Ophiuchus, the 13th Sign of the Zodiac

Here's the science

Here's some history and mythology

The Zodiac we know today is based on the zodiac developed in Greek and Roman times. This Zodiac is a solar zodiac, based on the movement of the sun through the skies during one year. But this is not the only Zodiac that was floating around.

Other zodiacs were based on lunar movements - the movement of the moon through the seasons. Lunar zodiacs generally had thirteen signs, one for each moon month.