Swinging Girl

Countdown to the Solar Eclipse


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Solar Eclipse Science

On Monday, August 21, a total solar eclipse will cover a swath of the continental United States, and people across the country are losing their minds.

It’s for good reason, though. When was the last time you viewed a total solar eclipse? Unless you’re a solar eclipse chaser—yes, there are solar eclipse chasers, just like there are storm chasers—then probably never. It’s not that solar eclipses are rare—they occur about once every 18 months—but the chances of viewing a solar eclipse are much slimmer than viewing a lunar eclipse. 

Scientists and astronomers devote their lives to studying solar and lunar eclipses, so it's no surprise the average person has no idea what these celestial gems are all about.

We asked one of our Extreme Scientists, Jeff Stevenson, to walk us through the science behind solar and lunar eclipses, and here’s what he had to say. 

What is a solar eclipse?

Find a light source near you such as a lamp or streetlight and imagine it is the sun. Your head is the Earth and your thumb nail is going to be the moon. Close one eye and stretch out your arm. Slowly cover the light source with your thumb by moving it into the path of the light. This is what is physically happening to our solar system. Just because your thumb is covering the light source does not mean the whole room went dark. It is just the unique perspective of your eye that makes it seem like it is happening.

Simply put, a solar eclipse is the moon casting it's shadow on a specific spot on Earth.

Why don't we get a solar eclipse every month?

Our thumb and lightbulb analogy is simple, but orbital dynamics is often very complex, with lots of moving parts. The moon does not orbit in exactly the same plane as the sun. Its orbit is tilted ~5 degrees from the solar plane. This means that, more often than not, the moon passes near the sun but does not cover it from the Earth's perspective.

There are, however, two nodes in the moon's orbit that it has to pass along the solar plane. One of those nodes gives the opportunity for a solar eclipse and the other a lunar eclipse. There is the potential for one lunar eclipse and one solar eclipse about every 18 months.

If this sounds confusing, don't worry. There are astronomers who spend their lives studying and predicting eclipses around the globe. 

Why are solar eclipses rarer than lunar eclipses?

Let's go back to our thumb and lightbulb analogy. Remember, you had to get the your thumb in just the right spot for your one open eye to be in the path of your thumb's shadow. In fact, if you close that eye and open the other eye you can get another perspective from "Earth" and you'll see the light source again. A solar eclipse is only visible on ~1/2% of the Earth's surface. This makes it very unlikely that any given human will see more than one or two solar eclipses in their lifetime.

Except for eclipse chasers, also called umbraphiles or shadow lovers, who make it their mission to see as many solar eclipses as possible. Some of them have seen as many as 27 solar eclipses in their life. Want to read more about these self-professed "eclipse junkies"? Check out this recent story on NPR.

If you want to understand why lunar eclipses are more common, let's go back to our thumb analogy. This time, rotate your thumb and body so that your head is between it and the light source. Your head should now be casting a shadow on your thumb. Notice that it doesn't matter if you close one eye or the other. Your thumb is still dark from your head shadow.

This is exactly how lunar eclipses work. The Earth casts a shadow onto the moon and anyone on the night side of Earth will be able to view the lunar eclipse. That's 50% of the planet that will get to experience this type of eclipse!

So solar eclipses happen more often than you realize, you will just need to rack up the frequent flyer miles to see them all!

I'm not crying, you're crying

So now that you have an idea of all of the science behind a solar eclipse, does that make them less special? Less remarkable? Less dramatic?

Consider this question: Why does the moon almost perfectly cover the sun? 

We could have lived on an Earth with a smaller moon or a larger sun, or maybe even an alternate Earth where the moon was much much farther away and solar eclipses were hardly even noticeable. But we live on this Earth, where for some reason, by happenstance, the sun is ~400X larger than the moon and is also ~400X farther away, making them almost identical in size from our perspective.

The moon is also constantly getting farther from the Earth with each passing orbit, at a rate of about 3.8cm/yr. We can actually measure this very accurately because of the technology left on the lunar surface after the moon landings.  With this information, we know that one day there won't be any total solar eclipses. 

Mind. Blown. 

By some galactic miracle, cosmic coincidence or higher power, we humans are lucky enough to inhabit this planet at just the right time to witness this phenomena and also understand it in terms of our place in the universe.

Happy viewing everyone!

warning icon

As crazy-cool as it might seem to sit and stare at the sun with your bare eyeballs, it’s mostly crazy. Never directly view the sun through anything but a certified solar viewer or solar telescope.

Viewing the sun through binoculars, normal telescopes or bare eyes can lead to significant or permanent damage to your eyes.

For more information on how and why to protect your eyes during a solar eclipse, check out our safety tips.