Precession of the earth
axis is tipped over through 23 degrees relative to the
plane of the earth's orbit round the sun (called the
ecliptic), and the orientation of the axis
relative to the stars remains
virtually constant (by conservation of angular
momentum) as the earth goes round the sun. The
equinoxes (roughly March 21 and September 21) are the
two times in the year when the earth is `sideways on'
to the sun, so that day length and night length
The sun and the moon exert torques on the bulge, so the
angular momentum changes. As the earth's axis slowly precesses, the
time in the orbit at which the equinox occurs also
moves slowly round the sun. Hence the precession of the
earth's axis is called the
precession of the equinoxes.
The zodiacal signs correspond to 12
constellations, equally spaced along the ecliptic. The sun
does the rounds of the constellations once per year.
When the constellations were named and
identified with times of year, Aries was the constellation aligned
with the spring equinox (vernal equinox).
Since that time (3000 years ago?), the
equinoxes have precessed through a substantial angle,
so now the spring equinox occurs when the sun is
aligned with a different constellation -- not
Aries, but Pisces.
However, birth signs are still allocated using
the mapping of dates to constellations that applied
3000 years ago. Since the equator is perpendicular to
the earth's axis, another way of saying where the
equinoxes are, is that the equinoxes are the intersections of
the equator and the ecliptic.
The fact that the earth precesses was known to the ancient Greeks
(get name and date), who had
sufficiently accurate historical data on the timing of the equinoxes
that they could detect the one degree per 72 years precession.