Experiment prepares to test a possible second target for Stonehenge: ScienceAlert

When it comes to its connection to the heavens, Stonehenge is best known for its solar alignments.

Every Midsummer’s Eve, tens of thousands of people gather at Stonehenge to celebrate and witness the rising sun, aligned with the Heel Stone standing outside the circle. Six months later, a smaller crowd gathers around the Heel Stone to witness the setting of the midwinter sun in the stone circle.

But there has been a hypothesis for 60 years that part of Stonehenge also coincides with the rise and set of the moon during what is called a great lunar standstill. Although a connection between the location of certain stones and the great lunar standstill has been known for decades, no one has systematically observed and recorded the phenomenon at Stonehenge.

This is what we want to do in a project that brings together archaeologists, astronomers and photographers from the universities of English Heritage, Oxford, Leicester and Bournemouth and the Royal Astronomical Society.

There is now an abundance of archaeological evidence suggesting that solar alignment was part of Stonehenge’s architectural design. Around 2500 BC, the people who erected the large stones and dug an avenue into the chalk seemed to want to cement the solstice axis into the architecture of Stonehenge.

Archaeological evidence from the nearby Durrington Walls, where scientists believe the ancient people who visited Stonehenge stayed, indicates that of the two solstices, the midwinter period attracted the largest crowds.

But Stonehenge contains other elements, such as 56 pits arranged in a circle, a rampart and ditch, and other smaller elements such as the four station stones. These are four sarsen stones, a form of silicified sandstone common in Wiltshire, which have been carefully placed to form an almost exact rectangle encompassing the stone circle.

Only two of these stones remain, and they pale in comparison to their larger counterparts as they are only a few meters high. So what could their purpose be?

Lunar standstill

The rectangle they form is not just any rectangle. The shorter sides run parallel to the main axis of the stone circle and this may be a clue to their purpose. The longer sides of the rectangle run along the outside of the stone circle.

These longer sides are thought to align with the great lunar standstill. If you were to mark the position of the moon’s rise (or set) over the course of a month, you would see that it moves between two points on the horizon. These southern and northern limits of the moon’s rise (or set) change on an 18.6-year cycle between a minimum and a maximum range – the so-called minor and major lunar standstills, respectively.

The great lunar standstill is a period of about one and a half to two years during which the northernmost and southernmost moonrise (or set) are furthest apart. When this happens, the moon rises (and sets) beyond the range of sunrises and sunsets, which may have given meaning and significance to this celestial phenomenon.

The strongest evidence we have for humans marking the great lunar standstill comes from the southwestern US. The Great House of Chimney Rock, a multi-story complex built over 1,000 years ago by the Ancestral Pueblo people in the San Juan National Forest, Colorado.

It sits on a ridge that ends at a natural formation of two rock pillars – an area that has cultural significance to more than 26 Native American tribes. From the vantage point of the Great House, the sun will never rise in the gap between the pillars.

However, during a great standstill, the Moon rises between them in an awesome way. Excavations unearthed preserved timbers, meaning researchers could date back to the annual installments of the Great House’s construction.

Of the six logging dates, four correspond to major lunar standstill years between the years 1018 and 1093 AD, indicating that the site was renovated, maintained, or expanded during successive major standstills.

Returning to southern England, archaeologists believe there is a link between the Great Lunar Standstill and the earliest construction phase of Stonehenge (3000-2500 BC), before the sarsen stones were brought in.

Several sets of cremated human remains from this phase of construction were found in the southeastern part of the monument in the general direction of the southernmost major moonrise, where three wooden posts had also been set into the bank. It is possible that there was an early connection between the location of Stonehenge and the moon, which was later emphasized when the station’s stone rectangle was built.

However, the main hypothesis about the lunar standstill raises more questions than it answers. We don’t know if the lunar alignments of the station stones were symbolic or if people were intended to observe the moon through them. We also don’t know which phases of the moon would be more dramatic to see.

A search for answers

In our upcoming work we will try to answer the questions raised by the main hypothesis of lunar standstill. It is unclear whether the moon would have been strong enough to cast shadows and how they would have interacted with the other rocks. We will also have to check if the alignments are still visible today or if they are blocked by forests, traffic and other elements.

The Moon will align with the station stone rectangle twice a month
approximately February 2024 to November 2025, which gives us plenty of opportunities
to observe this phenomenon in different seasons and phases of the moon.

To bring our research to life, English Heritage will livestream the southernmost moonrise in June 2024 and host a series of events throughout the year, including talks, a pop-up planetarium, stargazing and storytelling.

Across the Atlantic, our partners at the US Forest Service are developing educational materials about the Great Lunar Standstill at Chimney Rock National Monument. This collaboration will result in events showcasing and discussing the moon phases at both Stonehenge and Chimney Rock.

Fabio Silva, Senior Lecturer in Archaeological Modelling, Bournemouth University; Amanda Chadburn, Fellow of Kellogg College, University of Oxford and Visiting Fellow in Archaeology, Bournemouth University, and Erica Ellingson, Professor of Astrophysics, Emeritus, University of Colorado Boulder

This article is republished from The Conversation under a Creative Commons license. Read the original article.