Solar System: Sky Coordinates

Have you ever looked up at the sky from a dark place? You see lots of stars, as shown in the picture below. You can almost imagine that the stars are painted on the inside of a gigantic black sphere. In reality, this sphere doesn’t exist; stars are all at vast distances away from Earth and from one another. Even though it’s not real, the idea of the Celestial Sphere is useful, because it helps us point out specific places in the sky.

Part of the sky seen through the Sloan Digital Sky Survey telescope

The celestial sphere gives astronomers a coordinate system for the sky, based on the system that we use here on Earth. The North Celestial Pole is directly above the Earth’s North Pole, close to the North Star. There is also a Celestial Equator, which is directly above the Earth’s Equator. Finally, just as we can imagine that lines of Longitude and Latitude are painted on the Earth, we can also picture coordinates painted on the inside of the imaginary Celestial Sphere.

Both the stars and the coordinate lines on the imaginary Celestial Sphere appear to rotate around the Earth, even though it’s really the Earth that’s rotating. But because both the stars AND the celestial sphere seem to rotate, the same star will appear at the same place every night.

So if you asked an astronomer where she was pointing her telescope to see a specific star, she would answer with the coordinates of that star on the Celestial Sphere.