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Astronomical alignments were built into the design and orientation of Stonehenge – the imposing monument that dominates a flat plain in southwest England.

The central axis of the megaliths was – and still is – aligned with the sunrise at midsummer and sunset at midwinter, the stones perfectly framing the rising and setting sun when days were at their longest and shortest.

But it has long been thought the monument was used for ceremonial purposes rather than an accurate way to track the days, months and seasons.

However, a new study by Timothy Darvill, a professor of archaeology at Bournemouth University in the United Kingdom, has concluded that Stonehenge served as a solar calendar and identified how it may have worked.

Stonehenge is made of two types of stone: larger sarsen stones and smaller bluestone monoliths from Wales. The latter were thought to have been the first to be erected at Stonehenge 5,000 years ago, centuries before the larger sarsen stones, which came from a site much closer to the monument.

A ring of 30 upright sarsen stones, supporting 30 horizontal lintels, represent the days within a month. Distinctive stones in the circle mark the start of three 10-day weeks, according to the study.

Twelve such months would come to 360, but a group of “trilithons” – a structure formed of two large vertical stones supporting a third stone set horizontally across the top – were arranged in a horseshoe shape in the center of the site. Those represent the extra five days needed to match the 365-day solar year, Darvill said.

A map of the sarsens of Stonehenge shows how the calendar might have worked.

Four smaller stones that lay outside the circle in a rectangle were a way to keep track of a leap year, with an extra day every four years.

“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,” Darvill said in a news statement.

“A place where the timing of ceremonies and festivals was connected to the very fabric of the universe and celestial movements in the heavens.”

This perspective of Stonehenge, viewed from the northeast, shows the post-and-lintel construction of the sarsen stones.

However, other experts weren’t convinced by Darvill’s argument.

“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? And there’s selective use of evidence to try to make the numbers fit: some of the stones have been left out because they evidently can’t be made to fit,” said Mike Parker Pearson, a professor at University College London’s Institute of Archaeology and the leader of The Stones of Stonehenge research project, via email.

The starting point for Darvill’s explanation was new research that found the 30 massive sarsen stones were all sourced from the same area and added during the same phase of construction, suggesting they were a single unit.

And while only 17 of the 30 upright stones are in their original positions and 22 of the lintels are missing, archaeological work at the site has suggested it wasn’t the case that the monument was unfinished, rather these massive stones were lost in antiquity.

While the solar calendar is unfamiliar today, it was used in ancient Egypt and other cultures in the eastern Mediterranean around the same time. It was possible that the builders of Stonehenge were influenced by these people, Darvill said.

Recent discoveries of graves and artifacts near the stone circle have shown that Stonehenge was not home to one isolated group but part of a deeply interconnected world.

The findings were published in the journal Antiquity this week.