The twelve constellations through which the ecliptic passes form the Zodiac. The name is derived from the Greek term zodiakos kyklos meaning circle of animals, and comes from the fact that most of these constellations are named for animals, such as Leo the lion, Taurus the bull, and Cancer the crab. These names, readily identifiable on sky charts, are very familiar to the millions of people who read the daily horoscope in their newspaper, although many of them would probably be hard pressed to find them in the sky.

If we could see the stars in the daytime, we would see the sun slowly wander from one constellation of the Zodiac to the next, making one complete circle around the sky each year. Ancient astrologers were able to figure out where the sun was in the Zodiac by noting the last constellation of the Zodiac to rise ahead of the sun, or the first to set after it. Obviously, the sun had to be somewhere in between. In this way, for each month a specific constellation was conferred the title House of the Sun, and in this manner each month-long period of the year was given its own sign in the Zodiac.

Interestingly, the sign assigned to a given month in our modern horoscopes is not where the sun actually is in that particular month, but where it would have been thousands of years ago! This loss of synchronization is due to the wobble in the Earth's axis known as precession, which slowly changes the location in the sky to which the Earth's north pole points and so also changes the relative positions of all the stars. In spite of this, today's astrologers—who believe that the sun, moon, and planets mysteriously direct our lives—continue to adhere to star positions that for all intents and purposes are out of date by thousands of years.

The ecliptic also passes through a thirteenth constellation, Ophiuchus, which is not included among the signs of the Zodiac. In fact, the sun spends more time traversing Ophiuchus than nearby Scorpius. During 2003, for example, the sun technically resided in Scorpius for less than a week: from November 23 through 29. It then moved into Ophiuchus on November 30 and remained within its boundaries for more than two weeks, until December 17. Yet the venerable Serpent Bearer is not considered a member of the Zodiac and so must defer to Scorpius.

Also, since astrology interprets the positions of the moon and planets in relation to the constellations of the Zodiac, it's important to know that the orientation of their orbits relative to the ecliptic allows them to sometimes appear within the boundaries of constellations entirely outside the Zodiac. For instance, from May 15 through June 5 of 2003, the planet Saturn was well within the boundaries of Orion, passing across the Hunter's club. Other constellations that can be visited by the moon and planets include Auriga the Charioteer, Cetus the Whale, and Sextans the Sextant.