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Precession of the Equinoxes - Crystalinks (1737 words) |
 | Precession of the equinoxes refers to the precession of Earth's axis of rotation in inertial space.It was discovered by Hipparchus that the positions of the equinoxes regress (move westward) along the ecliptic compared to the fixed stars on the celestial sphere. |
| The precession of the equinoxes can cause periodic climate change (see Milankovitch cycles), because the hemisphere that experiences summer at perihelion and winter at aphelion (as the southern hemisphere does presently) is in principle prone to more severe seasons than the opposite hemisphere. |
| Precession causes the cycle of seasons (tropical year) to be about 20.4 minutes less than the period for the earth to return to the same position with respect to the stars as one year previously (sidereal year). |
| Archaeogeodesy, a Key to Prehistory III, by James Q. Jacobs (1665 words) |
| Precession is the motion of the earth's axis of rotation gradually changing direction in reference to fixed celestial reference. |
| Because the precession motion is retrograde, the axis realigns inclined towards the sun slightly before completion of an orbit, and therefore one year is shorter than one orbit by the inverse of the precession cycle, or about 1/25,780 (1/PR) in 2000. |
| Lunar eclipses are a unique opportunity for every longitude to observe a celestial event the timings of which can be compared to locally-referenced time. |
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