First, let’s explain the zodiac.
Imagine the sun as a basketball on a table. Imagine the earth as an apple with a pencil stuck through it. The earth is spinning like a top on the point of the pencil (its axis) while at the same time slowly circling the sun.* Thing is, the top isn’t straight up and down. It’s tilted, at an angle of 23.5 degree from the vertical, and it stays tilted at that same angle as it orbits the sun. Sometimes the bottom half is closer and gets baked a little more (summer in Australia) and sometime the top half is closer (summer in Canada.)
Now imagine you’re an ant on the surface of the apple, and imagine we could temporarily turn the sun’s light off. What you’ll see are the walls of the room. In space, there are no walls, but there are stars, which are far enough away that they can be considered stationary relative to the earth. However, as the Earth spins, the stars will appear to rotate around the point that lines up with the Earth’s axis, which you can now visualize as a laser beam shooting out of the top of the pencil. It so happens that there’s a star there; Polaris, the North Star. From the point of view of the ant, all the stars are rotating around Polaris, which stays in the same place.
The apparent rotation is slow, but fast enough to notice if you stay up all night looking at the stars. If you measure how much time it takes for a star to “move” from any apparent location all the way around the circle and get back to where it started, you’ll find that it’s about 23 hours, 56 minutes, and 4 seconds. This period of time is called sidereal day, from a Latin word meaning “star.”
Now let’s turn the sun back on. The 24 hour day we’re used to is called a solar day, and it’s the time it takes the sun to accomplish the same feat as the stars, that is, to appear to get back to where it appears to have started. The reason why it’s longer than the sidereal day is because the sun is much closer than the stars, so each day we’re in a slightly different position relative to it. This nifty diagram from Wikipedia gives a good illustration:
Incidentally, this whole optical illusion of the nearby sun appearing to be in a different position relative to the far-away stars is called parallax. It’s the same illusion you use when you pretend to squish the heads of distant people with your fingers.
Anyway, the very slight mismatch between the sidereal day and a solar day means that the sun isn’t quite in sync with its celestial backdrop. Over the course of the year, it appears to move in a slow circle in relation to the stars. Astronomers call this circle an ecliptic. Several thousand years ago, the Babylonians divided up the ecliptic into twelve pieces, each named after the closest constellation. Whichever of these twelve pieces the sun was “in” when you were born is your star sign. Note that you can’t actually see your constellation during the month you were born; since it’s “behind” the sun, it’s only out during the day, when we can’t see the stars.
Now for the precession part.
To go back to the top analogy; as the top slows down, it starts to wobble; the axis doesn’t always point in the same direction, like this gyroscope. This is called precession.
The wobble of the earth is caused by gravity. This is a bit complicated, but basically the earth’s spin means it isn’t perfectly spherical; it bulges at the equator. The gravity of the sun and moon pull on this bulge, causing the axis to precess in a slow circle, completing one revolution every 26,000 years.
This has all kinds of implications. For one, it changes the North Star. Polaris isn’t exactly lined up with the axis, it’s just the star that happens to be the closest right now. In the year 3000, Gamma Cephei will be closer, and 1200 years after that, the closest will be Iota Cephei. It’s equally true of the past as well; for the ancient Egyptians, the best North Star was Vega, and 12,000 years from now, it will be the closest again.
Precession also means that the ecliptic (which you will remember is the path that the sun appears to trace across the sky) has also moved since the ancient Babylonians invented the zodiac. However, because Western or “tropical” astrologers have been using the solar calendar instead of the sidereal one, they haven’t changed the dates to keep up. Thus, when I was born on October 21, 19 something-or-other, I was declared a Libra, even though the sun was actually “in” Virgo at the time. (Incidentally, “sidereal” astrologers in India and Japan do pay attention to precession and have adjusted their horoscopes accordingly)
Of course, “in” is a relative term; because of the non-uniform shapes of the constellations, the sun spends much more time in some than others, so it’s a bit arbitrary to decide that it moves from one to another 12 times a year. Alternate systems have been proposed throughout history, but the idea that we should add Ophiuchus as a 13th seems to originate with a guy called Stephen Schmidt, who wrote a book called Astrology 14 in 1970 (he also advocated adding Cetus, the whale). The idea proved quite popular among sidereal astrologers, particularly in Japan. The thing is, there is no international body governing astrologers in the same way that the International Astronomical Union does for astronomers. Even if there was, I have a funny feeling that all kinds of variations on the same general theme will be popping up as long as people continue to put their faith (and money) in astrologers.