Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news of fascinating discoveries, scientific breakthroughs and more.
CNN
—
To those who gather throughout the ages at Stonehenge – the imposing prehistoric monument that dominated the Salisbury Plain in southwest England for about 4,500 years—it was probably clear how the sun might have influenced its design.
The central axis of the stone circle was, and still is, aligned with the sun rising in summer and setting in midwinter, the stones dramatically outlining the rising and setting suns when the days were longest and shortest.
But does Stonehenge and potentially other megalithic monuments around the world also align with the moon?
The idea that Stonehenge was somehow connected to the moon gained ground in the 1960s. However, the concept has not been systematically investigated, said Clive Ruggles, emeritus professor of archaeoastronomy in the School of Archeology and Ancient History at the University of Leicester.
This summer, archaeologists are using a little-known lunar phenomenon that occurs every 18.6 years to investigate as part of their work to understand why Stonehenge was built.
Lunar stasis
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 stasis is when the moon’s northernmost and southernmost sunrise and sunset are the furthest apart.
“The moonrise changes every day and if you track this over a month you will notice that there is a northern and southern boundary beyond which the moon never rises (or sets),” said Fabio Silva, senior lecturer in archaeological modeling at Bournemouth University via e-mail.
“If you look at these limits over 19 years, you will notice that they change like an accordion: they expand to a maximum limit (major lunar stasis) and then begin to shrink to a minimum limit (minor lunar stasis).”
This major lunar stasis is due to 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, then you’ll occasionally see the moon rise or set much further north or south than most times. In the years in between, you never see him there,” Ruggles said.
Despite the phenomenon’s name, the moon doesn’t actually stand still during this period, he said.
“What stands still are these boundaries, and the time when that will happen is in January of next year,” added Ruggles. “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 made of two types of stones: larger sarsen stones and smaller blue stones that form two concentric circles. Ruggles said the Stonehenge station stones, which form a rectangle around the circle, roughly line up with the moon’s extreme positions during lunar stasis.
How this lunar alignment was achieved, whether it was designed, and its potential purpose are topics of debate that the team wants to explore.
Although there are no written documents that shed light on the meaning and significance of Stonehenge, archaeologists have long believed that its solar alignments were intentional. Such alignments have been identified in many places around the world and would have been relatively easy for ancient builders to identify, given that knowledge of the annual cycle of the sun and its relationship to the seasons would have been essential for subsistence.
However, it is much more difficult to say whether Stonehenge really has anything to do with the lunar stasis.
“I don’t think we can say definitively, but to me there is some evidence that made me think it was intentional,” Ruggles said.
One clue was the fact that archaeologists found cremated human remains clustered in the southeast, near where the southernmost moonrise would occur.
“I think there’s a possibility that they knew about that direction of the moon and then it became some kind of sacred direction,” Ruggles said.
Since April, Ruggles and Silva, along with colleagues from Bournemouth University, the University of Oxford and English Heritage, the organization that manages the site, have been documenting moonrise and moonset at key moments when the moon is aligned with the station’s stones. The moon was expected to line up with the station’s rocky rectangle twice a month from about February 2024 to November 2025, Silva said.
“This will happen at different times of the day and night throughout the year, with the moon in the right place at different phases each month,” Silva said in an April news release.
The team wants to understand what patterns of light and shadow the moon creates at Stonehenge and whether they might have had meaning for the people who built and used the monument.
Stonehenge is not the only megalithic monument potentially linked to a lunar stasis.
In the United States, Erika Ellingson, professor emeritus of astrophysics at the University of Colorado Boulder, is studying lunar alignments at Chimney Rock, a rocky ridge about 1,000 feet above the Colorado valley floor. The landmark features two large pillar-like rocks that frame the skyline.
Between 900 and 1150, the ancestors of the Pueblo people built multistory buildings and ritual spaces on this hard-to-reach high ground with a dramatic view, Ellington said, and it remains an important site for the 26 Native American groups that have traditional or cultural ties to the area.
“The extraordinary view of the sky between the twin peaks suggests an astronomical connection, but the gap is a little too far north for the Sun to ever shine through. However, the moon can be seen rising there when it is near its northernmost position, during the great lunar stasis season,” she said by email.
Additional evidence for moon sightings comes from the dating of wooden beams in nearby ancient buildings by wood rings, indicating that their construction is linked to the dates of the moon’s standstill nearly 1,000 years ago, she added.
The standing stones at Calanais, located on the Isle of Lewis in Scotland and predating Stonehenge, may also have a connection to the lunar stasis, Ruggles said.
Bradley Shafer, professor emeritus in the Department of Physics and Astronomy at Louisiana State University, said he is deeply skeptical that ancient people knew about the lunar stasis and built monuments to accommodate it. More likely, he suggested, it was a coincidence.
“Any ancient site has tens to hundreds of potential landmarks, and one or more will always point somewhere near one of the 8 stasis directions,” he said via email.
Lunar stasis is difficult for a casual moon observer to recognize, he added, and really only shows up in detailed moonrise and moonset observations.
Although the change in the moon’s position is subtle and historical records documenting a lunar standstill are rare and difficult to interpret, Ellington said he thinks the connection is plausible because many ancient people watched the sky very closely.
“An observer of the moon would see the moon begin to rise or set outside these boundaries, moving further and further outside the boundaries as the great lunar stasis approaches,” she said.
