According to the boundaries for the constellations adopted by the International Astronomical Union in 1921, there are 13 signs of the zodiac instead of the usual 12; time from Ophiuchus the snake handler is usually included in time given to Scorpius. If you already know your astrological sign from reading an astrology column in a newspaper or from consultation with an astrologer, you may be surprised that few of these dates correspond to the dates listed in the newspaper's astrological column. Why? Generally, listed astrological signs and the dates given by most astrologers assume that you were born around 140 AD. Not many people I know, personally, are that ancient. The common, but now incorrect, listing is:
 

Claudius Ptolemy, an astronomer and astrologer-in his day there was no distinction between astronomy and astrology-around 140 AD compiled the Tetrabiblos, which even today remains the most important work on astrology to most astrologers, in the west. In the time of Ptolemy the study of the stars was already very ancient, and at least 40 of the currently generally accepted 88 constellations were already named. In fact, Ptolemy even knew about the precession of the equinoxes, since the vernal equinox (the first day of spring) in his time occurred at the beginning of Aries and not in the middle of the constellation Taurus, as it had many millennia before his time. Today, as was mentioned above, the first day of spring occurs when the sun is in Pisces, and the relative position of the sun on this date is slowly processing, or moving, into Aquarius. Perhaps we are approaching the age of Aquarius, whatever that may mean. In fact, Ptolemy no doubt could have calculated all of this information from the data given in the Tetrabiblos and the Almagest.  I doubt that he thought that astrologers would be using his 140 AD table almost 2,000 years later without processing (adjusting) them.

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 l/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 sign of Ophiuchus, but your modern sign would still differ by one sign from the tradition designations.

Notice that Ptolemy's signs are not quite equal in time, since the earth does not travel at a completely uniform speed around the sun. This observational fact was known, too, by Ptolemy, though of course in his model the sun went around the earth. Observationally it does not make any difference which body moves, as long as you are describing relative motion. Of course, good Newtonians know when to use an inertial (nonaccelerating) reference frame; the earth and the sun go around their common center of mass, which is almost at the center of the sun because the sun is so heavy. (Good general relativists know that we can also use noninertial reference frames. From one relative point of view, the sun does rise and set, and the sun does go around the earth, and I am the center of the universe; this is one way of looking at the expansion motion resulting from the big bang. Of course, I have to be democratic enough to let anyone else be the center of the universe, too, from their own point of view.)

So, what difference does it make if your sign is different in the twenty century, and soon to be twenty-first century on January 1, 2001, than it would have been in 140 AD? It doesn't matter, unless you believe in astrology, a religious system founded in Sumerian, Babylonian, Greek, and Egyptian culture and clearly described in Ptolemy's Tetrabiblos. Some people find comfort in believing that there is some ancient knowledge or wisdom, not completely understood by modern people, that they can tap into to gain some advantage in their daily life. I personally think that what you think you know, but is not so, is even more potentially harmful than what you don't know. I agree with Geoffery Chaucer, who wrote in 1387 in the Treatise of the Astrolabe "Horoscopes are observances of judicial matters of the rites of pagans, in which my, spirit has no faith." Of course, I am a Christian along with Chaucer, so I openly declare my religious preferences and possible prejudice.

As Chaucer said, to slay envy, I too, am but a compiler. I do not wish to ignite religious warfare over calendar differences, as has happened several times in history. I am not as cynical as Johanas Kepler who said that God gave astrology to astronomers for their meat and drink, just as he gave lambs to the wolves for their substance. I don't cast horoscopes, since I don't believe in their validity.  But I will help you find your true astrological sign that you can make of as you wish.

If you would like to learn more about things astronomical and historical, but not astrological, Montgomery College at Takoma Park offers many opportunities. Montgomery College's planetarium offers free public lectures and planetarium shows monthly September through May; free grade-specific field trips for school groups; free special shows for youth, senior citizen, or other groups; and introductory astronomy courses taught from within a planetarium, the only place in the Washington Metro area where you can take an astronomy course from within a planetarium and get college credit. Of course, we do charge tuition for the astronomy courses, but our tuition is very reasonable, especially when compared to other institutions, and our class size is not so large that you need binoculars to see the instructor from the back of the classroom.