Stonehenge may have been a giant calendar and now we know how it works

Stonehenge has long been thought to be an ancient calendar due to its alignment with the summer and winter solstices, but exactly how the calendar system worked was a mystery. Now a new analysis shows that it could have functioned like the solar calendar used in ancient Egypt, based on a year of 365.25 days, with each of the stones of the large sarsen circle representing a day within a month.

“It’s a perpetual calendar that recalibrates every winter solstice sunset,” says Tim Darvill at Bournemouth University, UK, who carried out the analysis. This would have enabled the ancient people who lived near the monument in what is now Wiltshire, UK, to keep track of days and months of the year.

The key to unlocking this calendar system came from the discovery in 2020 that most of the sarsen stones were quarried from the same location 25 kilometres away, and were placed at Stonehenge at around the same time.

“All except two of the sarsens at Stonehenge come from that single source, so the message to me was that they’ve got a unity to them,” says Darvill. To him, this indicated that they were intended for a common purpose. To find out what, he looked for clues in the numbers.

The sarsens were arranged in three different formations at Stonehenge around 2500 BC: 30 formed the large stone circle that dominates the monument, four “station stones” were placed in a rectangular formation outside this circle, and the rest were constructed into five trilithons – consisting of two vertical stones with a third stone laid horizontally across the top like a lintel – located inside the stone circle.

“Thirty, 5 and 4 are interesting numbers in a calendrical kind of sense,” says Darvill. “Those 30 uprights around the main sarsen ring at Stonehenge would fit very nicely as days of the month,” he says. “Multiply that by 12 and you get 360, add on another 5 from the central trilithons you get 365.” To adjust the calendar to match a solar year, the addition of one extra leap day every four years is needed, and Darvill thinks that the four station stones may have been used to keep track of this. In this system, the summer and winter solstice would be framed every year by the same pair of stones.

This Stonehenge calendar system “makes a lot of sense”, says David Nash at the University of Brighton, UK.  “I like the elegant simplicity of it.”

Others aren’t so sure. “It’s certainly intriguing, but ultimately it fails to convince,” says Mike Parker Pearson at University College London, UK.” The numbers don’t really add up: why should two uprights of a trilithon equal one upright of the sarsen circle to represent one day? There’s selective use of evidence to try to make the numbers fit.”

Although a calendar with 30-day months and an extra “intercalary” month of five days might be unfamiliar to us today, such a system was used in ancient Egypt from around 2700 BC and other solar calendars had been developed in the eastern Mediterranean region at around that time.

In the Egyptian calendar, these five extra days were “very significant, religiously speaking”, says Sacha Stern, an expert in ancient calendars at University College London. This has led Darvill to think that the five trilithon structures at Stonehenge might have marked a five-day mid-winter celebration, an idea bolstered by the fact that the tallest stone at the monument, part of one of the trilithons, points to the sunrise on the midwinter solstice.

The similarity between the Stonehenge calendar and that used in ancient Egypt hints that the idea for the Stonehenge system may have come from afar. Recent archaeological finds support the idea of long-distance travel and trade around that time. Isotope analysis of the body of the Amesbury Archer, who was buried 5 kilometres from Stonehenge around 2300 BC, revealed that he was born in the Alps and came to Britain as a teenager, and a red glass bead found 2 kilometres from the monument appears to have been made in Egypt around 2000 BC.

However, Stern isn’t convinced by the argument that the Stonehenge calendar system originated elsewhere. “I wonder if you need to invoke the Egyptians,” he says. “Why can’t we just imagine that [the people who built Stonehenge] created the whole system by themselves? They certainly knew when the solstice was, and from that point onwards you just have to count the days, and it won’t take long to figure out how many days you need in the year.”