Is there really a 13th astrological sign?
A question I answered on Quora
There are indeed 13 star constellations in, or touching, the zodiac belt if that is what you mean – see Two Zodiacs below.
The thirteenth is Ophiuchus (the serpent bearer – formerly known as Serpentarius). It lies between the constellations of Scorpius and Sagittarius.
But regards Ophiuchus ever being included in the astrological zodiac, it is highly unlikely for reasons that:
The demarcation between constellations may be clear by agreed IAU standards but it is not at all so clear to the eye – which is what the ancients relied on – and it is very easy to see how Sagittarius could follow the constellation Scorpius.
While evidently Ophiuchus does cross the ecliptic, Scorpius in full occupies the same space as Ophiuchus along the ecliptic (in effect it wraps around Ophiuchus for that delineation).
I need to clarify something here:
While Eastern astrology (Hindu astrology) relies on the star constellations in the Zodiac belt (Sidereal Zodiac), most Western astrology uses the Tropical Zodiac which does not depend upon the star constellations. Rather for the Tropical Zodiac, the Ecliptic (path of the Earth around the Sun) is symbolically divided into 12 equal 30 degree parts starting from the Spring equinox.
For the record, there is only one star in most Western tropical astrology – that is the Sun. No star constellations.
Admittedly a few astrologers will include certain stars into the birthchart they draw up but that is their choice or way of working – and these stars are not necessarily from within the zodiac belt of constellations.
I’d suggest your question (re “signs”) probably applies to the tropical zodiac rather than the sidereal zodiac.
There are other symbolical reasons for why 13 was not accepted into astrology. These you can find out about by reading my article Get Real About Astrology Part 2.