Why is the 13th sign of the Zodiac not included in Astrology?

Answer 1

Ophiuchus is often mistakenly called the 'thirteenth sign of the zodiac' because the sign is thought of as an additional sign to the the twelve Tropical or Sidereal signs.

In fact, Ophiuchus is a Sun-sign in the Real Solar Zodiac, i.e. the Sun can be seen against the stars of Ophiuchus between 30th November and 17th December each year.

Ophiuchus is an Ancient Greek constellation. The Serpent Bearer is one of the original Ptolemy constellations, appearing in Al Magest Star Catalogue [c 130 - 170 AD].