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How Do We Know the Earth is Round?

Week of December 2, 1998

Ah, vacations. Most people, when they take vacations, forget about work and responsibilities and simply relax. Not so for the Bad Astronomer! Astronomy is everywhere, even on vacation in sunny Florida. No matter how hard I try to relax and get away from it all, astronomy finds a way to creep in to my life.

A quick digression: I get email every now and then asking for some proof that the world is round. Of course, photos from space make it pretty clear, but how can we tell from the ground that the Earth is not flat? There are lots of methods, actually. One is that the shadow of the Earth on the Moon is always round, indicating that the Earth must be a ball. I never liked this one, actually: it's possible to imagine the Earth is actually a flat disk and get the same results. Another is that a ship sailing away will appear to slip below the horizon, because it is going around the curvature of the Earth. While true, this one is a little unsatisfying because there are all sorts of weird optical effects as you look through all that air to the horizon.

There is an unambiguous effect, though, of the curved Earth, which brings me back to my vacation. My parents live in Sarasota, which is about 1600 kilometers south of where I live near Washington DC. This is equal to about 1/30 of the way around the Earth, or 12 degrees. When I am at home and go out to look at Polaris, the North Star, it is about 40 degrees above the horizon. If I lived at the North Pole, it would be 90 degrees above the horizon, or straight up. However, when I visit my parents, I travel south, and so Polaris appears lower. Much lower, 12 degrees worth! That is very noticeable to the naked eye. On the other hand, stars towards the south appear to be much higher in the sky when I am in Florida. Last year I could clearly see Canopus (the second brightest nighttime star in the sky) to the South, but it never gets high enough to see from my house.

If the Earth were flat, we'd never see this effect. If the Earth were a disk we'd only see it if we traveled along the edge, and not the face. Therefore we must live on a curved Earth, a big ball in space (as a matter of fact, this effect can even be used to determine the circumference of the Earth!). But you don't have to believe me. The next time you travel north or south, try it for yourself. You may be surprised at how different things look. As a matter of fact, if you go to the opposite hemisphere, familiar constellations (and even the Moon) look upside down! I've never been to the southern hemisphere, but someday I hope to see this for myself. What a vacation that would be!

©2008 Phil Plait. All Rights Reserved.

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