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The Question

(Submitted May 18, 1997)

Why do stars seem to travel across the sky at night differently than the moon moves? Do stars rise and set like the Sun and Moon? The stars seem to stay in the same place while the earth moves in circles, is this how it works? I appreciate any explanations about the change in star positions.

The Answer

When we look at the night sky we see stars in different locations depending on the time of night. We also see different groups of stars depending on the time of year. The reason we see stars move during the night is because the Earth spins. Because the Earth spins, we see the stars rise and set, just as the Sun and Moon do. In addition to rising and setting, however, the Sun and the Moon have an additional motion across the sky.

Try this experiment. Go out one night when the Moon is visible and try to find some stars that appear close to the Moon. You might want to draw a picture showing the Moon and the location of these stars. Try to go out the next night at the same time and compare your drawing to what you see. The stars you drew should be in about the same spot as the night before but the Moon will have moved. What happened? The stars are in the same spot because the Earth spun around once. The reason why the Moon isn't quite where you saw it the night before is because the Moon is orbiting the Earth. It takes about 27 days for the Moon to orbit the Earth one time. What happens when you go looking for the same stars a month later at the same time? You will probably notice they are in a slightly different part of the sky then they were before. Why? This is due to the Earth's motion around the Sun. The Earth orbits the Sun in one year and in one year you will see many different groups of stars in the sky. These groups are the constellations. Because the Earth orbits the Sun you will see different constellations, for example, at night in the winter months than in the summer.

So, the motion of the stars we see over one night is due to the Earth spinning. The motion you might see over several weeks or months is due to the Earth orbiting the Sun.

Leonard Garcia and Jim Lochner
for Ask an Astrophysicist

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