Even though the planets move on the celestial sphere, they do not wander all over it but are confined to a narrow strip, dividing it in half. Stars along that strip are traditionally divided into the 12 constellations of the zodiac. The name, related to “zoo," comes because most of these constellations are named for animals - Leo the lion, Aries the ram, Scorpio the scorpion, Cancer the crab, Pisces the fish, Capricorn the goat and Taurus the bull.
At any time, the Sun is also somewhere on the celestial sphere, and as the Earth turns, it rises and sets the same way as stars do.
Like the planets, the Sun, too, moves around the zodiac, making one complete circuit each year. Every month it covers a different constellation of the zodiac, which is the real reason why those constellations are 12 in number. Of course, during that month, this particular constellation is not seen, because the sky near the Sun is too bright for its stars to be seen (except, very briefly, during a total eclipse of the Sun).
One can however figure out where the sun is on the zodiac (as ancient astronomers have done) by noting which is the last constellation of the zodiac to rise ahead of the Sun, or the first to set after it. Obviously, the Sun is somewhere in between. In this manner each month-long period of the year was given its “sign of the zodiac."
Astrologers, who believe that stars mysteriously direct our lives, claim it makes a great difference “under what sign" a person was born. Be aware, however, that the “sign" assigned to each month in horoscopes is not the constellation where the Sun is in that month, but where it would have been in ancient times. The difference is discussed in the section on the precession of the equinoxes (See the chapter “The Precession").