What are the 12 signs of the zodiac?

What are the 12 signs of the zodiac?

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. 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 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.

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. The astronomer Ptolemy, author of the Tetrabiblos, which became a core book in the history of Western astrology, helped popularize these 12 signs.

“This whole idea that there were 12 signs along the zodiac that were 30° wide, and [that] the sun moved through these signs regularly during the year, that was codified by Ptolemy,” says Odenwald. Even the word “zodiac” comes from the Greek, from a term for “sculpted animal figure,” according to the Oxford English Dictionary, and the order in which the signs are usually listed comes from that period too. 

“Back at the time of the Greeks,” Odenwald explains, “the first day of spring started when the sun appeared in the constellation Aries and then everything was marked from that time forward around the circuit of the year.”

However, the Earth has moved on its axis since then, a process known as precession, so now the dates that are used to mark the signs don’t really correspond to the background constellations that give them their signs names. In fact, the chronology has really shifted one sign to the West. That means zodiac sign dates, based on the mathematical division of the year, basically correspond today to the presence of the sun in the constellations of the signs that come before them. (The set nature of the signs is also why the Minnesota Planetarium Society’s 2011 argument that there should be a 13th zodiac sign now, Ophiuchus, didn’t actually result in a big astrology change.)

What’s the difference between astrology and astronomy?

For centuries, astrology (looking for signs based on the movement of the celestial bodies) was considered basically the same thing as astronomy (the scientific study of those objects). For example, revolutionary 17th-century astronomer Johannes Kepler, who studied the motion of the planets, was at the time considered an astrologer. That changed around the beginning of the Enlightenment in the late 17th century.

Once Sir Isaac Newton basically turned the sky into a calculator, mathematizing the motion of the planets and realizing that gravity controlled everything, Odenwald says, “that started a whole new scientific approach to looking at the sky and the motion of planets and the earth.”

That’s the point at which astronomy came to be known as a science and astrology was acknowledged as not a science. But its popularity relies on factors that numbers can’t compute, and the appeal of looking to the stars for answers has not waned — in fact, in recent years, it seems to have expanded. After all, a 2014 National Science Foundation poll foundmore than half of millennials think astrology is a science.

And Odenwald argues that, even if astrology’s answers aren’t based on scientific study, the reason people keep turning to the sky does come down to something very real — a psychological phenomenon he calls the human tendency for “self-selection,” the search for interpretations that match what we already hope to be true.

“People magnify the positives, they forget the negatives,” he says, “and that’s just how we’re designed.”