Precession of the Earth's Rotation Axis
- The Earth's rotation axis is tilted 23.5 degrees relative to its orbit around the Sun.
- In the absence of any outside force the Earth's rotation
axis would always point in the same direction.
- Both the Sun and the Moon exert gravitational forces on the Earth which
cause the rotation axis to ``precess'' like a gyroscope.
- The pole remains tilted 23.5 degrees but points in different directions at different times
- It gradually traces
out a circle on the celestial sphere.
- It takes 26,000 years for the pole to make a full circuit
around the sky.
- It is only a fortunate circumstance that a bright star lies
above the Earth's north pole today.
- In a few thousand years the pole position will be far
from the North Star.
- Since the poles, which are the points around which all of the
stars appear to trace out circular paths as the Earth turns, can
shift by 47 degrees...
- stars that are now "circumpolar" may not be thousands
of years from now.
- stars not now visible because they always lie below the
horizon -- for example the Southern Cross -- will appear in
our skies in the distant future.
Updated September 18, 2002