Archaeologists working in Scotland have uncovered what they believe to be the world's oldest lunar calendar—a series of 12 large, specially shaped pits that were designed to mimic the various phases of the moon. The pits aligned perfectly on the midwinter solstice in a way that would have helped the hunter-gathers of Mesolithic Britain keep accurate track of the passage of the seasons and the lunar cycle.

At nearly 10,000 years old, these curious lunar-cycle-marking pits in Aberdeenshire are by far the oldest "calendar" ever discovered, pre-dating by several thousand years the Bronze Age monuments in Mesopotamia that until now had had that distinction. (Related: "Scientists Try to Crack Stonehenge's Prehistoric Puzzles.")

"What we are looking at here is a very important step in humanity's earliest formal construction of time, even the start of history itself," said Vincent Gaffney, professor of landscape archaeology at Birmingham University, who led the team that analyzed the pits and revealed their purpose.

The pits were dug in the shapes of various phases of the moon. "Waxing, waning, crescents, and gibbous, they're all there and arranged in a 50-meter-long (164-foot) arc," said Gaffney.

"The one representing the full moon is big and circular, about two meters (roughly seven feet) across and right in the center."

Intriguingly, this arc is aligned perfectly with a notch in the landscape where the sun would have risen on the day of the midwinter solstice 10,000 years ago. This was important, says Gaffney, because not only does this give further compelling evidence for the purpose of the moon-shaped pits, but also because without some form of calibration with the solar year a calendar based on 12 lunar months would soon be out of sync with the sun and become meaningless.

"Positioning their calendar in the landscape the way they did would have allowed the people who built it to 'recalibrate' the lunar months every winter to bring their calendar in line with the solar year."

And this is something they appear to have done, since the geophysical evidence suggests the pits had been maintained and periodically reshaped many dozens, perhaps even hundreds, of times over the succeeding millennia until at last the calendar-monument seemed to fall out of use around 4,000 years ago.

Keeping track of time and the seasons would have had enormous significance for the hunter-gatherer societies of Mesolithic Britain for both cultural and economic reasons, whether it was enhancing the perceived power of shamans and their ability to predict or "make happen" certain astronomical events, or knowing when the game would start to migrate or the salmon begin their run up the Dee River.

"The Dee Valley, where these pits are located, was an important crossroads and meeting area for a very long time," said Simon Fitch, a Mesolithic archaeologist who was involved with the discovery.

The pits themselves were first discovered by aerial photography in 2004, but it was only recently—using the latest-generation remote-sensing technology and specially developed software that worked out the positions of sunrises and sunsets in the landscape 10,000 years ago—that their significance was recognized.

"It shows that Stone Age society was far more sophisticated than we have previously believed, particularly up north, which until lately has been kind of a blank page for us," said Richard Bates, a geophysicist from University of St. Andrews who did much of the remote-sensing work for the project.