Celestial Sphere
What is a celestial sphere and what is it good for? Well, when we refer to the celestial sphere, we are imagining that everything we see on the sky is set on the inside of a huge spherical shell that surrounds the earth. This may sound rather similar to the early belief that all of the heavenly bodies were fastened to the inside of a bowl or shell which covered the earth. However, we are only using this picture for convenience, as a way to record the positions of objects on the sky. To begin drawing a celestial sphere such as the one above, you only need to know the lattitude of the observer. Then imagine that the spot where the observer is standing is the "top of the world"; draw circle for the earth, with the observer standing at the top. Now draw a much larger circle around that; this represents the sky. To place reference points on the circle, we start with the zenith. The zenith is the point directly above the observer's head. There is a corresponding point, called the nadir, on the opposite side of the circle from the zenith. The next important reference is the horizon. The horizon is a circle centered on the center of your outer circle, nintey degrees from the zenith. The next reference points we'd like to place are the North Celestial Pole (NCP) and the South Celestial Pole (SCP). If we're in the northern hemisphere, the NCP will be above the northernmost point on the horizon, and the SCP will be on the opposite side of the celestial sphere, below the southernmost point on the horizon. If we're in the southern hemisphere, the SCP will be above the southernmost point on the horizon, and the NCP will be opposite it, below the northernmost point on the horizon. The NCP and SCP are separated from the horizon by a number of degrees equal to your lattitude. For example, at 40 degrees north lattitude, the NCP will be 40 degrees above the northernmost point on the horizon.
Let's actually build a celestial sphere. First, consider the sky in
relation to the earth. Take the north and south poles and extend them
into the sky; these become the north and south celestial poles. The
earth's equator can be projected outward to form the celestial equator.
We'll get something that looks like this:
Now consider yourself as standing on top of the earth. Your zenith is
the point directly above your head; your horizon is a circle perpindicular
to the line between you and the zenith, and marks the boundary of the
hemisphere of the sky which you can actually see.
Now, to position the celestial poles with respect to your location
on the earth, you have to place them at an angle that will match your
lattitude.
When you're done, you should have a celestial sphere
very like the one at the top of this section.