Tag Archives: spheres

How they got all the planets inside that building I’ll never know

If you’re lucky enough to live near a planetarium, you should go see one of their shows.

As Ptolemy worked out the positions of the stars on a big sphere, he imagined smaller spheres that account for the movement of the Sun and the planets— operating like separate, smaller gears in a giant clock. The amazing thing is, even though Earth isn’t the center of the universe, Ptolemy’s geocentric model is still accurate. Weird, huh? If you ever visit a planetarium, you’ll sit in a round room with a domed ceiling above. At the bottom of the dome, all around the room, is the horizon. The night sky is projected onto the dome with all the fixed stars in their places—they rotate around, just like the real night sky. The planets are also projected onto the dome. The planet projectors operate on a separate gear system exactly like the spheres Ptolemy had proposed.

A rough sketch of a planetarium projector.

Here’s some interesting info about planetarium projectors. 

Here’s a listing of planetariums so you can find one near you.