Two thousand years ago, Greek astronomer Hipparchus noticed something very odd about the constellations in the night sky: they moved! Hipparchus took careful notes of the locations of the stars Spica and Regulus and compared them to his predecessors’ notes. He found the stars shifted by about 2 degrees from where they used to be. Cleverly, he deduced that Earth’s axis was undergoing precession and calculated the correct rate within 30% of its actual value. See the animation below for an example of precession.
Precession is one of three major cycles that the Earth experiences. It is also the fastest, with Earth completing a full precessional cycle every 20,000 years. The other two took much longer to be discovered since they are slower and a lot harder to observe in the nigh sky. Obliquity, or axial tilt, cycles with a period of 40,000 years. The shape of Earth’s orbit also cycles between being more circular and more elliptical with a period of 100,000 years. If you’re having trouble visualizing these cycles (they are tricky!), I highly recommend this short video which shows all three of them in action.
Each of these cycles change how Earth receives heat from the Sun. Precession controls how extreme the seasons…
