Babylonian Astronomy and Astrology: History, Legacy and Great Discoveries

Scientific summary

Astronoo's article traces the history of Babylonian astronomy, seen as a stellar science where observation and divination are closely linked. It details the systematic observations of planets and eclipses, the division of the zodiac into twelve sections, and the development of the first astronomical tables such as the Enuma Anu Enlil tablets. The article presents the discovery of the precession of the equinoxes and the functioning of the luni‑solar calendar, illustrated by a birth horoscope dated to 69 BC. It finally emphasises the decisive influence of this Mesopotamian heritage on Greek and Arab astronomy.

Why is Babylonian astronomy considered the cradle of modern science?

Why is Babylonian astronomy considered the cradle of modern science? The answer lies in its break with mythical explanations in favour of methodical observation and mathematical modelling. From the beginning of the 2nd millennium BC, the astronomer‑priests of Mesopotamia accumulated data on the movements of the Sun, Moon and planets, which they recorded on clay tablets. They identified precise cycles, such as the Saros for eclipses, and invented the sexagesimal system that we still use to measure time and angles. Their zodiac, a division of the sky into twelve sections, became the reference for later astrologies. But above all, they developed arithmetic methods to predict the future positions of the stars, thus inaugurating a predictive and quantitative approach that heralds modern science. This tradition, passed on to the Greeks and then to the Arabs, nourished the Copernican revolution and Newtonian physics. Far from being mere divination, Babylonian astronomy represents humanity's first attempt to decipher the hidden laws of the sky, laying the foundations of a rational thought that would revolutionise our understanding of the Universe.

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History of Babylonian Astronomy

Origins and historical context

Babylonian Astronomy, practised from the second millennium BCE, is one of the earliest forms of structured "science". It is based on meticulous observation of the stars, especially planets and stars, with a complex calendar based on lunar and solar cycles. The Babylonians introduced key concepts that influenced astronomy in later civilisations, such as the idea of the precession of the equinoxes, as well as mathematical models to predict the movements of celestial bodies. This immense culture has come down to us through thousands of Clay Tablets engraved with a stylus (a small part of which concerns astronomy).

A stellar science between astronomy and astrology

Historians of science consider that Mesopotamian astronomy was not a science because of the pre‑eminent place of astrology. However, astrology and astronomy in Mesopotamia belong to the same field of thought, the same form of celestial knowledge, or "Stellar Science".

Systematic observations of the stars

The oldest known data come from the Babylonians who lived in Mesopotamia 4,000 years ago. The phenomena measured as a priority were the risings and settings of the stars (first and last visibilities in the sky), but also the synodic periods (time to return to the same celestial configuration). Among the many regular celestial phenomena in astronomy are lunar and solar eclipses, changes in lunar phase, the periods of Mercury, Venus, Mars, Jupiter, Saturn, the inclination of the Earth's orbit, and oppositions and conjunctions of the planets. The distribution of stars in the sky and the movements of the wandering stars (synodic period) were known in the first half of the 1st millennium BC.

The zodiac and almanacs

From the 5th century BCE, the ecliptic was divided into 12 sections of 30 degrees containing the 12 constellations corresponding approximately to our current zodiac signs. The Zodiac was a reference system in Almanacs (calendars listing the main dates), Horoscopes (examination of the time of birth in relation to celestial events) and Ephemerides (astronomical tables by which the positions of the planets are determined for each day).

Example of a child's birth horoscope

A horoscope from the year 69 BC

In year 243, in the month of Nisannu, on the 20th day (16 April 69 BCE), during the 9th hour, a child was born. At that hour, the Moon was in the end of Capricorn at 18°, the Sun in the end of Aries at 30°, Jupiter in Sagittarius at 24°, Venus in Gemini at 13°, Saturn in Aquarius at 15°, Mars in Libra at 14°, Mercury, which had not risen, was not visible. The Moon set before the Sun rose on the 14th day; the last lunar visibility occurred on the 27th day.

That year, on the 28th day of the month Abu (20 August 69 BCE), an expected solar eclipse was not observed at the end of Leo. On the 13th day of the month Ululu (3 September 69 BCE), at sunset, a lunar eclipse exceeding one‑third of the disk occurred; the Moon was already eclipsed when it rose in Pisces.

The Main Discoveries of the Babylonians

Astronomical tables and ephemerides

Among the major discoveries of the Babylonians are the development of the first Astronomical Tables, such as the famous Enuma Anu Enlil Tablets, which consisted of series of observations and predictions related to eclipses and planetary positions. These tables served as a reference for astronomers of later cultures.

The precession of the equinoxes

The Babylonian theory of the Precession of the Equinoxes, according to which the positions of the stars slowly change over time due to the movement of the Earth's axis, is a fundamental concept in astronomy.

The luni‑solar calendar

Regular observations and empirical measurement of the cycles of the Moon and Sun were essential for determining calendars, although they already had a long history. The so‑called "Luni‑Solar" system had a year of 12 months (based on the Moon's cycle). This made it possible to predict when it would reappear in the sky. In this context, the month has 29 or 30 days (on average the synodic month is about 29.5 days).

At the end of the 4th millennium BCE, the month lasted 30 days and the year lasted 12 months. It was this simplified reference that was used for calculations. Then adjustments were made to the actual observed length of the year. From the 3rd millennium BC, an intercalary month was added empirically to correct the discrepancy between calculation and observation.

The legacy for later civilisations

Babylonian astrology and astronomy were at the origin of Greek, Byzantine, Syriac astronomy, and the medieval astronomy of the Muslims and Europeans. Classical Greek and Latin sources frequently refer to the astronomers of Mesopotamia as "Chaldeans", often presenting them as specialists in astrology and other forms of divination.

References

FAQ – Babylonian astronomy and astrology

What is Babylonian astronomy?

Babylonian astronomy is a body of knowledge and practices developed in Mesopotamia from the 2nd millennium BC. It is characterised by methodical observation of the movements of the Sun, Moon and planets, recorded on clay tablets. The Babylonians developed arithmetic models to predict the positions of the stars, thus laying the foundations of a mathematical astronomy that would lastingly influence Greek, Indian and Arab civilisations.

What is the difference between astronomy and astrology among the Babylonians?

Among the Babylonians, astronomy and astrology did not form two distinct disciplines, but a single "stellar science". Precise observation of celestial phenomena served both to establish calendars and to interpret divine will. While astrology aimed to decipher omens, astronomy provided the necessary calculations. The two practices were inseparable and exercised by the same scholars, often priests.

What are the main contributions of the Babylonians to modern science?

The Babylonians bequeathed to the world the sexagesimal system (base 60), which is still used for measuring angles and time. They discovered the precession of the equinoxes, developed the zodiac, and created the first astronomical tables enabling the prediction of eclipses. Their empirical approach and mathematical formalisation laid the foundations of the scientific method, later transmitted to the Greeks and Arabs.

How did the Babylonians predict eclipses?

The Babylonians identified the Saros cycle, a period of about 18 years that makes it possible to predict solar and lunar eclipses. By compiling centuries of observations, they established series of dates and conditions favourable to eclipses, which they recorded in ephemerides. These predictions, although not based on a physical theory, were remarkably reliable and served both religious life and the administration of the kingdom.