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Sky watch: Measuring the sky with circles

We can't see them, of course, but the heavens contain two "great circles" that can help us define the position of any celestial object.

The easiest to imagine is the one we know as the horizon. Starting at due north, scan your gaze east along the horizon and divide it into 360 equal pieces. One degree is the width of your little finger held at arm's length; your fist -- thumb to little finger -- also held at arm's length spans about 10 degrees.

We measure azimuth eastward along the horizon from due north. So something that lies due east is said to have an azimuth of 90 degrees east of north. And something in north-northwest might have an azimuth of 315 degrees east of north. circle we can imagine begins at any point on the horizon, passes directly overhead (the zenith) and continues down to the opposite side of the horizon. Since it's half a circle, we can divide it into 180 degrees and use it to measure the altitude of any celestial object. You can use your fist for this, too.

See it with Polaris, the North Star, which remains relatively fixed in our sky. If you live in San Diego, the star's altitude is about 33 degrees; from Chicago, Polaris appears about 42 degrees above the horizon because the altitude of the North Star equals your latitude.

There are other circles: the ecliptic, the meridian, the celestial equator . . . knowing these help astronomers grasp the layout and movement of our sky. It was back in the autumn of '66 that something clicked inside of me. Perhaps it was a clonk on the head during football practice; more likely, though, it was the skill of my geometry teacher who never gave up on the goofy kid in the third row.

Soon I became fascinated by how one can describe the world and universe with geometrical figures and angles, and how important this was in the field of astronomy.

And for that precious gift, Mrs. Moyer, I thank you!

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