A rare phenomenon may reveal Stonehenge's connection to the moon

For the people who have gathered over the centuries at Stonehenge - the imposing prehistoric monument that has towered over Salisbury Plain in south-west England for 4,500 years - it was probably clear how the sun influenced its design.

The central axis of the stone circle was - and still is - aligned with the midsummer sunrise and midwinter sunset, with the stones dramatically framing the sunrise and sunset when the days are longest and shortest.

But are Stonehenge and possibly other megalithic monuments around the world also oriented to the moon?

The idea that Stonehenge is somehow connected to the moon was confirmed in the 1960s. However, this concept has not been systematically investigated, says Clive Ruggles, emeritus professor of archaeoastronomy in the School of Archeology and Ancient History at the University of Leicester.

This summer, archaeologists are using the little-known lunar phenomenon, which occurs every 18.6 years, to study it as part of their work to understand why Stonehenge was built.

Like the sun, the moon rises in the east and sets in the west. However, the rising and setting of the Moon moves from north to south and back again within a month. The northern and southern extremes also change over a period of about 18 and a half years. Lunar solstice is when the moon's northernmost and southernmost sunrise and sunset are farthest from each other.

"The moonrise changes every day, and if you track this over a month, you'll notice that there is a northern and southern boundary beyond which the moon never rises (or sets)," says Fabio Silva, senior lecturer in archaeological modeling at Bournemouth University.

"If you observe these boundaries for 19 years, you will notice that they change like an accordion: they expand to a maximum boundary (the major lunar stasis) and then begin to shrink to a minimum boundary (the minor lunar stasis)."

This major lunar stasis should occur in January 2025, but between now and mid-2025, the Moon may appear, to the casual observer, unusually low and high in the night sky during the lunar month.

If you're in one of those 19 years, you'll occasionally see the Moon rise or set much further north or south than most times. In the years in between, you'll never see her there," Ruggles said.

Despite the phenomenon's name, the moon doesn't actually stand still during this period, he says.

"What's standing still are those borders, and the moment that's going to happen is in January of next year," Ruggles added. "But for about a year on either side, if you happen to catch the moonrise at the right time, you'll see the moon rise extremely low (in the sky)."

Stonehenge is built of two types of stones: larger and smaller blue stones that form two concentric circles. Ruggles said the Stonehenge station stones, which form a rectangle around the circle, roughly coincide with the final positions of the moon during the lunation.

How this lunar alignment was achieved, whether it was intended and what its potential purpose is - these are topics of discussion that the team wants to explore./BGNES