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The celestial sphere has been described, up until now, as a giant invisible sphere surrounding the Earth with no markings other than the stars themselves. But just as man has drawn imaginary lines on the Earth to describe position, he also has drawn similar imaginary lines in the sky. The lines on the celestial sphere are a projection of the latitude/longitude grid of the Earth. Look at a globe of the Earth. On the top and bottom will be the North and South Poles. There will be lines of longitude running from pole to pole, while perpendicular to the north/south longitude lines will be the east/west latitude lines, each one parallel to the other. Now imagine this grid of lines growing larger and larger, until instead of looking at it from the outside inwards, you are inside it looking outwards. If you let the grid grow until it surrounds not only you the but entire Earth, it has become the grid of the celestial sphere. Remember that you can see only half of the grid at any one time from the Earth’s surface because the rest is below the horizon.
The celestial sphere is oriented exactly like the Earth. A line draw along the Earth’s axis out of the North Pole will point to the North Celestial Pole. If stretched out in the other direction towards the South Pole, the line will point to the South Celestial Pole. Midway between the celestial poles is located the celestial equator. At the North Celestial Pole is located a star called the Pole Star or Polaris. Polaris is not particularly bright, but being located in an area of the sky with only very dim stars, it is quite distinctive. You may ask what makes that particular spot in the sky so special that we give it the honor of being “on top”. Other than the fact that the Earth’s axis points to the spot, there is nothing unique about the North Celestial Pole. The lines of the celestial sphere have been drawn for the comfort of those of us on Earth who wishes to locate objects in the sky. Since space does not have a center, it does not have a top or bottom either.