Over the years, human beings have turned to many things in the natural world for answers. The stars are just one of them. In Ancient China, noblemen looked at sunspots or eclipses as foretellers of bad or good times for their emperor. The Babylonians and Sumerians appeared to have had many divination practices. They might have had the idea that watching stars and planets as a way to keep a track of where gods were in the sky. It’s thought that all these ideas formed around 330 BC when Alexander the Great conquered Egypt.
Here’s how NASA has described how that logic led to the creation of the familiar zodiac signs known today:
Imagine a straight line drawn from Earth through the Sun and out into space way beyond our solar system where the stars are. Then, picture Earth following its orbit around the Sun. This imaginary line would rotate, pointing to different stars throughout one complete trip around the Sun — or, one year. All the stars that lie close to the imaginary flat disk swept out by this imaginary line are said to be in the zodiac. The constellations in the zodiac are simply the constellations that this imaginary straight line points to in its year-long journey.
It was during this Ancient Greek time that the 12 star signs of the zodiac — Aries (roughly March 21-April 19), Taurus (April 20-May 20), Gemini (May 21-June 20), Cancer (June 21-July 22), Leo (July 23-Aug. 22), Virgo (Aug. 23-Sept. 22), Libra (Sept. 23-Oct. 22), Scorpio (Oct. 23-Nov. 21), Sagittarius (Nov. 22-Dec. 21), Capricorn (Dec. 22-Jan. 19), Aquarius (Jan. 20 to Feb. 18) and Pisces (Feb. 19 to March 20) — were set down. These zodiac signs were named after constellations and matched with dates based on their apparent relationship between their placement in the sky and the sun.