December 3, 2021

I, Science

The science magazine of Imperial College

In this festive feature, Artur Donaldson takes a look at the role of the winter solstice in numerous cultural customs and celebrations at this time of year.

It is undeniable – the world is spinning off-kilter. No, I do not refer to yet another exciting addition to the history books before the close of this year’s jam-packed chapter. I argue a more perennial phenomenon is also influencing our culture – the winter solstice.

The 23.4° tilt of the Earth’s axis, as can be seen in the mounting of a desk globe, makes itself chillingly evident with long, dark nights and bitter weather for those in higher latitudes. But it is also why this time lends itself to celebration. In fact, mid-winter and solstice festivals (where there is no winter season) have arisen in almost all cultures.


Whether you are tucking into an almighty Christmas turkey, or a Yalda dinner with watermelon and dried fruit, or glutinous rice balls as part of Dongzhi, you are joining in a truly global feast.

But why do people unanimously choose to feast precisely when food is most scarce and difficult months lie ahead? The cold requires that animals that choose not to hibernate, such as humans, require more nutrients to cope with the winter. A much slower metabolic rate during the winter months allows these nutrients to be better absorbed.

Moreover, it makes sense to slaughter animals so that they do not have to be fed through the rest of the winter, and the solstice marks a perfect time to undertake the task. In fact, for much of history, mid-winter may have been one of the few times in the year when meat has been abundant.


To be pedantic, the solstices are the instants where one pole of the Earth is tilted directly towards the Sun. However, they are more helpfully thought of as the shortest and longest days of the year, since finding the solstice is like trying to decide exactly when a swinging pendulum stops at the edge of each swing – quite tricky without a nicely aligned marker (like a very large stone). This is one of the reasons why mid-winter celebrations, from the Roman “Dies Natalis Solis Invicti” (“Birthday of the Unconquered Sun”) to modern day Christmas, do not fall exactly on the winter solstice (20th-21st December in the North) but are generally within a few days.


Except … December 25th is not the birthday of Jesus, according to the biblical texts and Christian scholars themselves! The famed shepherds, for instance, would not be tending their sheep by night beyond September.

How did it come to be that the birthday of the central figure of one of the world’s most popular religions was forgotten? The most likely explanation is the social momentum behind the winter solstice. From the very beginning, Christianity assimilated mid-winter celebrations from the cultures it reached, from the Romans (“Dies Natalis Solis Invicti”) and Norse (Yule) whose message of birth and hope was reinterpreted in terms of the birth of Jesus – and all this because of the Earth’s tilt.

What does the Solstice do for us?

Wouldn’t it be far better if we did not have to endure the extreme cold altogether? What if the Earth was not tilted? And where did the tilt come from in the first place?

The axial tilt, or obliquity, of Earth arose from meteoric impacts as well as the gravitational influence of the Moon, Sun and other planets. Early on, the obliquity of the inner planets, and hence the length of days, is believed to have varied wildly. Gradually the Sun, and especially the Moon, reduced Earth’s wobble to a 41,000 year cycle in its tilt between roughly 22.1° and 24.5° (we are currently at 23.4°).

Furthermore, this wobble, known as a Milankovitch cycle, is a major long-term contributor to Earth’s climate, and consistently matches up with the timings of ice ages. Thus if the Earth wobbled too much or unpredictably, it could have led to very unstable climate and would have eliminated the possibility of life developing altogether.  Mars, for instance, still undergoes drastic changes in its tilt from 15° to 45°.

However, the seasonal variation in temperature has helped humanity tremendously. Winter eliminates many disease-bearing insects that plague tropical regions, who carry, for example, Ebola and malaria. Even more so, such a season-less planet might have been only sparsely populated by small communities, according to Don Attwood, a cultural anthropologist from Montréal university. The only inhabitable regions on a planet like this, the tropics, could not support crops such as wheat, maize and potatoes, which were crucial to the formations of large settlements on our “wonky” Earth.

Bah Hum-Bling!

Undoubtedly, such a planet, with vast polar wastelands, would have a much smaller inhabitable land mass and a very different climate. Thus humans would face different challenges in creating a civilisation and would miss one astronomical event to celebrate. It is holidays like Christmas that could only emerge in sophisticated civilisations. On days like that we unwittingly celebrate what we have our planet’s tilt to thank for – our own existence. It is humbling to think that if cosmic chance hadn’t given us a nudge, Christmas might have been cancelled.

Artur Donaldson is studying for an MSc in Physics

Banner image: Stonehenge, jb_pics