# Babylonian astronomy and the birth of the zodiac

The longest scientific record in history: Babylon's sky-watchers, from the Venus Tablet and the omen-series that founded astrology to the astronomical diaries, the Saros eclipse cycle, and the 360-degree circle. Their arithmetic passed through Hipparchus and Ptolemy into Greek, Islamic, and modern astronomy.

*This story parallel: How technology remade the world*
*This story part of: Babylon and its long shadow*
*This story part of: Science and technology*
*Astrology: from the Babylonian sky to the horoscope continues this story*
*Ptolemy: the map of the heavens and the earth continues this story*

## c. 1645 BCE — The Venus Tablet: the oldest planetary record

Under King Ammisaduqa, Babylonian scribes record the risings and settings of Venus over 21 years. It is the earliest surviving record of systematic planetary observation, the seed of a science that would run for two thousand years.

## c. 1595 BCE — The Enuma Anu Enlil: reading fate in the sky

Babylonian priests compile the Enuma Anu Enlil, a vast series of celestial omens linking events in the heavens to the fate of kings and nations. It is the founding document of astrology, the belief that the sky foretells events on earth.

## 747 BCE — The Nabonassar era begins continuous records

From the reign of Nabonassar, Babylonian astronomers keep unbroken dated records of the heavens. Centuries later the Greek astronomer Ptolemy would begin his own chronology from this very year, so precious were the Babylonian observations.

## c. 652 BCE — The astronomical diaries: seven centuries of the sky

Babylonian observers begin the astronomical diaries, nightly records of the moon, planets, weather, river levels, and prices kept for some seven hundred years. They are the longest continuous scientific record in human history.

## c. 600 BCE — The Saros cycle predicts eclipses

Babylonian astronomers discover the Saros, a period of about 18 years after which eclipses repeat, letting them predict eclipses in advance. Turning omen-reading into calculation was a decisive step from astrology toward astronomy.

## c. 500 BCE — Babylon measures the sky in 360 degrees

Using their base-60 numbers, Babylonian astronomers divide the circle of the sky into 360 degrees, each degree into 60 minutes of arc, each minute into 60 seconds. Every modern map, clock, and navigation system still uses these Babylonian divisions.

## c. 400 BCE — The 19-year cycle fixes the calendar

Babylonian astronomers regularize a 19-year cycle in which 7 leap months keep the lunar calendar in step with the solar year. The same cycle, later named for the Greek Meton, still sets the date of the Jewish calendar and of Easter.

## c. 350 BCE — Mathematical astronomy: Systems A and B

Babylonian scholars develop purely mathematical schemes, known to scholars as System A and System B, to compute the future positions of the moon and planets with columns of numbers. It is arithmetic prediction of the heavens, centuries before Greek geometry.

## c. 340 BCE — Kidinnu measures the year and the moon

The Babylonian astronomer Kidinnu (known to the Greeks as Cidenas) calculates the length of the year and the moon's motion with an accuracy not matched in Europe until the modern era. His constants passed into Greek astronomy.

## c. 140 BCE — Hipparchus builds on Babylonian records

The greatest Greek astronomer, Hipparchus, uses centuries of Babylonian eclipse and planetary records to build his own theories, borrowing their arithmetic and their degrees. Babylonian science is the hidden foundation beneath Greek astronomy.

## c. 75 — The last cuneiform tablet is an astronomical almanac

The very last datable cuneiform texts, written when Babylon was already a ruin, are astronomical almanacs. The science of the stars was the final thing the world's oldest written civilization kept recording, outliving its temples and its kings.

## c. 150 — Ptolemy's Almagest preserves the Babylonian sky

Ptolemy's Almagest, the astronomy textbook of the next 1,400 years, is built in part on Babylonian observations and their base-60 arithmetic. Through it, Babylonian science reached the medieval Islamic and European world and shaped astronomy until Copernicus.