The Constellations of the Zodiac
The ecliptic, or the position of the Sun as it’s perceived from the revolving Earth, passes through the constellations that formed the Zodiac - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Zodiac signs were originally determined by which constellation the Sun was "in" on the day you were born.
Early astronomers observed the Sun traveling through the signs of the Zodiac in the course of one year, spending about a month in each. Thus, they calculated that each constellation extends 30 degrees across the ecliptic.
However, a phenomenon called precession has altered the position of the constellations we see today.TAURUS = MAY 14 - JUNE 19
CANCER = JULY 21 - AUG 9
PISCES = MARCH 12 - APRIL 18
http://photosophize.blogspot.com/2009/01/ophiuchus-13th-zodiac-sign.html