If you were a steadfast moon observer, tracking moon rises and sets every day, you would have noticed something odd over the past year. When Earth’s companion first swings above the horizon in the evening hours these days, it does so further to the south then it usually would. If you were to catch its morning descent, you would have seen it taking place further to the north. While the exact spot of the lunar rise and set usually shifts along the horizon throughout every month, over the past year or so, it has barely changed. Although most of us, 21st century humans, haven’t noticed, we are in a unique period in the moon’s life cycle known as the major lunar standstill. 

Scientists, however, think that unlike us, ancient humans paid much closer attention to the moon’s dance and even built their monuments to reflect that rare still period, which takes place about every 19 years. Now, a team of archaeologists and astronomers in the U.K. is using the two-year standstill to prove whether one of the U.K.’s most famous and mysterious landmarks — the 5,000-year-old Stonehenge — reflects the celestial phenomenon in its design. 

Archaeologists have known since the 1700s that Stonehenge was built to align with the movements of the sun. Just like the moon, the sun’s point of rise and set shifts along the horizon throughout the year. The sunrise reaches its northernmost point on the summer solstice in June; the sunset gets to its southernmost position on the winter solstice in December. 

“That Stonehenge was aligned with the sun is almost beyond any doubt,” Stonehenge archaeologist and archeo-astronomer Amanda Chadburn told Supercluster. “At the center of the monument is a composition of large stones that has a shape of a horseshoe. And if you draw an axis through that horseshoe, it aligns exactly with the summer solstice sunrise and the winter solstice sunset.”

The summer solstice draws thousands of people to Stonehenge every year. Thanks to some rather aggressive campaigning by the British neo-druid community in the 1980s, English Heritage, which looks after Stonehenge, allows unrestricted access to the monument on the eve of every June 21st. Crowds gather to spend the night observing neo-druidic ceremonies evoking the sun, which then magnificently rises above one special stone opposite the central horseshoe, casting its first ray into the center of the stone circle. Archeologists, however, believe, that for the ancient neolithic architects of Stonehenge, it was the winter solstice sunset they intended to revere.

“On winter solstice, the sun sets behind what once used to be the tallest of the trilithons that make up the horseshoe, it sets behind the heart of the monument,” Jennifer Wexler, an English Heritage archeologist, told Supercluster. “We think that for those ancient people, who were mostly farmers, the winter solstice really was the key moment of the year. It’s the moment when days start getting longer again and the light begins to return.”

Stonehenge’s famous trilithons are made of two vertical chiseled sandstone blocks with a third placed flat on top of them. The central horseshoe used to consist of five such trilithons, archeologists know. Only three still stand today, surrounded by six remaining arches of what once used to be the outer circle of 30 sandstone pillars connected by overlying lintels.

It was only in the 1960s, Chadburn said, that some Stonehenge researchers observed that four of the monument’s lesser-known stones appear to mark the extreme positions of the moonsets and moonrises during the major lunar standstill. These so-called station stones lie outside Stonehenge’s magnificent outer circle, forming a rectangular outline within which the heart of the monument sits. They also mark the diameter of the outer ditch, the oldest component of the monument. 

“If you are standing in the middle of the circle, you will see that one of those stones aligns with the most southerly point on the horizon where the moon rises,” Chadburn said. “The opposite way would be the northernmost moonset.”

Chadburn and Wexler are part of a small team of researchers that set out to prove whether this conspicuous alignment could be intentional. Moon rises and moonsets, Chadburn said, can be hard to observe, especially in England’s notoriously wet and murky weather. The lines that the station stones mark are also directly perpendicular to the winter solstice sunset – summer solstice sunrise axis, suggesting the whole lunar standstill theory could be a coincidence. 

Wexler said she hadn’t originally been convinced whether the moon was part of the inspiration behind the monument’s construction, but witnessing firsthand one spectacular moonrise persuaded her that the ancient inhabitants of south England’s Salisbury Plain would unlikely be oblivious to it. 

“We had this really tremendous moonrise just after the summer solstice this year,” Wexler said. “It was one of those most southern moonrises on the horizon and it was a really bright full moon. It looked almost as if the sun was coming back up in the sky. It was so prominent that you just thought — wow, how would those people back then not notice that.”

Dependent on the elements for their livelihood and undistracted by the internet and television in their evenings, the farmers that worked the south English land in the neolithic period between 4,000 and 2,000 years BC must have been acutely aware of the two odd light balls taking turns in the sky above their heads. They would have wondered why the nighttime orb suddenly began showing up more to the south; an event that would remain etched into their memory for years. 

