There's a partial solar eclipse Thursday afternoon. Here's how to see it

On Thursday afternoon and early evening, people across the United States, Canada, and Mexico will be treated to a pretty cool astronomical phenomenon: a partial solar eclipse.

The moon will pass between Earth and the sun and appear to take a nicely rounded bite out of the sun for a period anywhere between a few minutes (for people on the East coast) to over an hour (for those on the West coast). The sky won't go dim (that only happens in a total eclipse), but it'll still be an interesting sight worth checking out.

There is, however, a caveat: you definitely shouldn't look at the eclipse directly, as it can cause permanent eye damage. Instead, there are several ways to watch the eclipse happen safely. Here's what you need to know to see this astronomical event.

What is a solar eclipse?

A solar eclipse occurs when the moon's orbit of Earth takes it directly in between us and the sun, causing the moon to temporarily block out some of the sun's light.

(Sagredo)

Places on Earth that fall within the moving path of the moon's umbra — the central part of its shadow — can briefly see a total eclipse, in which the entire sun gets blocked out by the moon, leaving just a faint shimmering ring of light in the otherwise darkened sky.

Partial eclipses are much more common, because they occur throughout the much wider penumbra region. Because of the alignment of the Earth, moon, and sun, in this case, only a partial eclipse will occur on Thursday — no part of the Earth will fall within the umbra.

What will the eclipse look like?

As the Earth rotates and your location enters the penumbra, the faint moon will gradually pass in front of the sun and block out a portion of it. Because it won't block the whole thing, the sky won't take on an eerie daytime darkness (as it does during a total eclipse), but it will dim slightly.

A composite image showing the stages of a 2007 partial eclipse. (Giul Maiolini)

Depending on your location, the moon could block out larger portions of the sun, reducing it to a crescent-shaped sliver. People on the West coast, who will see a much longer eclipse, will also get something a bit closer to this effect.

A composite image of a 2011 partial solar eclipse, taken in Germany. (Sgbeer)

Who will be able to see the eclipse?

People living across pretty much all of the United States, Canada, and Mexico will be able to see at least part of the eclipse, but in general, the farther north and west you are, the longer and more dramatic it will be.

That's partially because the sun will set shortly after the eclipse begins for people on the East coast. Additionally, the area of greatest eclipse is in extreme northern Canada, so people closer to there will see a greater portion of the sun blocked out by the moon.

The concentric blue rings show the degree of eclipse, with the greatest eclipse seen by residents of extreme northern Canada. The green lines show starting times. (NASA)

The eclipse will start first for people on the West coast. It'll also last longer for them, as it won't be cut off by sunset.

(NASA/SINCLAIR)

Here's a chart showing starting and ending times for some major cities. The timing depends on your precise latitude or longitude, so if your city isn't listed here, you can look at NASA's listed times.

This cool map, made by astronomer Geoff Sims, shows exactly how much eclipse you'll be able to see in any given location:

(Geoff Sims)

How can I watch the eclipse?

It bears repeating: do not look directly at the eclipse. It's never safe to look directly at the sun, even if it is partially blocked out. It might be tempting to try, but the light's intensity can quickly cause permanent damage to your retinas, potentially even leading to blindness. Using binoculars, a telescope, or a camera is an even worse idea, as the lenses focus the light further.

There are, however, a few different solutions. One is a pair of eclipse glasses, which are specifically made to block dangerous wavelengths of light. Don't try substituting normal sunglasses, as they're not designed to do this.

Alternately, you can make a pinhole projector by poking a small hole in the side of a cardboard box or on a piece of paper. Hold the cardboard or paper in the sunlight as the eclipse occurs, and it'll project a miniature image of the eclipse on to the ground. NASA has more detailed instructions here.

A tiny image of a partial eclipse, seen via a pinhole projector. (mr.irwin)

Finally, if it's cloudy in your area, you can just watch a livestream of the eclipse on the astronomy site Slooh.com.

How common are these eclipses?

Partial eclipses actually happen pretty regularly — they occur in regular cycles called saros, and during the 21st century, there will be a total of 224.

This one is notable because it'll be visible to people across North America. The next one most Americans will have the chance to see will be a total eclipse in August 2017.

On the other hand, total eclipses are much more rare than partial eclipses for a couple of reasons. One is that many eclipses (like this one) are partial for all viewers on Earth, because the moon's umbra doesn't actually hit our planet. Another is that even total eclipses only appear as total eclipses for viewers along a thin strip of Earth, where the umbra actually covers — everywhere else, they simply look partial.

For an idea of just how rare total eclipses are, here's a map showing the areas all total eclipses between 2001 and 2020 will be viewable from.

(Eclipse predictions courtesy of Fred Espenak, NASA/Goddard Space Flight Center)

In other words, mark your calendars for August 21, 2017.

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