UK archaeologist suggests Stonehenge may have been an ancient solar calendar

Scholars have long speculated that the famed prehistoric monument Stonehenge might have served as some kind of calendar that helped local people predict eclipses, summer and winter solstices, the equinox, and other relevant celestial events. Now, a British archaeologist has concluded that the site was designed as a solar calendar, and he describes his system in a recent paper published in the journal Antiquity.

"Finding a solar calendar represented in the architecture of Stonehenge opens up a whole new way of seeing the monument as a place for the living—a place where the timing of ceremonies and festivals was connected to the very fabric of the universe and celestial movements in the heavens," Bournemouth University archaeologist Timothy Darvill said.

Stonehenge is located on Salisbury Plain in Wiltshire, England. It consists of an outer circle of vertical sandstone slabs (sarsen stones), connected on top by horizontal lintel stones. There is also an inner ring of smaller bluestones and, within that ring, several free-standing trilithons (larger sarsens joined by one lintel). Radiocarbon dating indicates that the inner ring of bluestones was set in place between 2400 and 2200 BCE. But the standing arrangement of sarsen stones wasn't erected until around 500 years after the bluestones.

No contemporary written records exist concerning the monument's construction, and scholars have pondered its likely use and cultural significance for centuries. Stonehenge's form (and maybe its purpose) changed several times over the centuries, and archaeologists are still trying to piece together the details of its story and the stories of the people who built it and gathered in its circles.

Darvill, for instance, has championed the hypothesis that Stonehenge was a place of healing. Alternatively, University College London archaeologist Michael Parker Pearson speculates that the site was part of a broader ritual landscape, serving as a funerary monument (a possibility first proposed in the 12th century by Geoffrey of Monmouth). People apparently buried their dead (after cremation) at the site for several centuries. Those burial sites are located in the circle of pits now called the Aubrey holes.

In 2019, Parker Pearson and several colleagues reported the results of their investigation into the quarry source for the bluestones. They found that the 42 bluestones came all the way from western Wales. Chemical analysis has even matched some of them to two particular quarries on the northern slopes of the Preseli Hills.

One quarry, an outcrop called Carn Goedog, seems to have supplied most of the bluish-gray, white-speckled dolerite at Stonehenge. And another outcrop in the valley below, Craig Rhos-y-felin, supplied most of the rhyolite. When another group of archaeologists studied the chemical isotope ratios in the cremated remains of people once buried beneath the bluestones, those researchers found that many of those people may have come from the same part of Wales between 3100 and 2400 BCE.

But the sarsen stones hail from much closer to home. Since the 1500s, most Stonehenge scholars have assumed the sarsen stones came from nearby Marlborough Downs, an area of round, grassy hills 25 to 30km (17 miles) north of Stonehenge which has the largest concentration of sarsen in the UK. A 2020 study by University of Brighton archaeologist David Nash and colleagues confirmed that.

As Kiona Smith reported for Ars at the time:

About 99 percent of the average sarsen boulder is silica, but the other 1 percent contains trace amounts of other elements, like aluminum, calcium, iron, potassium, magnesium, and others. That extra material is different in different sarsen sources, as it depends on the minerals in the ground where the rock formed. Nash and his colleagues used those trace elements as a geochemical fingerprint to match the Stonehenge sarsens to their most likely source.

Nash and his colleagues used portable X-ray fluorescence to analyze the chemical makeup of all 52 sarsens at Stonehenge (the only survivors of the 80 sarsens that once stood at the site). Each element emits a slightly different wavelength of light when hit by X-rays, and by measuring those emissions, researchers can map the composition of an object without damaging it.

Fifty of the sarsens shared very similar chemical fingerprints, which means they probably all came from the same place, most likely one site in the southeastern Marlborough Downs: West Woods, about 25 km (16 miles) north of Stonehenge and just 3 km (2 miles) south of where most earlier studies had looked for Neolithic sarsen quarries. The other two surviving sarsens came from two different places, which archaeologists haven't pinpointed yet.

Nash's 2020 study turned out to be key to Darvill figuring out his solar calendar system. "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," he told New Scientist. That spurred him to take a closer look at the numerical significance behind the stones. Nash compared their numerology to other known calendars from the same period—most notably the solar calendar adopted by Egypt around 2700 BCE.

Darvill concluded that each of the 30 stones in the sarsen circle represents one day in a month, which in turn consisted of three weeks of 10 days each. Specific stones in the circle mark the start of each new week. To be consistent with the solar year, Darvill's system also incorporates an extra month of five days—represented by the five trilithons at the center of the site—as well as a leap day every four years.

"30, 5, and 4 are interesting numbers in a calendrical kind of sense," said Darvill. "Those 30 uprights around the main sarsen ring at Stonehenge would fit very nicely as days of the month. Multiply that by 12 and you get 360, add on another 5 from the central trilithons you get 365." He described it as "a perpetual calendar that recalibrates every winter solstice sunset."

"It's certainly intriguing but ultimately it fails to convince," Parker Pearson told New Scientist. "The numbers don't really add up—why should two uprights of a trilithon equal one upright of the sarsen circle to represent 1 day? There's selective use of evidence to try to make the numbers fit." Nash, however, said that he liked "the elegant simplicity" of Darvill's system, which made sense to him.

DOI: Antiquity, 2022. 10.15184/aqy.2022.5 (About DOIs).