Precession of the earth

The earth's 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 are equal.

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.