Watching the sky for a reason
Babylonian astronomy began as a practical and religious activity. Scholars at the temples of Babylon, Uruk and other cities watched the sky because they believed celestial events were signs sent by the gods about the fate of the king and country. The sky was read like a text, and good reading required good records. That is why a tradition that started with omens ended up producing some of the most careful long-term observations from the ancient world.
It helps to avoid two modern assumptions. First, the Babylonians did not separate astronomy from astrology; the same scholars, called in Akkadian tupsar Enuma Anu Enlil (“scribe of Enuma Anu Enlil”), did both. Second, they had no physical model of the heavens in the Greek manner. Their strength was arithmetic: they looked for patterns in numbers and used them to predict.
Early sources
Several early texts survive in later copies. The Venus Tablet of Ammisaduqa lists the risings and settings of Venus over the reign of a king of the First Dynasty, and scholars still debate how reliable its numbers are. Enuma Anu Enlil was a huge series of omens, running to roughly seventy tablets, linking the Moon, Sun, planets and weather to events on earth. MUL.APIN, a compilation whose material is generally dated to around the late second or early first millennium BCE, gathers star lists, the dates when constellations rise and set, and rules for intercalating a month in the calendar. These texts show a developed vocabulary of constellations well before the 7th century BCE.
The Astronomical Diaries
The most striking source is the group of tablets called the Astronomical Diaries. The earliest preserved example covers 652 BCE, and the series continues, with gaps, until about 60 BCE, so the practice lasted around six centuries. Scribes, working in Babylon’s temple Esagila, recorded the Moon’s position relative to reference stars, the movements of the planets, eclipses, and meteorological events. They also noted the level of the Euphrates, the price of barley, dates, wool and other goods, and sometimes political events.
The Diaries are not predictions. They are raw material, a logbook of what was seen, including missed observations when clouds hid the sky. Later scribes pulled information from them into thematic summaries and into so-called Goal-Year texts, which used the idea that planetary behaviour repeats after fixed periods to forecast what should be seen in a coming year by looking at what happened one cycle earlier.
Eclipses and the saros
Careful records of lunar eclipses showed that they recur in a pattern. After a period of 223 lunar months, a little over 18 years, the Sun, Moon and Moon’s orbit return to nearly the same relationship, and a similar eclipse tends to follow. The word “saros” for this period is a later, modern use of an ancient Greek-derived term, but Babylonian scholars knew the cycle in practice.
They could therefore say when an eclipse was possible, and their predictions of lunar eclipses were reasonably successful. Solar eclipses were harder, because whether one is visible depends on where an observer stands, so the tablets give warnings rather than confident forecasts. When an eclipse was expected, a king might take ritual precautions; this concern for protecting the ruler connects astronomy to the world of Babylonian omens.
Mathematical astronomy
From about the 5th century BCE, and especially in the last few centuries BCE, Babylonian scholars developed tables, called ephemerides, that calculated the positions of the Moon and planets in advance. Modern scholars distinguish two main approaches, called System A and System B. In System A, a body’s speed is treated as constant within one stretch of the zodiac and then changes abruptly; in System B, it rises and falls in a gradual zigzag. Both are arithmetical schemes with no geometry of circles, and both worked in the sexagesimal number system inherited from earlier Mesopotamian mathematics.
A 2016 study of cuneiform tablets from about 350 to 50 BCE argued that some texts compute Jupiter’s displacement by finding the area of a trapezoid under a speed-against-time graph. That is a geometric method which scholars had earlier associated with medieval Europe. The interpretation involves a small number of tablets, but it shows how sophisticated the tradition became.
The zodiac as a belt of twelve equal 30-degree signs also appears to have taken shape in Babylonia around the 5th century BCE. The earliest known horoscopes, which describe a person’s birth date and planetary positions, come from roughly the same era.
What happened next
Greek astronomers received this material. Hipparchus in the 2nd century BCE used Babylonian eclipse records and period relations, and Ptolemy in the 2nd century CE quotes Babylonian observations in his Almagest. Some of the cycles built into the Antikythera shipwreck device, including its eclipse dial, are of Babylonian origin. The habit of dividing the circle into 360 degrees and degrees into 60 minutes is generally traced to the same arithmetic tradition.
Other peoples developed their own systems, such as Egyptian sky watching with its star clocks, or the Dresden Codex Venus table kept by Maya scribes, so Babylonia was not the only place where careful observation led to prediction. What marks the Babylonian case is the length and continuity of the written record.
Astronomical cuneiform kept going into the Roman era. The latest dated astronomical tablet comes from about 75 CE, which also marks nearly the last use of cuneiform writing anywhere. The learned tradition of Babylon’s temples was thus one of the longest scientific traditions known from antiquity.