Precession

The third of the Milankovitch cycles is precession. Precession is the Earth's slow wobble as it spins on its axis. While the Earth rotates on its axis, it is not a perfect rotation. Like a spinning top winding down begins to wobble as it spins on its axis, the Earth does the same. The wobble slowly changes the direction toward where the Earth's axis is pointed. Today, for example, the Earth's north axis is pointed at Polaris, the North Star, but that has not always been the case, nor will it always remain the case. The precession of the Earth wobbles from pointing to Polaris to pointing at the star Vega. When this shift occurs, Vega is the Earth's new North Star. The Earth's wobble, or precession, has a periodicity of 23,000 years. This is important to the Earth's climate because unlike now where the Earth is closest to the Sun during the winter solstice, when Vega becomes the North Star, the Earth will be farthest away from the Sun during the winter solstice and will be closest to the Sun during the summer solstice. This will cause greater seasonal contrasts in the climate.

The Milankovitch cycles provide a theory of the potential influence of these factors on the occurrence of ice ages throughout time. Clima-tologists have found evidence of these orbital forcings in climatic data— the evidence of the 100,000-year eccentricity cycle being the strongest. Scientists believe they are significant because, although they do not affect the total solar energy received by the Earth, they do affect where and during which season the sunshine is received, which can ultimately affect climate. Much research is still needed in order to determine how big a role these orbital forcings have on global warming.

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