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How Far can you See?

Week of February 3, 1997
What's the farthest distance you can see? Looking around during the day, chances are the farthest you can see is only a few kilometers. If you are up in a tall building, you might be able to see objects as much as 100 kilometers away. In an airplane, that distance might double or triple.

But if you look up, you can see for about 150 million kilometers! That's how far away the Sun is. If you wait a few hours for nightfall, you'll see a lot farther. Saturn, the farthest planet from the Earth easily visible to the unaided eye, is about a billion kilometers away on average. But even that is closeby compared to the stars. The nearest star is 4 light years, or 40 trillion kilometers away. Betelgeuse, the bright red star that marks Orion's shoulder, is about 400 light years, or 4 quadrillion kilometers away (that's a 4 followed by 15 zeroes!).

picture of the Andromeda Galaxy But even that is small change. The Andromeda Galaxy, a collection of billions of stars much like our own Milky Way Galaxy, is not hard to see with the naked eye from a dark spot on a winter night, and it is 2.9 million light years away. That's about 20 million million million kilometers away, or a 2 followed by 19 zeroes!

So on a clear day, you can see quite a ways, but on a clear night, you almost can see forever.

The Students for the Exploration and Development of Space (SEDS) group has an excellent page with a thorough description of the Andromeda galaxy . It also has links to more info about one of our nearest spiral neighbors.

Thanks to Bitesize Reader Troy Hoffman for pointing out I had written `miles' at one point on this page instead of `kilometers'. Also, thanks to regular Bad Reader Russ Bogel for pointing out that I had an old distance to the Andromeda Galaxy listed. The new distance of 2.9 million light years came out after the Hipparcos satellite made better distance measurements to nearby stars.

©2008 Phil Plait. All Rights Reserved.

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