Oph - Ophiuchus and The Zodiac

Ophiuchus and The Zodiac

Ophiuchus is one of thirteen constellations that cross the ecliptic. It has therefore been called the '13th sign of the zodiac'. However, this confuses sign with constellation. The signs of the zodiac are a twelve-fold division of the ecliptic, so that each sign spans 30° of celestial longitude, approximately the distance the Sun travels in a month, and (in the Western tradition) are aligned with the seasons so that the March equinox falls on the boundary between Aries and Pisces. Constellations, on the other hand, are unequal in size and are based on the positions of the stars. The constellations of the zodiac have only a loose association with the signs of the zodiac, and do not in general coincide with them. In Western astrology the constellation of Aquarius, for example, largely corresponds to the sign of Pisces. Similarly, the constellation of Ophiuchus occupies most of the sign of Sagittarius. This is due to the fact that the time of year that the sun passes through a particular zodiac constellation's position has slowly changed over the centuries from when the Greeks and Babylonians originally developed the Zodiac. This phenomenon is due to the Earth's axial precession.