Not many people realise this, but, technically speaking, there are thirteen signs of the zodiac.
Let me explain.
As mentioned in a previous post, as the Earth orbits the Sun, the Sun appears to move across the sky relative to the stars. The path that the Sun traces is called the ecliptic.
The concept of the ‘zodiac’ – the division of the ecliptic into 12 segments – may have been used as long ago as 7th century BC by the Mesopotamians (or Bablyonians). Each of these 12 segments is nicely represented by a constellation, the ram of Aries or the bull of Taurus, for example. Each of the 12 segments of the sky is associated with one of the months of the Mesopotamian calendar, the months then being recognised when the Sun moves into a different constellation. Thus, the constellation that the Sun occupies when a child is born is equivalent to the birth month of the child. This is the concept we know today as star signs.
(As an aside, the whole superstition behind star signs is related to the religious ceremonies and superstitions practiced by the Mesopotamians and Egyptians, a tradition that stubbornly refuses to die out)
Now, the Earth’s axis of rotation is tilted with respect to its orbital plane, and it is this tilt which gives rise to the seasons (or alternatively, the inclination of the ecliptic with respect to the celestial equator). This tilt is not fixed however, and, due to a phenomenon called precession (similar to the way a spinning top twists on a flat surface), traces out a conical shape every 40,000 years.
The effects of precession include the changing position of the North Celestial Pole, which is currently quite nicely marked by Polaris (the north star), and the changing inclination of the ecliptic.
Historically, when the 12 signs of the zodiac were originally drawn out, the ecliptic passed through 12 constellations. Quite handy really.
Presently, the ecliptic passes through 13 constellations. The extra constellation being Ophiuchus. A bit of a mouthful.
So, technically speaking, anyone born between November 30th and December 17th was born under the sign Ophiuchus, not Saggitarius. But alas, tradition is stubborn and people don’t like change. Not to mention it would mess with all astrologers and mystics everywhere if we suddenly had an unlucky 13 zodiacal constellations. The truth hurts.
