We have just entered into the zodiac constellation Aquarius. There's actually a little more to it than vague, fabricated fortunes from the daily paper; the zodiac is a good way to keep track of the sidereal year. Cider what, you ask? Simply put, the sidereal year is one complete orbit around the sun. The name comes from the latin word for star. Since the stars remain relatively fixed (compared to closer objects such as the moon and planets), they serve as a good metric to measure the year and determine dates. The zodiac are 12 constellations evenly spaced along the ecliptic--the plane formed by the Earth's orbit around the Sun. They can be used to gague our planet's position during its orbit. This might just sound like a year, why the fancy adjective? Because it turns out that there's more than one type of year.
Allow me to sidetrack by bringing up that there is more than one type of day. Ask your typical person on the street to define a day and tell you how long one is, you'll probably get an answer like "A day is he time it takes for the Earth to rotate once about its axis, and it lasts 24 hours." One rotation around its axis is a sidereal day since we can use the stars to track the rotation. However, the 24 hour day is the time the spinning Earth takes to line up the Sun to the same longitude. In other words, it takes the Earth just under 24 hours to spin one complete (sidereal) rotation. But during that interval, it has moved along ts orbital pass and so the Sun isn't lined up where it was at the start of the day. It must continue spinning a little longer to reach this point. Phil the Bad Astronomer has a great post explaining all this much better than I ever could hope to do.
Back to the year, besides the sidereal year, we also have a tropical year. This would be, for example, the time between winter solstices. This is determined by the tilt in the Earth. During the winter solstice (from the latin for "sun stop" or "sun pause" because it is the date when the sun's maximum position in the sky stops progressing and changes direction), the Earth's southern hemisphere is tilted towards the Sun while the northern is tilted away. The summer solstice is the opposite. The equinoxes occur when the tilt is perpedicular to the line with the sun, hence both hemispheres are equidistant from the sun. If the tilt of the earth were constant, then the sidereal and tropical years would perfectly jibe. But alas, it's not.
Which brings us to the precession of the equinoxes. (cool picture) Precession is a wobble. Think of a spinning top. Besides the primary spin, the top has a wobble which causes the axis to draw out a circle on the ceiling (ignoring proper movement of the top). The Earth is basically doing the same thing. It's tilt is slowly wobbling so that as time passes, the equinoxes and solstices fall at different times during the sidereal year. Therefore a time travelling astronomer should be able to tell you what century he's in by looking at the stars.
This brings me to the interesting case of The Farnese Atlas.
The Farnese Atlas is a 2nd-century Roman marble copy of a Hellenistic sculpture of Atlas kneeling with a globe weighing heavily on his shoulders. It is the oldest extant statue of the Titan of Greek mythology, who is represented in earlier vase-painting, and more importantly the oldest known representation of the celestial sphere. The sculpture is at the National Archaeological Museum (Museo Archeologico Nazionale ) in Naples, Italy. It stands seven feet (2.1 meters) tall, and the globe is 65 cm in diameter.
In 2005, at a meeting of the American Astronomical Society in San Diego, California, Dr. Bradley E. Schaefer, a professor of physics at Louisiana State University, presented a widely reported analysis concluding that the text of Hipparchus' long lost star catalog may have been the inspiration for the representation of the constellations on the globe, thereby reviving an earlier proposal by Georg Thiele. The constellations are fairly detailed and scientifically accurate given the period of its creation, implying that the globe was modeled after a scholarly work. The position of these constellations is consistent with where they would have appeared in the time of Hipparchus - leading to the conclusion that the statue is based on the star catalog.
In other words, Dr. Schaefer used his knowledge of precession to figure out where the constellations would've been at the time the atlas was made, and determine that it was a star map.
As the Earth precesses through the zodiac, we go through different ages which are determined by which constellation we're in around the begining of the year. When I first found out that we we entering the age of aquarius, I got so excited because I thought that I had finally figured out what that song from Hair meant. Boy was I disappointed when I went to listen to the song again and heard nothing but astrological hippie nonsense. I certainly sympathize with Julia Sweeney and her revelation about Deepak Chopra.