The zodiac is composed of 12 constellations that fall along the ecliptic with the Moon and planets. The ecliptic is the path of the Sun over the course of one year. These 12 constellations form a circle and are divided into separate zones of celestial longitude. Thus they are a coordinate system when you put the Sun at their center as the longitude and the ecliptic acting as the latitude.
The zodiac was discovered hundreds of years ago by the Babylonians possibly in the 7th century BC. They divided it into 12 zones and created the coordination system around the constellations. Additionally, the Babylonian calendar was based on the constellations, linking each month with one of the twelve constellations. The twelve constellations in order are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces.
Many cultures from around the world – including India, Egypt, Greece, Babylon, and the Chinese – have used the zodiac for astronomy. The Bible also contains references to the zodiac. The 12 signs of the zodiac correspond to the 12 tribes of Israel. However, in Hebrew tradition, the Scorpius is replaced by the symbol of the Eagle.
The constellations make it easier to keep track of the Sun’s path as well as the moons and planets location along the ecliptic. Although modern astronomy still uses the tropical coordinates, the longitude is measured from 0° to 360°. In the old system, it was measured from 0° to 30° within individual constellations. Another important thing to realize is that the constellations themselves do not fall within these 30° units. When the Babylonians first developed the zodiac, they did not realize that the Earth actually shifts its orientation throughout the years, which is called the precession of the equinoxes. Thus, modern astronomers were faced with the choice of making the zodiac coordinate system sidereal or tropical.
If the system was sidereal, the constellations would be fixed in the sky in relation to other stars. In the tropical system, the constellations would use the vernal equinox, which is a cardinal point for the Sun on the ecliptic, as the point of reference. Because Western astronomy uses the tropical system, the zodiac actually moves apart gradually by about 1.4° every century. Even though Aries is considered to be the vernal equinox listed at 0° longitude, it has drifted throughout the centuries and currently it is actually in Pisces’ territory.
Astronomy Cast has an episode on the constellations.