The zodiac is a belt of constellations that circles the sky along the ecliptic, the path the Sun appears to trace over a year. The planets and the Moon travel within this same belt, which is why ancient astronomers watched it closely. The word comes from the Greek zodiakos, meaning circle of little animals.

Babylonian astronomers divided the belt into 12 equal sections, each 30 degrees wide, and named them after the constellations they contained. Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. Those names became the astrological signs, which are still used today even though astronomy has moved on.

The constellations and the signs no longer align. Earth's axis precesses, shifting the Sun's position against the stars by about one degree every 72 years. When astrology was codified, the Sun was in Aries at the spring equinox. Today it is in Pisces. The signs are fixed to the seasons, not to the stars.

Astronomers use the zodiac as a convenient marker along the ecliptic. Astrologers use it for other purposes. The two fields share a vocabulary but not a method.