How to Tell Time Using Stonehenge

For millennia since its erection in 3300 BC, Stonehenge served as an important ritual structure to the Chalcolithic people of Southern England.

For millennia more after its abandonment as a place of worship, those who have studied the structure and lived in its shadow have speculated as to the exact significance of its design -- its ring of sarsens, 30 four-meter-tall stones joined together with lintel stones topping them, that surround a smaller ring of bluestones which themselves surround a set of five large trilithon stone formations. The central trilithon lines up with the points where the sun rises in the area on the summer and winter solstices.

The sunrise at the solstices aligns perfectly with the center of Stonehenge's Sarsen Ring.

The conjectures as to Stonehenge's purpose have ranged from the practical (the bluestones have particularly acoustic properties that may have allowed them to serve as musical instruments, according to an 02013 study) to the more fantastical (the monument was built by the sorcerer Merlin using healing stones carried by giants, according to 12th century chronicler Geoffrey of Monmouth). The archeological consensus, though, is that Stonehenge served at least in part as a calendar, tracking the progression of the year through the passage of celestial bodies across its architecture.

A new study published in Antiquity sheds light on the precise mechanism through which Stonehenge may have kept track of time for the people of Southern England. The study, written by Professor Timothy Darvill of Bournemouth University, notes that Stonehenge’s construction is more complex than contemporary archaeoastronomical sites in the British Isles. Unlike sites like Boscawen-Un in Cornwall, which consists only of a single ring of stones and a central marker, or sites like Maeshowe in Scotland, which has just a single opening that corresponds to the solstice, Stonehenge consists of multiple combined stone structures arranged in a variety of formations.

Timothy Darvill's reconstruction of Stonehenge's possible calendrical setup. Courtesy of Timothy Darvill/*Antiquity*

According to Darvill, these different arrangements allowed for the builders and maintainers of Stonehenge to use it as a remarkably accurate solar calendar with a mechanism for keeping track of leap days. The core of the calendar was in the structure’s outside ring of 30 Sarsen stones, each of which marked a single day within a 30 day solar month. Notably, the gaps between Sarsens 30 and 1, 10 and 11, & 20 and 21 are slightly larger than the rest, indicating the possible division of the 30 day month into three 10-day weeks. While the paper provides no architectural evidence for how the operators of Stonehenge kept track of the 12 months required to reach a solar year in accordance with the solstices, it is possible that a now-destroyed or moved element of the structure corresponded with the months.

Yet a cycle of 12 months of 30 days each only results in a cycle of 360 days. To represent the five missing “epagomenal” days required to match the earth’s orbit around the sun, Darvill proposes that the five Trilithons at the center of the Sarsen ring were used by ancient Britons as a way of marking the five days of winter solstice rituals that may have given rise to the more recent pagan celebration of Yule, which later became associated with the modern Christmas holiday.

Outside of the Sarsen circle, the Stonehenge site may also possess a tool to wrangle the trickiest aspect of the solar calendar: the need for an additional day every four years to match the 365.25 day procession of the earth around the sun. Nearby the ring lies four additional spots for Sarsens. While only two of these smaller external stones remain, the full set of four could have been used as a way to keep tally of when an extra day needed to be added to the calendar.

The Clock of the Long Now's Solar Synchronizer, much like Stonehenge's solstice aperture, uses the sun to correct error induced by human time-keeping practices over extended periods of time. Courtesy of Danny Hillis and Alexander Rose.

All told, Darvill’s research on Stonehenge paints a picture of a remarkably sophisticated Chalcolithic calendar that could have been used to inscribe time itself into the landscape of ancient England using principles that would seem familiar to a modern time-keeper. Its design — based around human ritual technology but aligned to the rhythm of the earth’s annual orbit around the sun — is even reminiscent of the Clock of the Long Now, which features an analog time-keeping mechanism — the Solar Synchronizer — that is regularly re-aligned with the pattern of the Sun.