Ever wonder why astronomy books refer to the precession cycle as being 26,000 years long while discussions of the Milankovitch cycles in the Earth’s orbit, which almost certainly affect the climate of our planet, invariably have a shorter precession cycle, averaging around 23,000 years? The problem is that they are using the same word for two slightly different things.

Milankovitch precession. Note that positive values correspond to minimum seasonal contrast in the Northern hemisphere.

Astronomical precession refers to the gradual change in the direction the earth’s axis is pointing relative to the fixed stars. This is a very slow change relative to a human lifetime, but obvious enough when records are kept that the ancient Greeks were aware of it. We know now that this precession is due to the pull of the sun and the moon on the equatorial bulge of the Earth. They can’t actually pull the rotating earth straight, any more than leaning a little sideways on a rapidly moving bicycle will make it topple sideways. What it will do is make the bicycle turn. The corresponding effect on the earth is that the direction the axis points describes a very slow circle in the sky. Right now the earth’s axis points at Polaris, the North Star, but this has not always been true. In about 12,000 years the pole star will be Vega, which is a much brighter star.

The stars other than the sun, however, have little to do with our climate and weather. For that, the important factor is not the direction that the earth’s axis is pointing in the heavens, but how that direction interacts with the earth’s distance from the sun. The earth’s orbit is not a circle, but an ellipse. Currently perihelion (which just means closest to the sun) is around January 4. Half the year later is aphelion (farthest from the sun.) If the earth’s axis is pointing most nearly toward the sun (summer solstice) at perihelion, the summer will be hot but short and the winter will be long and cold. This is currently true of the South pole. The North pole, however, is pointing almost away from the sun at perihelion, so the northern hemisphere has relatively cool, long summers. The effect is slight, especially since the eccentricity (a measure of the difference in distance from the earth to the sun between perihelion and aphelion) is currently very small.

The difference between astronomical precession and the interaction above is due to the fact that the earth’s elliptical orbit itself rotates in space due to the pull of the other planets, especially Jupiter and Saturn. This rotation is not as uniform as astronomical precession, hence the combined precession (which is what is used in the Milankovitch cycles) does not always have the same cycle length. On average it is around 23,000 years, but this varies. The strength of the effect also varies, due to the change over time in eccentricity.

The two figures (if I can figure out how to get them from Excel into the blog) show the precession effect and the eccentricity for the last 200,000 years. Note that the precession shown here is positive when the northern hemisphere summers are cool and long, and negative when the northern hemisphere summers are hot and short. Also note that the precession effect is out of phase in the two hemispheres. The different distribution of land and ocean in the two hemispheres is generally though to be the reason that the seasonality is most important in the northern hemisphere.

If I can figure out how to do a better job with Excel charts, I’ll do it.

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