Moon

Why Is A Total Solar Eclipse Such A Big Deal?

Curiosity's coverage of the 2017 eclipse is brought to you by Rocket Mortgage by Quicken Loans.

Darkness completely overtaking a sunny afternoon sky sounds like an apocalyptic storyline. But don't ration your food just yet. That's exactly what occurs during a total solar eclipse. Imminent doom? Nah. Try once-in-a-lifetime celestial event.

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Darkness Falls Across The Land

Let's do a quick science experiment. Shine a flashlight on a basketball. Move your fist between the flashlight and basketball, in front of the beam of light. Congrats! You've just made a (very not-to-scale) solar eclipse diorama, where the flashlight is the sun, your fist is the moon, and the basketball is the Earth. A solar eclipse occurs when the moon blocks any part of the sun's disc as it passes between the Earth and the sun. (Not to be confused with a lunar eclipse, which occurs when the moon passes behind Earth's shadow.)

The real star performer of the whole event is the moon. That's the thing getting in the way to jazz up your sunny day, after all. A solar eclipse can only occur during a new moon. During this phase, the moon isn't visible because the illuminated side of the moon faces away from the Earth. If we could see any bit of the moon during this phase, a solar eclipse would be impossible. How could the sun illuminate any part of the moon the same time that the moon blocks out part or all of the sun's disc? Get out your flashlight and basketball, and try to simultaneously create a shadow on the basketball and shine some light on the basketball-facing side of your fist. Not gonna happen.

The Rarest Common Event

A new moon isn't the only requirement for a solar eclipse. If it were, we'd get a solar eclipse once every 29 ½ days. (Bummer.) Blame the moon's goofy orbital path. The moon's orbit around Earth is tilted five degrees to Earth's orbit around the sun, which means the moon's shadow usually misses Earth as it passes just above or below us during a new moon. So this must mean solar eclipses are rare, right? Well, yes and no. The chances that you'll experience a total solar eclipse are slim to none. A total solar eclipse occurs over any given location once in about 375 years, on average. So, you know, don't hold your breath.

Because that's just an average, some spots will get a longer or shorter eclipse drought, though. Take Princeton, New Jersey, for example. The last total eclipse visible there was in 1478 and the next one isn't until 2079, a difference of 601 years. On the other hand, Carbondale, Illinois gets a total solar eclipse in 2017 and again just seven years later in 2024. How's that for good luck?

The reason experiencing a total solar eclipse where you live is so rare is because the eclipse's path of totality (where on Earth the moon totally blocks the view of the sun's disc) is very narrow, as little as 70 miles across. That's roughly the distance between Chicago and Milwaukee.

This doesn't mean total solar eclipses are especially rare, though. "Total solar eclipses happen about every 18 months somewhere in the world, on average." Amir Caspi, a senior research scientist at the Southwest Research Institute in Boulder, explains. "But, the Earth is mostly water, so most of the time eclipses are going to be happening over water." Even when an eclipse does cross over land, that doesn't make it a sure bet either. "Most of the Earth is unpopulated," Caspi adds, "and it requires a great deal of effort to go and catch an eclipse when it's crossing over a remote part of another continent."

(Psst — Want to know when your town's time to shine, er not shine, will be? NASA's Javascript Solar Eclipse Explorer lets you calculate the visibility of solar eclipses from any city for hundreds of years in the past and future.)

You'll Burn Your Eye Out, Kid

A total solar eclipse is a dramatic celestial event, but you'll be surrounded by eerie evidence down here on Earth. It'll look like nighttime in the middle of the day, temperatures will drop, and confused birds might stop singing to prepare for nighttime. "If you happen to be in some place where you can see for many miles away, like on top of a mountain," says Caspi, "you'll notice that the ground isn't going to be dark far away because you're in a shadow, but it's not nighttime. The horizon may actually be bright."

But, of course, during a once-in-a-lifetime celestial event, you're going to want to look up. Just be careful, very careful. If the eclipse is not in totality, you can permanently damage your eyes by looking up. You know not to stare straight at the sun, don't you? This is no different. But with certified protective eyewear, you'll be good to watch the whole process. (Here's how to know your glasses are up to the task.)

If you happen to be standing in the right place, you can safely observe the eclipse with your bare eyes, only during totality. And you should, because you'll be seeing the coolest part of the whole event. "The solar corona is basically the thing that makes the total solar eclipse so amazing to see," says Caspi. "It is the outer atmosphere of the sun, and it's about as bright as the full moon. Normally, you can't really see it during the day because it's washed out by the very, very bright light of the sun." Reminder: Once totality is over and the moon begins exposing the sun's disc again as it moves away, slap those protective glasses back on!

"One other cool thing to look for is Baily's Beads," says Paul Bryans, a solar physicist at the High Altitude Observatory of the National Center for Atmospheric Research. "This happens just before and just after totality, when the moon doesn't quite cover the Sun entirely and pockets of light escape to us through the valleys of the moon."

One-Way Ticket To The Eclipse, Please

Pro tip: If you ever get the opportunity to experience a total solar eclipse, do it. Like we've mentioned, depending on where you live, it could be a rarer than once-in-a-lifetime opportunity. "I'd encourage everyone else to make an effort to get into the path of totality if they can," says Bryans. "The difference between a total eclipse and a 99 percent partial eclipse is night and day. Literally. You might not ever get another chance to see it."

Take-Home Notes

Want to tell all your friends about solar eclipses? Of course you do. Let's go over some vocabulary that might come in handy. (For an exhaustive list of eclipse terminology, check out this NASA glossary.)

Penumbra: This is a partial shadow, kind of like the haziness you'll see around the edges of a shadow.

Umbra: This is the darkest part of a shadow.

Antumbra: This is a partial shadow caused by a light-blocking object that is far away from you. Imagine making a shadow puppet very far from a wall versus right up against one.

Partial solar eclipse: This occurs when the moon's penumbral shadow traverses Earth.

Annular solar eclipse: This occurs when the moon's antumbral shadow traverses Earth, meaning it is too far from Earth to completely cover the sun's disc.

Total solar eclipse: This occurs when the moon's umbral shadow traverses Earth, and the moon is close enough to Earth to completely cover the sun's disc.

Hybrid solar eclipse: This occurs when the moon's umbral and antumbral shadows traverse Earth (eclipse appears annular and total along different sections of its path). Hybrid eclipses are also known as annular-total eclipses.

Totality: This is the maximum phase of a total eclipse. It can last from a fraction of a second to 7 minutes 32 seconds.

Path of totality: This is the area where total is visible, and measure around 70-100 miles in width.

Want to learn more about the eclipse? See our other articles here. And to hear an astronomer give even more insights into the eclipse, check out our special podcast episode here or click below to stream.

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Why a Total Solar Eclipse is Such a Big Deal

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