Astrology: history, basics, tropical or sidereal zodiac, limits
Born in Mesopotamia and systematised in the Hellenistic era, astrology is a symbolic language with no scientific validation. History, zodiac, precession, tests.
Key points
| Origins | Mesopotamia: celestial omens from the 2nd millennium BCE, individual horoscopes from around the 5th century BCE |
|---|---|
| Systematisation | Hellenistic period (after 332 BCE), then Ptolemy (2nd century CE) |
| Separation from astronomy | 17th century; the official French almanac La Connaissance des temps stopped including astrology in 1679 |
| Precession of the equinoxes | About 1 degree every 72 years, a cycle of roughly 25,800 years |
| Tropical/sidereal gap | About 24 degrees today |
| Controlled tests | No effect shown beyond chance (Carlson, Nature, 1985, and others) |
Definition: astrology and astronomy
Astrology is a symbolic system that links the positions of the planets, the Sun and the Moon in the sky with human traits or events. Astronomy is the science that studies these celestial bodies. For centuries the two were intertwined: sky-watchers were often astrologers too, and astrology partly funded astronomy, including in the medieval Islamic world and during the Renaissance. The split came in the 17th century with the scientific revolution: the official French almanac La Connaissance des temps stopped including astrology in 1679.
On this page we treat astrology as a subject of history and culture. We present its principles, its variants and its limits, without promoting it as a way to predict the future.
Mesopotamia: the sky as omen
The oldest traces of astrology come from Mesopotamia. From the early Babylonian period (early 2nd millennium BCE), scribes recorded sky omens: eclipses, planetary appearances, phases of the Moon. They gathered them into a large compendium, the Enūma Anu Enlil, which contains some seven thousand omens meant to interpret what the sky foretold for the king and the country. This was not yet personal astrology.
The division of the sky into twelve signs of thirty degrees each took shape in Babylon between the 6th and 5th centuries BCE. The oldest known individual horoscope, that is, a record of the positions of the stars at a person's birth, dates from around 410 BCE. At the same time, Babylonian astronomers became able to calculate planetary positions precisely.
Hellenistic era: astrology of the horoscope
Alexander the Great's conquest of Egypt in 332 BCE brought the Babylonian, Egyptian and Greek traditions together. In Alexandria, horoscopic astrology was born: signs, planets, "houses" (twelve sectors of the sky) and the ascendant (the sign rising on the eastern horizon at birth) were combined. Around 280 BCE, the Babylonian priest Berossus settled on Kos and spread the doctrines of his homeland. Astrology reached Rome in the 2nd century BCE; Cicero criticised it in 44 BCE in his treatise De divinatione. The emperor Tiberius is the first to whom a court astrologer, Thrasyllus, is attributed.
In the 2nd century CE, the Alexandrian Claudius Ptolemy wrote the Tetrabiblos, which for centuries became the foundation of Western astrology. Ptolemy was also the author of the Almagest, a major treatise on astronomy. He himself acknowledged that predictions of planetary positions were more reliable than those of astrology.
From the Arab world to Europe
In the 8th century, the Abbasid caliphs founded Baghdad and encouraged the translation of Greek texts. Abū Maʿshar (Albumasar in Latin, died 886) became the most influential astrologer in the Islamic world. Observatories funded by princes also served astrology. From the 12th century, Latin translations (the Tetrabiblos was translated in 1138 by Plato of Tivoli) spread Arabic knowledge across Europe. Around 1277, Guido Bonatti wrote a major astrology treatise. In the Renaissance, physicians consulted the Moon before bloodletting their patients, and leading astronomers, such as Johannes Kepler, cast horoscopes.
With Copernicus's heliocentric system (1543), Kepler's laws and Newton's physics, the theoretical framework of astrology gradually lost its intellectual support. It survived in popular culture, with a new revival in the 20th century thanks to newspaper horoscopes, which reduce a birth chart to the single sun sign.
Tropical zodiac, sidereal zodiac and precession
Most Western astrologers use the tropical zodiac, whose starting point (0 degrees Aries) is fixed on the spring equinox. It is therefore tied to the seasons, not to the stars. Indian (Vedic) astrology uses the sidereal zodiac, anchored on the fixed stars.
