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2.5.2.3. Precession

The third orbital variation is that of precession. The Sun lies at one of the focal points of the Earth's orbital ellipse. Due to the gravitational interaction of other planetary bodies in the solar system, primarily the Moon and the planet Jupiter, the perihelion (the point at which the Earth passes closest to the Sun) moves in space with a consequent shifting or precessing of the elliptical orbit. This phenomenon is known as the precession of the equinoxes, and effects the intensity of the seasons.

Precession has two components: an axial precession, in which the torque of the other planets exerted on the Earth's equatorial bulge causes the rotational axis to gyrate like a spinning top; an elliptical precession, in which the elliptical orbit of the Earth itself rotates about one focus. The net effect describes the precession of the equinoxes with a period of 22,000 years. This term is modulated by eccentricity which splits the precession into periods, 19,000 and 23,000 years (Crowley & North, 1991).

Like obliquity, precession does not affect the total amount of solar energy received by the Earth, but only its hemispheric distribution over time. If the perihelion occurs in mid-June i.e. when the Northern Hemisphere is tilted toward the Sun, then the receipt of summer solar radiation in Northern Hemisphere will increase. Conversely, if the perihelion occurs in December, the Northern Hemisphere will receive more solar radiation in winter (see Figure 2.1). It should be clear that the direction of changes in solar radiation receipt at the Earth's surface is opposite in each hemisphere.