Here's the Wiki article I found in that particular constellation: http://en.wikipedia.org/wiki/Ophiuchus_%28astrology%29
I really wouldn't worry too much about accuracy or inaccuracy without that sign, especially since the Tropical system we use here in the West is more determined by the seasons (solstices and equinoxes) and the planets' path in our solar system around the sun than that of the relative position of the planets to the actual constellations. The year is divided up in twelve equal parts with each season divided up in into three astrological 'months' per season starting with the cardinal months of Aries (spring), Cancer (summer), Libra (fall) and Capricorn (winter). Each cardinal month is then followed by a fixed month (Taurus, Leo, Scorpio, Aquarius, respectively) and then a mutable month right before the season change (Gemini, Virgo, Sagittarius, and Pisces, respectively). The months seem to be more named after the constellations that may have been around at the time versus an indication that these constellations are actually hanging in the sky right now at this time of year. Each of the planets'/satellites' paths in the solar system are also divided up into twelve equal parts (at least this is my understanding here--if anyone smarter wishes to correct me, go for it
Here's a couple of Wiki articles on Sidereal and Tropical systems that can launch more research if interested: http://en.wikipedia.org/wiki/Sidereal_a ... astrology; http://en.wikipedia.org/wiki/Western_astrology
. It does mention the 13th constellation in the chart, but I do understand why at least it's not included in the Tropical system as it wouldn't make too much sense to include it.
EDIT: Here's a better Wiki article that is more complete and doesn't have the citation flags: http://en.wikipedia.org/wiki/Zodiac