The apparent path of the Sun around the Earth (we all know the Earth really goes round the Sun) is called the Ecliptic – because eclipses take place along this line – and the planets (with the exception of Pluto) are usually found within plus or minus 8° of it. This band around the ecliptic is called the Zodiac.
So far so simple, but there are many zodiacs, how do we know where each starts? Now we come to the ancient question “Where is the beginning of a circle?” Where do we put the start point?
Western Astrology uses the tropical zodiac which is defined by the interaction of the Sun and Earth. There are no stars or constellation involved in this zodiac. It starts at the vernal point, that is the crossing point of the Celestial Equator and the Ecliptic when the Sun is travelling north. The Celestial Equator is the Earth’s equator projected into space. The Sun on its journey “around the ecliptic” reaches these crossing points at the spring and autumn equinoxes. The spring equinox is also the first point of Aries and it is this point which is chosen to be the starting point of the Tropical Zodiac. The signs are divisions of 30 degrees from this point.
The Sidereal Zodiac is based upon the stars. The exact start point is an area of discussion and so there are several calculations which mark the start of the sidereal zodiac. The signs are divisions of 30 degrees from the selected point even though the actual constellations are not all the same length.
The other common zodiac is the Draconic Zodiac which uses the North Node of the Moon as its starting point. Again the signs are divisions of 30 degrees from this point. Pam Crane is one of the foremost proponents of draconic astrology.
These three zodiacs are very rarely in line with each other. When they do line up, some would say they herald significant times. The last time the sidereal and tropical zodiacs lined up was around the time of the birth of Christ.