"a 13th sign of the Zodiac named for it"
Public DomainOphiuchus - Credit: Wikimedia Commons

There is in fact a thirteenth sign of the Zodiac. Ophiuchus has been overlooked in astrology because the constellation is behind the sun between November 29 and December 17. Mainstream astrology is based on dividing the ecliptic into 12 equal parts.

In the 1970s, there were moves to make Ophiuchus a full sign of the Zodiac, and the idea has taken off in Japanese astrology.