The ancient monument of Stonehenge has been studied for a long time to understand its possible links to ancient astronomy. The site lines up with the direction of the sunrise on the summer solstice and the sunset on the winter solstice.

Early work

Stonehenge has an opening in its earthwork facing northeast. Some people believe the builders intentionally aligned the monument with the solstices and equinoxes. For example, during the summer solstice, the Sun rises near the Heel Stone, and its first light shines into the center of the monument between the horseshoe-shaped stones. While this alignment might be a coincidence, it was noticed as early as 1720, when William Stukeley mapped the site and identified its main axis pointing toward the midsummer sunrise.

Stukeley observed that the Heel Stone was not perfectly aligned with the sunrise. Changes in Earth's tilt over time do not fully explain this misalignment. Recent discoveries suggest there may have been a second stone near the Heel Stone, which no longer exists. This missing stone might have been part of a "solar corridor" designed to frame the sunrise.

Stukeley and astronomer Edmund Halley tried to date Stonehenge using early scientific methods. Stukeley believed the builders used a magnetic compass, but the compass’s direction at the time did not match true north. He calculated the difference between the observed and expected sunrise position to estimate the monument’s age. His earliest calculation suggested 460 BC, though this was incorrect. This attempt, however, marked an important step in archaeology.

Earlier efforts to date Stonehenge relied on changes in the stars’ positions. In 1864, H. Broome proposed the monument was built in 977 BC, when the star Sirius would have risen over Stonehenge’s Avenue. Sir Norman Lockyer suggested 1680 BC, based on a mistaken alignment of the Avenue with a modern survey marker. William Petrie estimated a later date of 730 AD, but he noted the stones were leaning, making his findings less reliable.

In 1963, Gerald Hawkins published Stonehenge Decoded, claiming the monument had many solar and lunar alignments and could predict eclipses. His use of a computer for calculations made his work widely known. However, archaeologists questioned his findings, especially after other experts, like C. A. 'Steve' Newham, Sir Fred Hoyle, and Alexander Thom, proposed alternative theories. Later, Richard J. C. Atkinson and others criticized these ideas, pointing out practical challenges in the "Stone Age calculator" theory.

Gerald Hawkins' work

Gerald Hawkins first shared his research about Stonehenge in a scientific journal called Nature in 1963. He used a computer from Harvard-Smithsonian to analyze the monument. Hawkins discovered that many features of Stonehenge aligned with the positions of the Sun, Moon, planets, and bright stars as they would have appeared in 1500 BC. He studied 165 important parts of the monument and compared their positions to these celestial events. His findings showed that 13 solar and 11 lunar connections were very accurate when compared to older parts of the site, but later parts were less precise. Hawkins also suggested using the Aubrey holes to predict lunar eclipses by moving markers between them. In 1965, Hawkins and J.B. White wrote a book called Stonehenge Decoded, which explained his research and proposed that the monument acted as a "Neolithic computer."

In 1966, Atkinson wrote an article titled "Moonshine on Stonehenge" in Antiquity, challenging Hawkins' work. Atkinson argued that some pits Hawkins used for his sight lines were likely natural depressions, not intentional markers. He also noted that Hawkins had allowed for an error of up to 2 degrees in his measurements. Atkinson calculated that the chance of seeing so many alignments from 165 points was about 50%, not the "one in a million" possibility Hawkins claimed. Atkinson also pointed out that the Station Stones were built on top of the earlier Aubrey Holes, which made some of Hawkins' alignments between these features misleading. The article also criticized the idea that Aubrey Holes were used as astronomical markers and questioned the work of Fred Hoyle.

There is still debate about whether the weather in ancient England would have allowed people to observe astronomical events accurately. Modern researchers looked for alignments with events they already knew about, but the people who used Stonehenge long ago did not have this advantage.

Newham and the Station Stones

In 1966, C. A. "Peter" Newham studied how the equinoxes might be aligned at Stonehenge. He drew a line between one of the Station Stones and a posthole near the Heel Stone. He also found that the long sides of the rectangle formed by the four Station Stones matched the positions of the moon rising and setting during its major standstill. Newham also believed that postholes near the entrance were used to observe the saros cycle, a pattern that helps predict eclipses.

Two of the Station Stones are broken, and while their positions could form a rough rectangle, their exact age and connection to other parts of Stonehenge are unclear. Stonehenge's latitude (51°10′44″ N) is special because only near this location (within about 50 km) do the solar and lunar alignments described earlier form right angles. If Stonehenge were more than 50 km north or south of its current position, the Station Stones could not be arranged as a rectangle.

Alexander Thom's work

Alexander Thom studied stone circles beginning in the 1950s to find evidence of astronomical alignments and a unit of measurement called the megalithic yard. He started focusing on Stonehenge in 1973. Thom did not consider alignments between structures inside the monument because he believed they were too close to be accurate. Instead, he searched for natural landmarks that might indicate lunar and solar events. However, one of Thom's important sites, Peter's Mound, was later discovered to be a 20th-century waste site.

Later theories

A study published in 2017 suggests that the average size of the moon and Earth might match the sizes of the stone circles and Earthworks at Stonehenge. While this similarity could be a coincidence, the same size relationship between the moon and Earth is also seen in the Earthwork at Stonehenge and the nearby circle at Durrington Walls.

Although Stonehenge is now a popular place to visit during the summer solstice, with 20,000 people visiting in 2005, researchers have found increasing evidence that prehistoric people may have visited the site only during the winter solstice. The only large stone structures in the British Isles with clear solar alignments are Newgrange and Maeshowe, both of which are aligned with the winter solstice sunrise.

Recent evidence supporting the idea of winter visits includes bones and teeth from pigs that were killed at nearby Durrington Walls. These remains show the pigs were killed in December or January each year. In 2005, Mike Parker Pearson of the University of Sheffield said, "There is no evidence that anyone was in the landscape in summer." Later, after more research, he suggested it is "reasonable to assume that they came to celebrate both the midsummer solstice and the midwinter solstice."