Every one of us is an astronomer, in a sense. Every time you look up and see the stars, you're doing astronomy, just like humans have done for thousands of years. Let's give astronomy an official definition. Astronomy is the study of everything that lies beyond the Earth's atmosphere. That's a lot of stuff. The Celestial Sphere Here's a little something to help us put the heavens in perspective—the celestial sphere. When people first started looking at the sky, they imagined that all those little lights out there were hanging on the inside of a large bowl. Well, not so much a bowl, really, but a sphere that surrounded the Earth. We call that imaginary sphere the celestial sphere. All the stars are on the surface of the imaginary celestial sphere and the Earth is inside in the center. Now, we know that all the stars and planets and everything are different distances from us, but the idea of the celestial sphere is still convenient for pinpointing things in the sky. To use the celestial sphere, we'll pretend that everything in the sky is attached to the inside of this big imaginary sky ball that encloses the Earth, even though we know everything isn't attached to it. If we imagine that all the stars and stuff are attached to the celestial sphere, it's easier to visualize their position in the sky when we're looking up at it from the Earth. Let's run down the different parts of the celestial sphere. Oui? The celestial sphere has poles in the same places as the Earth. Imagine that we set up big searchlights on the north and south poles of the Earth. The north and south poles of the celestial sphere are where the searchlights hit the celestial sphere. In between the celestial poles, right on the surface of the celestial sphere, we draw an equator, just like the Earth's equator. Now, imagine that we draw a line from one pole of the celestial sphere to the other. It goes from the north celestial pole, through the middle of the Earth, and right down through to the south celestial pole. The line between the north and south poles of the Earth is the axis the Earth revolves around. When we look at the stars on the celestial sphere over a period of hours, they seem to revolve around this axis. Just a few more terms. You may have heard of these two before: zenith and nadir. The zenith is the point on the celestial sphere that is exactly above your head. The exact opposite point, the one underneath your feet all the way through the Earth and on the other side of the celestial sphere, is called the nadir. No matter where you are on the Earth, the point above you is the zenith, and the point below you is the nadir. Pretty cool! Okay, last term. Imagine that we draw a great circle on the celestial sphere from one pole to the other, through the zenith of our position. This line is called the meridian.