To some, the zodiac is just something that determines your horoscope when you open your newspaper, but of course that would be astrology.
In actuality, the zodiac is the group of constellations that serve as the direct backdrop to the sun’s yearlong light show (meaning it’s Scorpius and Capricornus, not Scorpio and Capricorn). Of course, since the sun completely drowns out any surrounding competitors for screentime and the constellation active at the time disappears with the sun at night, your chances of actually seeing your sign during your special month are slim.
The thirteenth sign most people don’t know about would be the Snake Charmer, Ophiuchus. It’s settled right between Scorpius and Sagittarius and vaguely resembles a snake in a pot. Most horoscopes tend to remove it from the lineup, mostly since ancient civilizations still followed the current “twelve month calendar” thing and deemed it easier to just round it down to a more even number.
Or, considering we’re talking about horoscopes here, perhaps they were just superstitious.