The Celestial Sphere

In today's class we took the metaphor from the Game of Science and started applying it to the night sky.  The first step is to watch the game being played, so we used an open source planetarium program called Stellarium to observe the motion of the stars across the night sky as seen from Rome, GA.

The students quickly spot some patterns.  The stars all move together as a bunch, not each on their own independent path.  That, of course, is why we can speak of "constellations".  But they find other things.  One star doesn't move much at all, and its angle above the horizon just so happens to be the same as the latitude of Rome, GA.

Eventually we they are led to their first theory: the stars are all stuck on the inside of a giant sphere, with the vastly smaller Earth at the center.  The Celestial Sphere theory is really quite amazing.  It explains the motion of all of the stars across the night sky.  Not one of them, not most of them, but all of them.  Not just tonight, but tomorrow night and every night for the foreseeable future.  (As long as you don't measure to high enough precision, or wait around long enough for precession effects to be large, then it pretty much works all the time.)

Such a simple, beautiful theory.  It explains so much with so little.  If you had not been taught something different since Kindergarten it would be difficult not to believe that this wonderful theory is true.  You would be totally convinced that you had found one of the "rules of the game".

We also learned about coordinate systems: for our sky (altitude and azimuth - using the EJS Local Coordinates Model) and for our newly proposed Celestial Sphere (right ascension and declination - using the EJS Equatorial Coordinates Model).  These coordinate systems are useful and important, but they are so obviously human inventions.  They just don't have the beauty of the whole Celestial Sphere idea.  There is no sense in which one is tempted to think of these coordinate systems as "true" - they are just useful in practice.  The Sphere is useful too, but it seems to transcend mere practical utility.  One can easily believe that it was "discovered" rather than "invented".  If your brain wasn't clouded with a modern education, you might really believe that the Celestial Sphere is up there in the great big sky, spinning inexorably about us.

No wonder people held on to this idea for 15 centuries, and even then gave it up only with extreme reluctance.

On Thursday we are going to take a look at what's up with the Sun.  Is it, too, stuck to the Celestial Sphere?