The two do not coincide because the Earth's axis slowly wobbles, like a spinning top: this is the precession of the equinoxes, attributed to Hipparchus in the 2nd century BCE. The spring equinox moves back about 50 arcseconds per year, that is roughly 1 degree every 72 years, and completes a full cycle in about 25,800 years. As a result, today's tropical "Aries" no longer lies in front of the constellation Aries: the gap with the sidereal zodiac is about 24 degrees, almost one full sign.
The signs are therefore not the constellations of the same name. The constellations have unequal sizes, and the Sun actually passes through thirteen constellations, including Ophiuchus, which is not one of the twelve signs. Astrologers reply that the signs are sectors of thirty degrees, not constellations. However, this assumes that these sectors carry meaning, which is precisely what is debated. See the guide to the signs.
What the tests show
Astrology has been tested several times. The test most often cited is that of physicist Shawn Carlson, published in the journal Nature on 5 December 1985. Twenty-eight well-known astrologers had agreed to a protocol defined in advance with astrologers: matching birth charts to psychological profiles established by a recognised questionnaire, the California Psychological Inventory, with each person presented with three possibilities. The result: about one third of correct answers (34% in the first test), that is, the level of chance. Carlson concluded that the astrological hypothesis was refuted.
The protocol was debated, including by researchers favourable to astrology, who proposed other statistical analyses. But no controlled study has since established a reproducible link between the positions of the stars at birth and personality. As for the "Mars effect" of statistician Michel Gauquelin (1955), which claimed a link between the position of Mars and sporting success, it was not confirmed by independent teams.
The scientific community therefore regards astrology as a pseudoscience: it offers no physical mechanism consistent with current knowledge, and its predictions do not hold up under testing.
Why it still appeals
- The Barnum (or Forer) effect. In 1949, psychologist Bertram Forer gave his students the same personality profile made up of sentences taken from an astrology book. The students rated it very accurate (average of 4.2 out of 5). Statements that are both flattering and vague seem personal.
- A shared language. Signs and planets offer a rich vocabulary for talking about oneself and others, like a reading grid.
- Conversation. "What's your sign?" is a gentle way to start a conversation.
- A story. Astrology gives a narrative to periods of life and meaning to chance events.
These psychological drivers are nothing to be ashamed of, but they explain how a description can feel very "right" without being based on any astronomical reality.
How to approach it with a critical mind
If astrology interests you, you can practise it as a game of reflection or as cultural knowledge, keeping a few principles in mind: never make an important decision (health, money, separation, work) based on a chart; never let anyone tell you that your sign "condemns" you to a particular behaviour; be wary of those who sell fear or precise predictions; learn the difference between sun sign, ascendant and full birth chart.
To get started: the basics of astrology (planets, elements, houses, aspects), the page on the ascendant, and how to read a birth chart. The correspondences between signs and stones are modern traditions: see the guide to the signs.
Frequently asked questions
Is astrology a science?
No. It has no known physical mechanism, and controlled tests have shown no effect beyond chance. It is a symbolic language and a cultural tradition.
Where does astrology come from?
From celestial omens recorded in Mesopotamia from the 2nd millennium BCE. Individual horoscopes developed in Babylon and then in the Hellenistic era, with Ptolemy in the 2nd century CE.
What is the difference between the tropical and sidereal zodiac?
The tropical zodiac is anchored on the spring equinox, the sidereal zodiac on the stars. Because of the precession of the equinoxes, they are offset by about 24 degrees today.
Do the signs match the constellations?
No. The signs are sectors of 30 degrees; the constellations have unequal sizes, and the Sun passes through thirteen constellations, including Ophiuchus.
What was Carlson's experiment (1985)?
A double-blind test published in Nature in which astrologers had to match charts to psychological profiles. The results were at the level of chance, which was seen as a refutation.
Can you be interested in astrology without believing in it?
Yes. You can study it as history, as an art and as a symbolic language, keeping a critical mind and without using it to make important decisions.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Major topics.