surprises in the tablets. In 2016, historian Mathieu Ossendrijver found that the priests were using geometric techniques in their astronomy too. He reported a Babylonian tablet that recorded a calculation of the distance Jupiter had travelled using a method equivalent to plotting its velocity against time and then calculating the area under the graph. This method was previously thought to have been invented by European astronomers in the fourteenth century AD.
8 Instead of a decimal number system, the Babylonians used a sexagesimal system with sixty as its base (as we still use for degrees and time today). Both the Babylonians and Hipparchus used exactly the same figure of 29;31,50,8,20 days, which converts to 29.5306 days (29 days, 12 hours and 44 minutes). The modern value is also 29.5306 days.
4
FAITH
On 28 October 312, two armies clashed just outside Rome. Two brothers-in-law, Maxentius and Constantine, were fighting for control of the western Roman Empire, which stretched from Britain to north Africa. Maxentius held Rome, while Constantine advanced across the Alps from Gaul. On the eve of the battle, he set up camp a few kilometres north of the city walls.
What happened to Constantine that day, described by ancient authors such as Eusebius, Bishop of Caesarea, has acquired legendary status as one of history’s great turning points. At around noon, the power-hungry emperor saw a divine vision: a flaming cross of light above the sun, emblazoned with the words ‘Conquer by this’. It was enough to convert Constantine from paganism to Christianity. He ordered the sign – made from the first two letters of Christ’s name, Chi and Rho, superimposed – to be painted on his soldiers’ shields.
Maxentius, meanwhile, claimed protection from Mars, the Roman god of war. Rattled by his rival’s military victories, Maxentius had tried everything he could think of to halt Constantine’s progress, from conducting temple rituals and sacrifices to reading animal entrails and omens in the sky. He prepared for a siege behind the capital’s impregnable walls, stockpiling grain and destroying the stone arches of the Milvian Bridge, Constantine’s route across the Tiber into Rome. But he wasn’t sure the citizens would remain loyal; at chariot races in the Circus Maximus on 27 October the crowds shouted Constantine’s name. The next day, after consulting a collection of oracles called the Sibylline Books, Maxentius chose instead to meet his rival in open battle.
With the bridge disabled, Maxentius’s men crossed the Tiber on a temporary platform made from wooden boats, meeting Constantine’s army a few kilometres north of its banks. The attacking forces, though much smaller in number, pushed Maxentius’s troops back. With no escape route, thousands of his soldiers were forced into the river and drowned, until, according to one account, the water could barely penetrate the piles of bodies. Maxentius perished too as he tried to flee, weighed down by his armour. Constantine fished his body out of the mud and paraded his severed head through the streets.
The victory gave Constantine undisputed control of Rome’s western territories. Fighting under the Chi-Rho symbol, he later gained its eastern lands too – from Macedonia as far as Syria and Egypt. After generations of instability and civil war, he finally united the Roman Empire. Although the west fell within a century or two as Rome’s political influence over its vast territory gradually disintegrated, the eastern empire, ruled from his new capital Constantinople, lasted for another thousand years.
The significance of Constantine’s victory goes far beyond geopolitics. Throughout his reign, the emperor broke with centuries of religious tradition, single-handedly transforming his chosen faith from a minor, persecuted sect into a hugely powerful church. His conversion paved the way for Christianity, rather than the old planet-based gods, to become the dominant religion not just for Rome but the entire western world. The Battle of the Milvian Bridge, then, marks a key moment in an even greater clash in human history: between the sky worship of early civilisations and the monotheistic religions that dominate today.
To commemorate his victory over Maxentius, Constantine built a huge stone arch near the Colosseum in Rome. It still stands, spanning the grand, ceremonial route taken by emperors as they entered the city in triumph, and it features a giant inscription, originally cast in bronze, that attributes his success to ‘divine inspiration’. Many historians have taken that phrase as referring to Constantine’s epic moment of conversion on witnessing the flaming cross in the sky. But over the last few years, scholars like the art historian Elizabeth Marlowe have pointed out that the marble sculptures and reliefs that cover the arch include no Christian symbols.
Instead, they show the Roman sun god, Sol. On the eastern side of the arch, Sol rises from the ocean in his four-horse chariot, balanced on the west side by the descent of Luna, goddess of the moon. Sol is identifiable elsewhere on the arch, too, from a band of light rays around his head – known as a radiate crown – and a raised right hand; in several places Constantine mirrors this pose. What’s more, Marlowe has shown that the arch was carefully offset from the road so that for approaching crowds, the view beyond it centred on a colossal bronze statue dedicated to the sun. Far from affirming Constantine’s Christianity, she says, ‘the favoured deity is unambiguously Sol’.
In other words, the emperor’s famous conversion isn’t everything it first appears. But then, neither is the victory of monotheism over the gods of the sky.
•
Most early societies worshipped the sky in some form, or associated their gods with celestial bodies. There are earthly gods too, of course, representing everything from animals and ancestors to rivers and crops. But the vast majority of religions – from all periods of history, anywhere in the world – have a prominent role for celestial beings. The very word ‘deity’ derives from a root that means ‘shining in the sky’.1
According to the twentieth-century Romanian historian Mircea Eliade, who surveyed hundreds of religions around the world, the sheer size and power of the sky drives spiritual experiences. Simply by being there, the heavens reveal how tiny we are in the cosmos at the same time as putting us in touch with the vast, unimaginable whole. ‘The sky, of its very nature, as a starry vault and atmospheric region has a wealth of mythological and religious significance,’ he wrote. ‘Atmospheric and meteorological “life” appears to be an unending myth.’
Some sky gods are associated with specific celestial bodies, such as Babylonia’s Marduk and Ishtar, or the Egyptian sun god, Ra. Others are supreme creator beings that are embodied by or live in the heavens. The goddess Mawu, worshipped in Benin, West Africa, wears the blue sky as a veil and clouds for clothes. Debata, known in Sumatra, releases lightning when he opens his mouth to smile. Qat, supreme being of the Banks Islands in Melanesia, created the dawn when he cut into night’s darkness with a red obsidian knife. But with the rise of the three great monotheistic religions – Judaism, Christianity and Islam – this wealth of celestial personalities was swept away. Their divine dramas were replaced, for much of humanity, by the concept of one, unchanging God.
It’s a revolution that started in Canaan, a region centred on Palestine, between the Jordan Valley and Mediterranean Sea. Ruled by the Egyptians for much of the Late Bronze Age, a people called the Israelites emerged here around 1250 BC. Texts and archaeological evidence suggest that early on the Israelites worshipped celestial bodies among a pantheon of gods, led by Yahweh (linked by some scholars to the sun) and his wife Asherah (associated with trees, and later Venus).
The region split into two kingdoms: Israel in the north and Judah in the south. The Assyrians destroyed Israel in 722 BC and exiled thousands of its people; in 586 BC, the Babylonians did the same to Judah. After Israel’s fall, a religious group emerged in Judah, centred in Jerusalem, which recognised Yahweh alone as the sole creator of the universe: a deity who couldn’t be depicted, and who forbade worship of all other gods. Surveys of religious texts from the period suggest this was initially a minority view, but that such ‘Yahweh-alone’ beliefs then strengthened among the exiles in Babylon.
In 538 BC, Babylon was in turn conquered by the Persians, who helped the exiles to return home and rebuild their Temple of Jerusalem. With Persian support, this monotheistic group came to control Judaean religious institutions, and they assembled and edited the documents that became