The precession of the Earth its causes, and its ramifications
2 answers
http://en.wikipedia.org/wiki/Axial_precession_%28astronomy%29
Bob's reference tells you tha cause of precession. It is a consequence of the Earth not being a perfect sphere. The sun and moon are able to exert a torque on the equatorial bulge, and this causes the spin axis to slowly move relative to the stars, because of angular momentum laws.
The ramifications include:
Polaris will not be the north star forever. It is already drifting away from its closest alignment with the Earth' axis direction.
Most star coordinates in an atlas change by about one degree per century. Every couple of decades, new atlases are issued with more coordinates.
The Gregorian calendar year that we use is NOT the time it takes the Earth to complete orbit around the sun. If it were, the dates of the equinoxes and solstices would slowly change. In about 10,000 years, summer and winter months would switch. This does not happen because the leap years are chosen (or omitted) so that the average year takes the precession of the equinoxes into account. This is called picking the synodic year instead of the sidereal year.
The ramifications include:
Polaris will not be the north star forever. It is already drifting away from its closest alignment with the Earth' axis direction.
Most star coordinates in an atlas change by about one degree per century. Every couple of decades, new atlases are issued with more coordinates.
The Gregorian calendar year that we use is NOT the time it takes the Earth to complete orbit around the sun. If it were, the dates of the equinoxes and solstices would slowly change. In about 10,000 years, summer and winter months would switch. This does not happen because the leap years are chosen (or omitted) so that the average year takes the precession of the equinoxes into account. This is called picking the synodic year instead of the sidereal year.
1 answer