The precession of the Earth its causes, and its ramifications
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 precession of the Earth refers to the slow, cyclic movement of the Earth's axis of rotation over a period of approximately 26,000 years. This means that as the Earth spins on its axis, the orientation of that axis changes over time.
The primary causes of Earth's precession are the gravitational interactions between the Earth, Moon, and Sun. There are two main components that contribute to this phenomenon:
1. Axial Precession: This is the main component and is caused by the gravitational pull of the Sun and Moon on the Earth's equatorial bulge. The gravitational force exerted by these celestial bodies creates a torque that gradually alters the orientation of Earth's rotational axis. It takes approximately 26,000 years for the axis to complete one full precessional cycle.
2. Precession of the Earth's Orbit: In addition to axial precession, the gravitational influence of other planets in the Solar System causes the Earth's orbit around the Sun to slowly change shape. This phenomenon is known as the precession of the Earth's orbit or apsidal precession. It has a much longer cycle of approximately 112,000 years.
The ramifications of the Earth's precession are primarily observed in changes to the positions of celestial objects over long periods of time. Here are a few important implications:
1. Shift in the North Star: As the Earth's axis precesses, the North Star, or Polaris, changes over time. Currently, Polaris is our North Star, but in approximately 13,000 years, the star Vega will take its place.
2. Changes in Constellations: Due to precession, the positions of stars in constellations can shift slowly over thousands of years. This means that the arrangement of constellations visible from Earth today will be different in the future.
3. Climate Changes: Over very long periods of time, the Earth's precession can influence climate by altering the distribution of sunlight received at different latitudes. This can affect the timing and intensity of seasons, resulting in long-term climate variations.
To understand and observe the effects of Earth's precession, scientists use astronomical measurements and mathematical models to calculate its magnitude and predict future changes. Various techniques are employed, including precise measurements of star positions, satellite data, and simulations based on the principles of celestial mechanics.