?Ophiuchus is one of the thirteen constellations that cross the ecliptic.[65] It has sometimes been suggested as the "13th sign of the zodiac". However, this confuses zodiac or astrological signs with constellations.[66] 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 always falls on the boundary between Pisces and Aries.[67][68] 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.[69] In Western astrology the constellation of Aquarius, for example, largely corresponds to the sign of Pisces. Similarly, the constellation of Ophiuchus occupies most (29 November – 18 December[70]) of the sign of Sagittarius (23 November – 21 December). The differences are due to the fact that the time of year that the Sun passes through a particular zodiac constellation's position has slowly changed (because of the precession of the Earth's rotational axis) over the centuries from when the Babylonians originally developed the Zodiac.[71][72]