Ancient astrologers thought of the sky which surrounds us as a giant
sphere, of which the earth is the center. Only the movements of the
visible stars count: the sun, moon, and the eight known plantets rotate
around at greater or lesser distances and at varying speeds. The sun,
moon, Mercury, Venus and Mars, relatively close to us, move rapidly;
Jupiter, Saturn, Uranus, Neptune, and Pluto rotate slowly as they
are further away.
Like every circle, the ring which circumscribes the zodiac is divisible
into 360 degrees; these are divided into twleve equal sections of
30 degrees each; the signs of the zodiac.
Astrology includes all of the stars of the solar system, including the planets and even the sun and the moon, even though they are less well known, their symbolism is as detailed as that of the signs of the zodiac.
Note, however, that, depending on the year, the sun may pass through these signs with one or two days difference.
By tradition, the twelve signs are grouped into categories of the
four elements: fire, air, earth, and water. Tradition also holds that
for each sign there is : A planet : Aries is ruled by Mars, Taurus
by Venus, etc.