are the real deal before you use them to look at the Sun. Welder’s glass of at least level 14 will also provide sufficient protection. It is also possible to indirectly observe a solar eclipse without actually looking towards the Sun.

Eclipses, whether of the lunar or solar variety, are absolutely incredible events to experience. Just make sure you don’t wind up sacrificing your vision to do it. Observe wisely, my friends!