The Zodiac was invented thousands of years ago to study the movement of the heavenly bodies and how they relate to mans affairs. Astronomy is a recent development and has yet to relate the stars to man to the extent that astrology does.
The very basics are that the Earth revolves causing day and night, that the moon orbits Earth causing the tides and the Earth orbits the sun causing the seasons. These things affect man. The whole solar system runs like clockwork with orbiting planets, Astrology will tell you that these also affect man, astronomy has yet to grasp this.