says historian of astronomy James Evans. The scribes no longer needed to rely on long lists of past observations, just a small set of numerical parameters to define the behaviour of each celestial event.7 Epping had uncovered the moment when humanity transitioned from simply experiencing phenomena in the sky to explaining them.
And that wasn’t the only surprise hidden in the crumbling clay.
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In 336 BC, more than 1,600 kilometres northwest of Babylonia, a young prince called Alexander ascended the Macedonian throne. Over the next five years he carved out a huge empire, winning Greek states, then Asia Minor, then Egypt. And in October 331 BC, after a decisive battle against Persian forces on the plains near Nineveh, he marched his armies to Babylon.
According to the later Roman historian Quintus Curtius Rufus, while many of the inhabitants climbed the city walls to watch Alexander the Great arrive, most went out to meet him as he approached the blue-glazed gate. Officials carpeted the ceremonial road with flowers, and lined it with silver altars, heaped with perfume. They sent out gifts – herds of cattle and horses; lions and leopards in cages – and showed off their cultural treasures with a procession of musicians, wise men and the scribes of Enuma Anu Enlil. Surrounded by his armed guard, Alexander entered the gate by chariot and went straight to the royal palace. Taken by the city’s beauty and antiquity, he made it his new capital. His victory ushered Babylon into the Greek world – and brought the scribes into contact with the astronomers and philosophers of the west. Their two views of the cosmos could not have been more different.
Whereas the temple priests saw celestial events as written on a flat tablet, Greek scholars were interested in three dimensions; they wanted to know how the solar system was arranged. And while the Babylonian belief in omens meant precision mattered above all else, the Greeks had little tradition of accurately observing the sky. They based their models on lofty, philosophical ideals.
In the fourth century BC, the dominant figure in Greek thinking was Alexander’s tutor, Aristotle. His fundamental assumption was that since the heavens are divine, they must be structured in the appropriately perfect and efficient way: a series of spheres. He proposed a spherical Earth at the centre of the cosmos, surrounded by concentric circles or spheres that carried the orbits of the sun, moon, five known planets and fixed stars. The only imaginable heavenly motion was constant speed in a perfect circle, but that couldn’t explain why the planets sometimes stop and change direction. In the third century BC, western astronomers came up with an elegant solution: the planets move in small circles, called epicycles, at the same time as tracing a larger loop around the Earth. Off-centre orbits were suggested to explain the varying speed of the moon and sun. These geometric theories included no accurate numbers; the principle was what mattered. Until the second century BC, that is, when an astronomer named Hipparchus changed everything.
Born around 190 BC, Hipparchus worked on the island of Rhodes and seems to have conducted a one-man revolution of Greek astronomy, essentially transforming this philosophical art into a practical science. He made extensive astronomical observations, and is credited with compiling the first star catalogue. He also slated his peers for their sloppiness, arguing that their models of the cosmos were useless if they didn’t accurately match what happened in the sky. His attitude, according to James Evans, ‘represented a radically new way of regarding the world – at least among the Greeks’. Hardly any of Hipparchus’s work survives directly, but the later mathematician and astronomer Ptolemy reports that Hipparchus used astronomical observations to derive accurate numbers – period relations – to describe the cyclic behaviour of the sun, moon and planets. Then he used the new maths of trigonometry (and possibly even invented it; Hipparchus was the first we know of to use such techniques) to plug these numbers into the existing geometric models.
‘Hipparchus turned a broadly explanatory geometric model into a real theory,’ says Evans. He wasn’t able to fully explain the motions of the planets using Aristotle’s perfect circles. But for the first time, the Greeks could calculate the position of the sun or moon in the zodiac for any given date.
The next great astronomer of the ancient Greek world was Ptolemy. Working in Alexandria in the second century AD, he built on Hipparchus’s work in a monumental text, the Almagest, in which he set out a logical, mathematical explanation for all the movements seen in the sky, derived step by step from observations. It included the planetary theory that eluded Hipparchus: Ptolemy suggested that epicycles move through the sky at a constant speed not as seen from Earth or from the centre of their orbit but from a third point, which he called ‘the equant’. Though complicated, this scheme was impressively accurate, and the Almagest proved to be one of the most influential science books ever written, defining a view of the cosmos that lasted for 1,500 years.
For much of recent history, then, this chain of events was thought to explain the origin not just of western astronomy but of scientific thinking in general, part of the so-called ‘Greek miracle’, as Evans puts it, ‘as if the Greeks had in one swoop invented science, along with history, poetry and democracy’. But in 1900, Epping’s colleague and successor, Franz Kugler, read something unexpected in the Babylonian tablets, the full significance of which would not be realised for many decades to come.
Kugler, from a landowning family in Königsbach, Bavaria, was square-jawed, determined and difficult. Another ex-student of Epping, he was appointed as maths professor at a Jesuit college in the Netherlands, and taught himself Akkadian in order to take over analysis of Strassmaier’s drawings in 1897, a few years after Epping died. He had a strained relationship with Strassmaier, complaining that his colleague’s frequent linguistic suggestions were not helpful for his astronomical analysis. He was also a scathing critic of ‘Panbabylonism’, a school of thought that emerged in the late nineteenth century which argued that the Hebrew Bible was directly derived from Babylonian culture and mythology, and that the Babylonians had developed highly sophisticated astronomy as early as the third millennium BC.
It was Kugler who worked out much of the detail of the astronomical theories that Epping had unearthed. And he noticed something odd about the period relations that the Babylonians used to calculate the behaviour of the moon.
This was the priests’ most complex theory. To fully describe the moon and predict all-important lunar eclipses, they had to combine several different lunar cycles: the moon’s variation in speed (anomalistic month); progression through its phases (synodic month); and the time it takes to travel between the ‘nodes’ where it crosses the sun’s path (draconitic month). To do this, the Babylonians ultimately used a cycle of nearly 350 years, from which they derived the average length of the synodic month as precisely 29.5306 days.8 Kugler noticed that the numbers in this theory were identical to those used by Hipparchus. In other words, Hipparchus didn’t derive the numbers in his theories from observations at all. He took them from the astronomers in Babylon.
In fact, over the last few decades it has emerged that pretty much all of the numbers on which Hipparchus’s theories were based, including his period relations for the planets, come from Babylonian tablets. Historians already knew that some aspects of Babylonian maths and astronomy had filtered to the Greeks, including the zodiac signs and the base-60 number system (which Hipparchus was one of the very first Greeks to use). But the Babylonians were still seen as primitive stargazers, inferior to the scientifically minded Greeks. The French Assyriologist George Bertin, for example, responding to Epping and Strassmaier’s findings in 1889, insisted that even if the Greeks had adopted some of the priests’ terminology, it was the Babylonians who had learned astronomy from the Greeks: ‘The Babylonians . . . soon discovered the accuracy of their new masters in science.’
The discovery of Hipparchus’s numbers embedded in older Babylonian models turns that view upside down, proving that the fundamental ingredients for his theories came from the temple tablets. More evidence is still being uncovered. In 2017, Australian researchers claimed that a Babylonian
