[troutman]

http://skepdic.com/astrology.html

In fact, there should be 13 signs, not 12.

Precession of the equinox is caused by the fact that the axis of the Earth's rotation (which causes day and night) and the axis of the Earth's revolution around the Sun (which marks the passage of each year) are not parallel. They are 23 1/2 degrees away from lining up; that is, the Earth's axis of rotation is tilted. This tilt also causes our seasons, a fact that Ptolemy did understand but that many people do not understand even today. Ptolemy understood that the rotation axis of the Earth was slowly precessing, or moving in a circle, with an angular radius of 23 1/2 degrees with a period of around 26,000 years. He deduced this from comparisons of data taken by the ancient Sumerians 2,000 years before his time. He did not understand what was pushing the precession, but he did understand the motion. We now realize that the Sun is rotating with a period of around 30 days and that this causes the Sun to bulge at the equator, which causes a torque to be exerted on the top like motion of the Earth's day and night cycle. There is also a small 18.6-year variation caused by the Moon's orbit around the Earth, and the Moon also has a small effect on precession; however, the Sun's equatorial bulge is the main cause of the precession of the equinox, which is why your sign listed in the newspaper, by Sidney Omar for instance, in most cases is removed by one sign from the modern, actual position of the Sun at your birth.

The modern signs as listed here are further complicated when their boundaries are those of the current constellations. A neater way of dividing the signs would be to divide the ecliptic into 30-degree slices, as Ptolemy did, but to keep the slices centered on the star patterns. This would make the time interval for the signs more nearly 30 days each and eliminate the [13th] sign of Ophiuchus [off ee oo' kus], but your modern sign would still differ by one sign from the tradition designations.*