“This whole idea that there were 12 signs along the zodiac that were 30° wide, and the sun moved through these signs regularly during the year, that was codified by Ptolemy,” says Odenwald. The astronomer Ptolemy, author of the Tetrabiblos, which became a core book in the history of Western astrology, helped popularize these 12 signs. The Babylonians had already divided the zodiac into 12 equal signs by 1500 BC - boasting similar constellation names to the ones familiar today, such as The Great Twins, The Lion, The Scales - and these were later incorporated into Greek divination. These Western, or tropical, zodiac signs were named after constellations and matched with dates based on the apparent relationship between their placement in the sky and the sun. It was during this Ancient Greek period that the 12 star signs of the zodiac with which many people are likely familiar today - 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. The constellations in the zodiac are simply the constellations that this imaginary straight line points to in its year-long journey. All the stars that lie close to the imaginary flat disk swept out by this imaginary line are said to be in the zodiac.
This imaginary line would rotate, pointing to different stars throughout one complete trip around the Sun - or, one year. Then, picture Earth following its orbit around the Sun. Imagine a straight line drawn from Earth through the Sun and out into space way beyond our solar system where the stars are. Here’s how NASA has described how that logic led to the creation of the familiar zodiac signs known today: “There must have been a lot of exchange that got the Greeks on-board with the idea of divination using planets,” says Odenwald, and because they were deep into mathematics and logic, they worked out a lot of the rules for how this could work.” It’s thought that all of these ideas came together when Alexander the Great conquered Egypt around 330 BC. The ancient Egyptians contributed the idea that patterns of stars made up constellations, through which the sun appears to “move” at a specific times during the year. This tablet, which is dated to the first millennium BC and tracks the motion of Venus, is one of the earliest pieces of what’s been called Babylonian planetary omens. (Odenwald points out that in societies where people in the lower classes had less control over their lives, divination could seem pointless.) The Sumarians and Babylonians, by around the middle of the second millennium BC, appeared to have had many divination practices - they looked at spots on the liver and the entrails of animals, for example - and their idea that watching planets and stars was a way to keep track of where gods were in the sky can be traced to The Venus tablet of Ammisaduqa. In Ancient China, noblemen looked at eclipses or sunspots as portents of good or bad times for their emperor, though it’s thought that those signs had less application to the lives of other individuals. Some form of astrology shows up in various belief systems in ancient cultures. That was taken over by the idea of divination, where you can actually look at things in nature and study them carefully, such as tea-leaf reading.”
“There’s some indication that cave art shows this idea that animals and things can be imbued with some kind of spirit form that then has an influence on you, and if you appease that spirit form, then you will have a successful hunt. “We don’t really know who first came up with the idea for looking at things in nature and divining influences on humans,” says astronomer Sten Odenwald, the director of Citizen Science at the NASA Space Science Education Consortium. The stars are just one of the many things in the natural world that human beings have turned to for answers over the years. If you don't get the confirmation within 10 minutes, please check your spam folder. Click the link to confirm your subscription and begin receiving our newsletters. For your security, we've sent a confirmation email to the address you entered.