About 2000 years ago the Greek astronomer Hipparchus discovered that the position of the Sun at any season, measured against the background stars, migrates in a slow cycle of about 26000 years. This precession of the equinoxes also shifts the position of the celestial pole (so that our pole star would not have been a good guide for the ancient Greeks) and is caused by the rotation axis of the Earth slowly moving around a cone. Precession may be one of several astronomical processes contributing to the ice ages, as proposed by the Serbian astronomer Milutin Milankovich. The file Lprecess.htm has a related lesson plan.


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