Definition
The zodiac is the region of the sky within about 8° of the ecliptic, divided into constellations through which the Sun, Moon and planets pass. There are 12 traditional signs, each spanning 30° of ecliptic longitude in the tropical system.
Tropical vs sidereal
The tropical zodiac is fixed to the vernal equinox (0° Aries at the equinox). The sidereal zodiac is fixed to the stars and drifts due to precession — currently offset by about 24° from the tropical signs.
