Saturday, March 10, 2007

Precession of the equinoxes


The Earth goes through one complete precession cycle in a period of approximately 25,800 years, during which the positions of stars as measured in the equatorial coordinate system will slowly change; the change is actually due to the change of the coordinates. Over this cycle the Earth's north axial pole moves from where it is now, within 1° of Polaris, in a circle around the ecliptic pole, with an angular radius of about 23.5 degrees (exactly 23 degrees 27 arcminutes [3]). The shift is 1 degree in 180 years, where the angle is taken from the observer, not from the center of the circle.

The precession of the equinoxes was discovered in antiquity by the Greek astronomer Hipparchus, and was later explained by Newtonian physics. The Earth has a nonspherical shape, being oblate spheroid, bulging outward at the equator. The gravitational tidal forces of the Moon and Sun apply torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. The portion of the precession due to the combined action of the Sun and the Moon is called lunisolar precession.

Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years, the so called great Platonic year, during which time the equinox regresses over a full 360°. Precessional movement also is the determining factor in the length of an Astrological Age. Which means you are not the astrological sign you were believed to be. I'm supposed to be a taurus but I'm actually an Aries.

Does the Precession of the equinoxes have a relation to the ice ages?



1 comment:

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