Show caption ‘I like to go for wintry walks, on crisp days in pale winter sunshine.’ Photograph: Alamy Stock Photo Opinion The religious case for Christmas is well known. But there’s a scientific one too Alice Roberts Our urge to mark midwinter and celebrate the days growing longer precedes Christianity by thousands of years Fri 20 Dec 2019 11.51 GMT Share on Facebook

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I love Christmas. At this very special time of year, when the sun appears only fleetingly to those of us living in the northern hemisphere, I feel a deep connection with ancient ancestors. I like to go for wintry walks, on crisp days in pale winter sunshine, finding a Neolithic monument or an iron age hillfort to explore with my children. It also feels right to hunker down, to be with friends and family, to celebrate friendship with the exchange of gifts, and to give to good causes. I love creating cosiness at home – a bit of Danish hygge – with candles and firelight, holly and pine cones. It’s all “good for the soul”.

Occasionally, friends who know I’m a humanist – that I don’t believe in any sort of gods, or indeed an immortal soul – have questioned why I would celebrate Christmas. But while Christians may choose to commemorate the birth of Jesus on 25 December, I don’t see any reason why others shouldn’t enjoy a midwinter ritual whose roots go back much further than the origins of Christianity.

In the late third century AD, the Roman emperor Aurelian established an official religion, based on a popular sun cult, to help bond the empire together. Priests of Sol were elevated to membership of the senatorial elite – rather like bishops gaining automatic seats in the House of Lords today. In the fourth century, the emperor Constantine was still stamping the solar deity on his coins, as well as declaring dies Solis, Sunday, an official day of rest. But the sun–god just didn’t seem to have the power to prevent the Roman empire from falling apart.

Meanwhile, a fairly obscure Jewish apocalyptic cult from Roman Palestine had grown, over three centuries, to become part of the fabric of Roman towns and cities. Constantine seems to have had a change of heart, or at least of policy. Perhaps this relatively new–fangled religion could help give the empire the common focus and identity it needed. In AD313, Constantine decriminalised various cults, including Christianity. Twelve years later, he convened the Council of Nicaea – a gathering of Christian bishops from across the empire – in what is now Turkey. He processed through the assembly, resplendent in purple robes decorated with gold and precious gems. Christianity had gained an imperial seal of approval. Just before Constantine died, he was baptised. After his death in AD337, coins were minted, not with Jupiter or the sun–god, but with the hand of a Christian god reaching down towards the deceased emperor.

By the end of the fourth century, Christianity had become the state religion of the Roman empire. And the early church had selected four “official” gospels – about the life of Jesus – for inclusion in the New Testament. Only two – Luke and Matthew – describe, with some contradictions, the birth of Jesus. The historian Ronald Hutton describes how the Jesus of those two gospels enters the world as an archetypal hero, fulfilling Hebrew prophecy. In Luke, shepherds go to find Jesus. In Matthew, an unspecified number of wise men, sometimes portrayed as kings, arrive. Nativity plays usually throw all the elements together, with kings and shepherds beating a path to the stable.

‘Nativity plays usually throw all the elements together, with kings and shepherds beating a path to the stable.’ Photograph: Daniel Berehulak/Getty Images

Nowhere in the gospels is the date of Jesus’s birth specified. The first time it’s pinned down to 25 December, or at least to the “Eighth day before the Kalends of January” is the year AD354, in the Roman calendar of Philocalus. The date seems to have been chosen to replace the prior festival, decreed by Aurelian, of the unconquered sun – Sol Invictus.

As a midwinter festival of the sun, the date makes celestial sense. It falls just after the solstice, when the days are perceptibly lengthening. Pagan Romans started their midwinter celebrations with the feast of Saturnalia on 17 December, ending them with a new year festival, the Kalendae Januariae, at the start of January – both were celebrated with parties and the exchange of gifts.

It’s difficult to know what festivals were celebrated by our pagan ancestors in pre–Roman times – as this takes us into prehistory, which is defined by a lack of history. A few prehistoric monuments are lined up with the position of the sun on the horizon at key times: the solstices and the equinoxes. The monumental Neolithic passage grave at Newgrange, in Ireland, is aligned with sunrise around the time of the winter solstice, and Maes Howe on Orkney is lined up with sunset on that day. In Dorset, there’s a huge, six–mile cursus, defined by parallel banks and ditches, and perhaps used for processions, which is also lined up with the midwinter sunset.

Stonehenge is famously aligned with midsummer sunrise, and possibly also intentionally with midwinter sunset. Archaeological excavations at nearby Durrington Walls suggest that people were gathering and feasting there during winter months. So perhaps these monuments were used as part of solstice–related festivals. Despite all of those, and others, it’s important to note that most prehistoric monuments have no such solar alignments.

Old pagan myths make their way down to us in early medieval Irish and Welsh literature – but those ancient stories were written down by Christians, and contain few hints about earlier festivals. St Patrick gave us a clue about surviving pagan beliefs, though, in the fifth century, when he condemned sun worship: “All those who adore it shall descend into misery and punishment.” In Anglo–Saxon literature, the feast of the nativity is simply described as “midwinter” – which could possibly hark back to the pre–Christian festival. It’s not until the 11th century that the festival gets its familiar English name, Cristes Maesse – Christmas. In the same century, with the Danish king Cnut ruling England, the Scandinavian name for the midwinter festival – juul or Yule – also became popular. There are plenty of historical hints – if not definitive traces – of a pre–Christian midwinter festival in northwest Europe.

So I don’t see any hypocrisy in celebrating a festival which predated Christianity, and which reflects a real, celestial rhythm. It comes at a time of year when we can – in a very scientific frame of mind – celebrate the moment when we reach that point in our orbit when the days have contracted and are just starting to lengthen again.

I love the sparkle of sunlight on frosty hedgerows, the crunch of icy grass underfoot, choosing gifts for my family and friends, the smell of cinnamon and cloves in mulled wine, decorating the tree. I even have a soft spot for a nativity play. You can enjoy a good myth without believing in it, after all.

So – have a joyful Yule, happy Solstice, merry Christmas – whatever you believe in and however you choose to celebrate it!

• Alice Roberts is professor of public engagement in science at the University of Birmingham