"Such a Scorpio." "Of course, he has Leo rising." Astrology has never circulated so widely: apps, memes, profile bios, daily horoscopes. Many people know their sign, some know their rising sign, few really know how a birth chart works. A quick tour of the sky, from Babylon to today's science.
From Babylonian priests to Ptolemy
Astrology was born in Mesopotamia. For centuries, Babylonian priests watched the sky for omens meant for the king and the kingdom: an eclipse or the position of a planet foretold a war or a good harvest. Around the 5th century BC, they divided the band of sky crossed by the Sun, the Moon and the planets into twelve equal parts of 30 degrees: the zodiac was born. The oldest known birth horoscopes, written on clay tablets, date from the end of that same century.
The Greeks took up this knowledge and expanded it. In the 2nd century AD, the astronomer Claudius Ptolemy, in Alexandria, brought it together in a treatise, the Tetrabiblos, which remained the reference work of Western astrology for more than a thousand years. At the time, astronomy and astrology were one and the same: people studied the movement of the heavens and looked for what it foretold.
What a birth chart contains
A birth chart is a map of the sky at the exact time and place of birth. It has three main ingredients.
The planets. Traditional astrology counts seven, meaning every body visible to the naked eye that moves across the sky: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Uranus (discovered in 1781), Neptune (1846) and Pluto (1930) were added after their discovery. Each one symbolizes an energy: the drive of Mars, the gentleness of Venus, the rigor of Saturn…
The signs. They show where each planet sits on the zodiac. Your sun sign, the one in the newspaper horoscope, is simply the position of the Sun at your birth. That is why your date of birth is enough to find it.
The ascendant and the houses. The ascendant, or rising sign, is the sign rising on the eastern horizon at the precise moment of birth. As the Earth turns, it changes on average every two hours: two people born on the same day in the same place can have different rising signs. It is the starting point of the twelve houses, which link each portion of the sky to an area of life (relationships, work, family…). To calculate it, you need the time and place of birth.
The sky has moved since Ptolemy
There is a detail horoscopes tend to leave out. The Earth's axis does not always point in the same direction: it slowly traces a circle, like a spinning top, over roughly 26,000 years. This is the precession of the equinoxes, discovered by the Greek astronomer Hipparchus in the 2nd century BC.
The consequence: since the zodiac was fixed, the constellations have shifted by almost a whole sign relative to the calendar. Someone who thinks they are an Aries was, in most cases, born while the Sun stood in front of the constellation Pisces. And from late November to mid-December, the Sun also crosses a thirteenth constellation that astrology ignores: Ophiuchus, the Serpent Bearer.
Western astrologers have an answer: their zodiac, called tropical, follows the seasons rather than the constellations. Aries always begins at the spring equinox, whatever star lies behind the Sun. Indian astrology, on the other hand, uses a sidereal zodiac that follows the constellations, so the same person often has a different sign there.
Astrology put to the test
If astrology really describes personality, it should be possible to check. Scientists have tried many times.
In 1985, the American physicist Shawn Carlson published a double-blind experiment in the journal Nature, designed with the help of astrologers. They received a volunteer's birth chart and three personality profiles drawn up from a psychological questionnaire, only one of which was correct. The astrologers had predicted they would succeed more than half the time. They found the right profile about one time in three: exactly what chance would give.
Another study, by Geoffrey Dean and Ivan Kelly and published in 2003, looked at more than 2,000 Londoners born in March 1958, often minutes apart. These "time twins" share practically the same chart. Across around a hundred characteristics measured from childhood to early adulthood (personality, intelligence, abilities, social situation…), they were no more alike than people born on different dates.
The French statistician Michel Gauquelin, for his part, believed he had found a "Mars effect": great sports champions were supposedly born more often when Mars had just risen or reached its highest point. His results sparked decades of controversy, but attempts by other researchers to check them did not confirm them.
Today the scientific consensus is clear: no rigorous study has shown that the position of the stars at birth influences personality or destiny.
Why people hold on to it anyway
If astrology has survived centuries of criticism, it is because it meets a need other than proof. Its descriptions benefit from the Barnum effect: profiles general and flattering enough for everyone to recognize themselves. It offers a shared language for talking about yourself and others, often with humor. It brings people together: comparing signs is a great way to start a conversation. And it provides a reassuring story in uncertain times.
You can perfectly well take it that way, as a game of mirrors and a repertoire of symbols, without asking it to predict the future. That is the spirit of our astro quizzes: Are you really your sign, or just its cliché? looks at your relationship with astrology, and others start from your habits rather than your date of birth to match you with a sign, a planet or an archetype.
In short
- Astrology was born in Mesopotamia; the twelve-sign zodiac dates from the 5th century BC, and Ptolemy set down its rules in the 2nd century AD.
- A birth chart combines the planets, the signs and the ascendant, which depends on the time and place of birth.
- Because of precession, the signs no longer match the constellations; Western astrology follows the seasons.
- Scientific tests have never shown any influence of the stars on personality. Astrology remains a language and a game, not a science.
