The ancient star watchers could only see 7 heavenly bodies from the earth! The zodiac signs originally corresponded to only the classical 7 heavenly bodies (the Seven Heavens), what were called the wandering stars. Other than the Sun and the Moon, we call the wandering stars, planets, today. Each sign was affected more by the positive or the negative traits of a shared ruling planet. Only the signs of Leo and Cancer didn't share a heavenly body with another sign. They were ruled by the Sun and the Moon, respectively.

         What used to be called the four corners of the earth or the four watchtowers, correspond to the four royal stars in the fixed signs of the zodiac - Taurus, Scorpio, Aquarius, and Leo - each is also associated with one of the four elements: Earth, Water, Air and Fire. This is why the Eagle, a constellation near Scorpio, is used in the Rider-Wait-Smith Tarot deck to represent the element of Water, while the Man represents Aquarius, which is an Air sign.

          For more details and explanation, I strongly suggest  The Weiser Concise Guide to Alchemy  by Brian Cotnoir, edited by James Wasserman, which you can get at Edge of the Circle Bookstore.