"The Geodetic Effect" If you ever had a spinning top, you know that its axis tends to stay lined up in the same direction--usually, vertically, though in space any direction qualifies. Give it a nudge, however, and the axis will start to gyrate wildly around the vertical, its motion tracing a cone (drawing). The spinning Earth moves like that, too, though the time scale is much slower--each spin lasts a year, and each gyration around the cone takes 26 000 years. The axis of the cone is perpendicular to the plane of the ecliptic. The cause of the
precession is the equatorial bulge of the Earth, caused by the
centrifugal force of the Earth's rotation (the centrifugal force
is discussed in a later section). That rotation changes the Earth
from a perfect sphere to a slightly flattened one, thicker across
the equator. The attraction of the Moon and Sun on the bulge
is then the "nudge" which makes the Earth precess.
Through each 26 000-year cycle, the direction in the sky to
which the axis points goes around a big circle, the radius of
which covers an angle of about 23.50. The pole star to which
the axis points now (within about one degree) used to be distant
from the pole, and will be so again in a few thousand years (for
your information, the closest approach is in 2017). Indeed, the
"pole star" used by ancient Greek sailors was a different
one, not nearly as close to the pole. Much of this explanation was drawn from "Precession," a web site at the Goddard Space Flight Center. |