How did the ancient inhabitants of Stonehenge make use of this famous prehistoric monument? A new study discusses further evidence that it was a calendar and quite a special one – a 365.25-day solar calendar similar to others seen emerging in the Eastern Mediterranean, in particular in Egypt.

The idea that the circle of stones is a calendar is far from new but there has been very little agreement on how it worked. A number of solar and lunar calendars have been proposed to explain the organization of the Sarsen, the technical name for the standing stones. The stones are oriented towards the sunrise of the summer solstice, hence the solar connection.

The study, published in the journal Antiquity, argues that the Sarsen circle was made of 30 stones, each representing a day of the month, which was divided into “weeks” made of 10 days and marked by anomalous stones. Twelve months of 30 days make 360 days. The final five days to complete a solar year were marked using the five structures of the Trilithon Horseshoe, which are located prominently in the center of the monument. Thanks to the four station stones located outside, an extra day could be added every four years.

The function of this calendar would not have been to say "today is Wednesday, March 2, 2022 CE" or even to tell farmers when to plant their seeds or when to harvest their crops. The function of such a monumental calendar would be to mark the important celebrations, the festivals of those people.

“The calendar is there so that things happen on the days that the gods decree. Or turn it around, they are there when the god is there waiting to be appeased,” author Professor Timothy Darvill, from the University of Bournemouth, told IFLScience. “It is that relationship between the universe and the day-to-day life of the everyday people, which is why a calendar is an important part of that. And the people who control time are the people who control the deities.”

A 365.25 days solar calendar is awfully familiar and it brings forth the question, assuming that the interpretation is correct, of where this idea came from. Darvill suggests that it was imported to Britain as people moved across Europe.

“That pattern is one that we can see in other parts of the old world at the same time as Stonehenge is being built. It’s often referred to as the Egyptian Civic calendar although it was used well beyond Egypt,” Professor Darvill explained. “That calendar, of course, became the basis for a lot of the modern calendars that we have come to use.”

Around the time of the construction of Stonehenge, solar cults spread across Europe originating from the Egyptian cult of Ra. Thanks to mobile cultures originating in the Mediterranean, ideas spread and something like a calendar might have made it all the way to Britain. In the last 20 years, ancient DNA has shown that people really did move about.

“Stonehenge is a very long-lived and complicated monument. Deciphering it is difficult. And it is great that there is a lot of work going on in that area just at the moment,” Professor Darvill continued. “It’s like a great medieval cathedral or a great shrine from any religion... These buildings, these structures have a life of their own, and they change over the course of time... But at the end of it is not the building that is important, it’s the people who are in the building... They built a monument to do something with.”