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How Far?

Here’s a great story of someone using their noggin to figure out something about nature. For a long time people have been fascinated by comets passing by the earth. However, nobody knew how far away they were. Aristotle, for example, figured that comets were in the upper atmosphere, just a few miles high. Other ancient thinkers thought this was probably wrong. How could you tell?

Can you think of a way? Imagine that you don’t have any modern instruments to use. No hubble telescope, no airplanes, no radar. What might you do?

A Danish astronomer named Tycho Brahe figured this one out. In 1577 everyone was talking about a comet that was then in the sky. Since a lot of people had seen it, Brahe traveled around gathering reports. He figured if the comet was only as high as the upper atmosphere, it should appear to be in different parts of the sky to different observers.

That’s because moving around under the comet would change your perspective. The stars are so far away that we can’t change our perspectives on them, so Brahe realized he could use them as a reference.

He found that everyone said it was in the same part of the sky on the same day. This showed that comets are much farther away than the upper atmosphere. Brahe decided they were even farther than the moon!

That was a bold statement in his day, but his experiment showed him right. Now we know that comets are much, much farther away than anyone thought, and that they only pass by our planet while orbiting the sun.

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