“It’s suddenly coming up in a position that it normally doesn’t come up,” said Wexler. “We think that’s something they would possibly tell their kids about. That they saw this happening 18 or 19 years ago, and they would notice it happening again. We think they may have marked it in the construction of the monument as something potentially significant.”

What role exactly Stonehenge played in the ancient people’s lives is still shrouded in mystery. The neolithic farmers that erected the mighty blocks left no written records, so archaeologists can only make inferences.

Scientific research shows that many of the blocks, chiseled with basic tools such as hammerstones and antler picks, are not indigenous to the south of England. The most prominent sarsen stones — the enormous sandstone monoliths that make up the inner horseshoe and the outer ring — have been traced to an area known as West Woods, some 21 miles to the north of Stonehenge. 

That the ancient builders, most likely relying on tools as simple as plant-based ropes and wooden rollers, managed to transport the blocks over such a distance inspires awe. But the sheer enormity of the Stonehenge construction project is revealed in the origins of some of the less conspicuous elements of the monument. The so-called blue stones — smaller boulders complementing the circle — have been hauled from as far as the Preseli Hills in Western Wales, a mind-boggling 140 miles away from Stonehenge. Although the bluestones weigh only between 2 and 5 tons each, a fraction of the mighty sarsens, their transport to Stonehenge must have been a complicated affair requiring the cooperation of hundreds of people over many years. 

In a surprising paper published last year in the journal Nature, a team of British scientists claim they found mineralogical evidence that another of the monument’s building blocks — the seven-ton Altar Stone — came from as far as northeastern Scotland, over 430 miles away. The Altar Stone, in the past positioned in front of the largest trilithon of the inner horseshoe, likely played a major role in the sun worship rituals. It is exactly behind the Altar Stone that the sun disappears on the shortest day of the year. 

The country's wide representation at Stonehenge made researchers reconsider the significance of the monument in the lives of the people who built it. It also made them rethink their beliefs about the ancient British society that put the magnificent stone circle at its heart. 

Building on the Altar Stone paper, the University College London archeologist Mike Parker Pearson, the U.K.’s leading Stonehenge expert, postulated that Stonehenge, the most complex among the UK’s 900 stone circles, may have been constructed as a symbol of the unity of Britain’s neolithic inhabitants.

“The fact that all of its stones originated from distant regions suggests that the stone circle may have had a political as well as a religious purpose,” Parker Pearson said in a press release. “As a monument of unification for the peoples of Britain, celebrating their eternal links with their ancestors and the cosmos.”

Wexler said that archeological excavations in nearby Durrington Walls suggest that people from all over Britain would descend upon Stonehenge in winter months to survive the cold, barren part of the year and pray for the sun’s return. 

“Durrington Walls was a settlement a few miles away from Stonehenge where people lived as they were building Stonehenge,” Wexler said. “We found huge amounts of animal bones in that area that suggest that every winter, these people would kill huge amounts of animals and feast on a massive scale. It was an off season for them, so this was something akin to our Christmas.”

Once the days got longer and temperatures warmer, the overwintering crowd would disperse all over the UK to graze their animals and tend to the land, only to return to Durrington Walls the following winter.

Parker Pearson suggests that the national unification exercise that the construction of Stonehenge could have been may have prompted by arrivals of new tribes from mainland Europe. These new migrants, equipped with superior technology including the wheel and metalworking, gradually replaced the Stonehenge peoples shortly after the monument’s construction, spanning 1,500 years, had concluded. 

What exactly destroyed the culture that adorned the British Isles with such stunning megalithic monuments as Stonehenge is somewhat a mystery. They seem to have disappeared shortly after the completion of Stonehenge at around 1,600 BC. The greatest of their monuments, however, continues to stir the human imagination to this day. 

Scientists will now have almost until the end of this year to crack one of the stone circle’s lasting mysteries — to find out whether the monument reflects the movements of the moon. 

Wexler said the area around the station stone that marks the southernmost moonrise had been used as a burial ground when Stonehenge had first been conceived around 3,000 to 2,500 BC.

“In the early centuries of the monument before they built the stones, they had cremation burials in a series of holes that go along the outer perimeter,” said Wexler. “There seems to be a concentration of these burials around the southernmost position at Stonehenge where three wooden posts were placed in the past.”

Chadburn said that the whimsical nature of the British weather complicates the research, frequently obscuring the views of the rising and setting moon. On top of that, the orbit of the moon with respect to Earth has slightly shifted since the time of the Stonehenge builders, which means the alignments with the station stones cannot be expected to be perfect anyway. Chadburn, however, thinks that locations of the station stones with respect to the motions of the moon “seems very deliberate.”

If the researchers fail to obtain enough evidence, they will have to wait until 2043 to have another look.