Now that you know how to find latitude and longitude on Earth, you can use the same technique to talk about the positions of stars. First, you'll have to imagine a huge hollow sphere that surrounds Earth. Think of all the stars as dots painted on the sphere. Of course, we know this analogy isn't really correct—some stars are much further away than others—but the visual image presents a handy way of thinking about the sky.

The imaginary hollow sphere is called the celestial sphere. You can imagine that the celestial sphere rotates around Earth once every 24 hours. We know that it is actually Earth that rotates on its axis once every 24 hours, making the stars (as well as the Sun and Moon) appear to rise and set, but it's easier if we just imagine the celestial sphere as the object undergoing rotation. Since Earth actually rotates from west to east, we perceive the celestial sphere as rotating from east to west above us.

The Celestial Poles

Now that we've imagined the celestial sphere, we need some reference points in order to define a celestial coordinate system. First, the stars on the celestial sphere seem to rotate around the north and south celestial poles. These are the points on the celestial sphere that are directly above Earth's North Pole and South Pole.

If you drew a line from the North Pole extending down through the center of Earth and then out the South Pole, then continued that line up and down in both directions until it intersected the celestial sphere, it would hit the celestial sphere at the north celestial pole and the south celestial pole.

FIGURE 5-3:The celestial sphere

Of course, the north and south celestial poles are just imaginary points in the sky, but luckily for those of us in the Northern Hemisphere, there's a fairly bright star that's very close to the north celestial pole! This star is Polaris, also sometimes called the North Star. If you imagined that you were looking down on Earth's North Pole, Earth would seem to rotate around it.

Similarly, the stars on the celestial sphere seem to rotate around Polaris. If you take a long-duration exposure of the sky with a stationary camera pointed at Polaris, the stars around Polaris will all make circular streaks but Polaris will stay in the same place.

Another interesting fact about Polaris is that its height in degrees above the horizon is the same as your latitude! If you were in Boston, Polaris would be about 42 degrees above the horizon. If you were at the equator, Polaris would be right on the horizon, and if you were at the North Pole, Polaris would be at the top of the sky. Early explorers, especially those on boats who needed to keep track of their position, used this information. Interestingly, Polaris wasn't always the North Star. Earth's axis has wobbled and dipped with time, and this variation means that the north celestial pole has appeared to change places on the sky.

FIGURE 5-4:Star trails around Polaris

Celestial Equator

Another important imaginary line on the sky is the celestial equator. This line is actually an imaginary circle on the celestial sphere, directly above Earth's equator. It is always exactly 90 degrees from the celestial poles, just like Earth's equator is 90 degrees from its poles. As the celestial sphere appears to rotate above us, all the stars rotate on paths that are parallel to the celestial equator. At the North Pole, where Polaris and the north celestial pole are exactly overhead, the celestial equator is on the horizon.