The Earth's Wobbles

It was suggested in 1842 that astronomical cycles would change the amount of sunlight that goes to the far north, and to the far south.

In the 1920's and 1930's, astronomer Milutin Milankovitch calculated three cycles which would have the strongest effect on sunlight. (Specifically, he calculated the effect on insolation.)

He claimed that the most important effect was the tilt of the earth's spin axis, relative to the plane of our orbit around the sun. The tilt is now 23.5 degrees from vertical, but it fluctuates between 21.5 and 24.5 degrees, with a cycle time of 41,000 years. A larger tilt should make summers hotter, and winters colder.

The next most important cycle is the shape of the earth's orbit. Over a period of 100,000 years, it becomes more elliptical, and then more circular again. An elliptical orbit makes one hemisphere have more intense seasons, and the other hemisphere have more moderate seasons.

The third cycle is a wobble of the earth's spin axis. Every 23,000 years, our axis really does point at Polaris, the North Star. Sometimes this cycle intensifies the effects of the others, and sometimes this cycle weakens the effects of the others.

Notice that none of the effects ever changes abruptly. From one century to another, everything is pretty well the same.

If you are interested in learning more, Dr. Hyde recommended "Quaternary Paleoclimatology" by R. S. Bradley.

Last modified: 16 May 1997

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