THE 13RD ZODIAC SIGN
Yes, any Astronomy software shows we have 13 Zodiac Signs, and not 12. There is indeed a constellation of Ophiuchus (or Serpentarius) between Scorpio and Sagittarius. And these 13 Zodiac Signs are not uniformly distributed in the sky, despite the astrologic compendiums. And all the Zodiac Signs are no longer in the place pointed by astrologers. For instance, the Sun transits under Gemini between June 21 and July 20, nowadays, precisely one month after the astrologic definitions. The astrologers stopped updating their registries on a day of the remote 11th century. Despite them, the Earth and the Sky didn't stay static from 1000 A. D. to now. As would say Galileo Galilei, "Despite the astrologers, the Earth yet moves". Look at the astronomic reality about the position of the Sun, under the zodiacal constellations, on the beginning of this 21st century. Take notice of the 13th constellation and of the fact that the "signs" have different duration, from 7 to 45 days:
See, below, the sky at 2008 December 3. It's an accurate copy of an image from the amazing RedShift astronomy software. Look at the Sun just joining Serpentarius from where it will get out at December 18, toward Sagittarius. See, also, the ephemeral transit of the Sun under Scorpio (just 7 days, about a third part of the transit under Libra or Serpentarius):
Besides, we have to admit that constellations and its borders are nothing but human and temporal inventions. We could have twelve, thirteen or thirty zodiacal constellations, depending on the mood of the astronomic authorities. The same that happened about our solar system: once we had seven planets, later we had nine, today we have eight.
I have many additional informations about that on my portuguese webpage.