Any Astronomy software shows we have 13 Zodiac Signs, not 12. There is a constellation of Serpentarius, between Scorpio and Sagittarius. Besides, the Zodiac Signs are not uniformly distributed in the sky, despite the astrologic compendiums: they have many extensions, from 7 to 45 degrees. Moreover, we see that the astrologers stopped updating their registries more than two thousand years ago, despite the "equinoxes precession" teaches us that the Sun and the Constellations relative position changes 1 degree each 72 years. Look at the astronomic reality about the position of the Sun, under the zodiacal constellations, on the beginning of this 21st century:

The 12 astrological signs have, nowadays, the following initial dates, incorrectly:

Aries-Mar21  Taurus-Apr20   Gemini-May21       Cancer-Jun21     Leo-Jul22       Virgo-Aug23
Libra-Sep23  Scorpio-Oct23  Sagittarius-Nov22  Capricorn-Dec22  Aquarius-Jan20  Pisces-Feb19

I have additional informations about that on my portuguese webpage.