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How to Block Out The Sun

August 6, 2008 on 11:19 am | In Astronomy, Solar System |

Eclipses are one of those events that when they happen, everyone who can wants to watch it. They’re rare enough that I’ve never had a total solar eclipse happen where I was, but I once had an almost-total eclipse happen when I was in New York back in 1995 — we had an 88% eclipse.

It was strange, because the Sun didn’t look any different with the naked eye, but it felt cooler, like the Sun was giving off light but not heat. What’s more, is that if I made shadows on the ground, I could see that the sunlight that shone through was misshapen, like it had a chunk taken out of it, like this picture of a kitchen skimmer’s shadow:

I later learned that this was working like a pinhole camera, which is a way of seeing how much of the Sun is blocked without looking at the Sun itself, by making an image of it instead.

So how do solar eclipses happen, anyway? When the Moon passes in front of the Sun, from the point of view of someone on the Earth, it can make a shadow on the Earth, and we’ve actually seen it from space:

When this happens, we get what’s called a total solar eclipse, where everyone who’s in that shadow sees the Moon entirely block out the Sun, during the day. During this time (and no other), you can see the Solar Corona (the super-hot part of the Sun’s atmosphere) with the naked eye:

But sometimes, even when the Moon passes between the Sun and the Earth, it doesn’t make a shadow on the Earth! This is because the Moon has a huge variety in its distance from the Earth. When it’s closest to the Earth (perigee), it’s 220,000 miles away. But when it’s farthest from the Earth (apogee), it’s 252,000 miles away, which means it appears about 15% smaller in diameter in the sky:

Well, when the Moon looks large (near perigee) and passes between the Sun and Earth, it makes a shadow on Earth and we get a total solar eclipse. But when it’s near apogee, the Moon looks smaller, and sometimes even appears smaller than the Sun in the sky. When that happens, even when the Moon passes between the Sun and the Earth, we don’t make a shadow on Earth:

But instead, what we see, rather than a total eclipse, is an annular solar eclipse, where the Moon blocks out the central part of the Sun, but appears so small that a thin ring (or annulus) of the Sun can always be seen:

So the next time you want to block out the Sun, know that the best way to do it is not just with the Moon, but with the Moon near perigee, or closest to the Earth!


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  1. Everybody knows that the way to block out the sun is with a big disk and a remote control:

    image link


    Comment by brian — August 6, 2008 #

  2. […] How to Block Out The Sun | Starts With A Bang! […]

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  3. […] Siegel, Astrophysicist, sent in his article “How to Block Out the Sun“, referring to the properties of an eclipse and how they work. He also looks at solar […]

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