The debt to Babylon 

Long before Rhodes, scribes in Babylon were keeping night-by-night notes. The tablets modern scholars call the Astronomical Diaries record positions of the Moon and planets, eclipses, weather, river levels and market prices, in a series that runs with gaps from the eighth century BC into the first century BC. It is one of the longest continuous observational records any civilisation has produced.

Out of that archive came predictive mathematics of real power. Babylonian astronomers isolated periodicities: the Saros, an interval of about eighteen years after which a similar eclipse pattern recurs, and cycles governing lunar month length and planetary appearances. Their methods were arithmetical rather than geometrical. They did not build a model of the cosmos in space; they built number sequences that reproduced what the sky did, and they worked.

They also bequeathed a system of counting. Sexagesimal notation, base sixty, is why a circle has three hundred and sixty degrees, why a degree has sixty minutes and a minute sixty seconds, and why we still divide the hour the same way. Every Greek astronomical table, including those of Hipparchus and Ptolemy, is written in fractions of sixty. When you set a clock you are using Mesopotamian arithmetic.

The transmission route was political. Alexander's conquests and the Seleucid kingdoms that followed put Greek-speaking scholars in contact with Babylonian temple archives for generations. Cuneiform astronomy continued under Greek rule, and material moved westward: cycle lengths, eclipse dates, methods. Ptolemy explicitly cites Babylonian eclipse observations from the eighth century BC, and modern scholarship on the parameters in Greek lunar theory finds values that match Babylonian ones closely enough to indicate borrowing rather than coincidence.

For Hipparchus this mattered enormously. His most celebrated results depend on comparing his own measurements with much older ones, and old measurements had to come from somewhere. A dataset spanning a few decades tells you very little about slow change in the heavens. A dataset spanning several centuries lets you detect motion so gradual that no single lifetime could reveal it. Historians credit him with using both Babylonian records and observations by earlier Greek astronomers, notably Timocharis and Aristyllus at Alexandria in the early third century BC.

It is worth naming the shape of the achievement honestly. Hipparchus is often described as the man who turned astronomy into a quantitative science, and there is truth in that: he combined Greek geometrical modelling with Mesopotamian numerical data and demanded that the model reproduce the numbers. But he inherited more than he invented, as everyone does.

Before he could compare old positions with new ones, though, he needed a way to record something the older records treated only loosely: how bright each star appeared.