Astrology is about studying the interaction between the planets (including the Sun and Moon) and the Signs. The relationships between them and their interactions are mathematically based, and Astrology studies these mathematical cycles. Each Sign represents a different aspect of the whole human; Aries starts the cycle, representing the Self, and Pisces completes it, representing the mass unconscious of all mankind. In between, each other Sign carries the energy of a different phase of man's evolution within the universe.
In some ways, the forces between the planets involved in Astrology can be simplified into one word: gravity. The Sun has the greatest gravity and the strongest effect in Astrology, followed by the Moon, the Earth's satellite. The other Planets are not truly satellites of the Earth, but nevertheless, they have gravity and so affect the Earth. The Sun controls the Earth's motion and the Moon controls its tides, but the other Planets have their own effects on the Earth -- and on the people who live on Earth. Sometimes their influences can be so strong that they outweigh the Sun's energy!