Introduction to Spherical Astronomy

Terms: celestial sphere, horizon

If you go out in an open field on a clear night and look at the sky, you have no indication of the distance to the objects you see. A particular bright dot may be a airplane a few miles off, a satellite a few hundred miles off, a planet a many millions of miles away, or a star more than a million times further away than the most distant planet. Since you can only tell direction (and not distance) you can imagine that the stars that you see are attached to a the inside of a spherical shell that surrounds the Earth. The ancient Greeks actually believed such a shell really existed, but for us it is just a convenient way of talking about the sky.

In the below diagram I've placed a stick figure of a person in a green open field, and marked the cardinal directions on the horizon with the appropriate letters (NESW). Unless you live in Kansas, real horizons are partially obstructed, but we can imagine what the view would look like without the bell banner, etc. getting in the way. The horizon would then be a 360° circle where sky meets ground.

Perspective drawing can be difficult to interpret (particularly when non-artists like me draw them). In the below drawing, I mean the red star to be on the far side of the dome, and hence a little above the NE horizon. The yellow star, on the other hand, I mean to be on the front side of the dome, and hence a bit above the NW horizon as viewed by the stick figure.