When it comes to analysing potential alignments at Stonehenge, there's one problem that always gets in the way. Well, there are several actually but the one I'm going to describe here is pretty fundamental.
The Sun, today, does not rise and set at the solstices at the same positions on the horizon that it did when Stonehenge was built.
This is due to an oddity in the way the Earth's rotational axis behaves. We're all familiar with the idea that Earth's axis is tilted away from the vertical at an angle of about 23.5° - it's in all the diagrams of the planet, it's there in every globe you've ever seen on a desk and it's why we have any seasons at all.
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| The archetypal desk globe with the planet's characteristic 23.5° tilt |
We might imagine that it's always been 23.5°, but it hasn't. Over very long periods of around 41,000 years this angle varies from between 24.2° and 22.5° and - we assume - back again. This is called the "Change in the Obliquity of the Ecliptic" (but don't worry about terminology for now).
4,500 years ago, when Stonehenge was built, the angle of tilt was roughly 24° give or take a couple of minutes of arc. As a result the Sun at summer solstice rose and set fractionally further round to the north than it does today, and at winter solstice it rose and set fractionally round to the south than today.
The different isn't much - about a degree or so, but it really affects observations of solstice rises and sets at Stonehenge.
For example, today the Sun at summer solstice rises out of the tip of the Heelstone, but in 2,500 BCE it rose off to the left of it - as seen from the main axis of the monument between Stones 30 and 1 on the NE side.
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| Summer Solstice Sunrise position today and 4,500 years ago |
What this means is that there is no way to make direct observations of the Sun at the solstices as it would have been seen by the builders of the monument. Instead, we have to calculate its prehistoric position (which we can do to an excellent degree of accuracy) and create mockups like the photo above to visualise how it would have looked.
(Incidentally, if you want more about what would have happened and what you would have seen at summer solstice sunrise back in the day then take a look at the article on this site called "The Shadow of the Heelstone")
Frustrating! Except there's a way around the problem thanks to the movements of the Moon.
The Sun's rising position on the eastern horizon varies throughout the year. In summer it's in the NE then it turns around at solstice and starts heading south. By the time of the winter solstice it's reached the SE before it turns back and begins its journey northwards again.
Back and forth, regular as the pendulum of a clock, and in a human lifetime the endpoints of this swing (and the corresponding ones on the western horizon for sunsets) are effectively fixed.
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| The Sun's annual swing along the eastern horizon from Summer Solstice (left) to Winter Solstice (right) and back |
The Moon also swings back and forth along the horizon between endpoints, but it does it every month. Each month there's a northernmost limit for rising (and setting) and two weeks later a southernmost limit. NE to SE to NE to SE to NE to SE.... tick tick tick.
What's different about the Moon is that the endpoints are not fixed like those of the Sun. The furthest north and south positions move back and forth as well. Every 18.6 years these endpoints get as far apart on the eastern horizon (for risings) as they ever can do, and the Moon can rise and set way further north (and south) than the Sun can ever reach in our era.
In the animation above, the yellow diagonal lines on the far left and right are the endpoints of the Moon's swing when it reaches its maximum extent. See how they're further apart than the Sun's endpoints (the orange diagonal lines)? The one in the middle marks the Equinox.
We're at that point in the Moon's cycle now, in 2024/25. It's called the Major Lunar Standstill. It's a poor label since it's not the same sort of "standstill" than the Sun experiences at the solstices, but we're stuck with the name. Never mind.
As always the Moon still keeps swinging along the horizon from day to day over the course of a month so it can rise anywhere between the endpoints of the swing. It's just that the endpoints are further north and south than the Sun's can ever be today. I know I just said that, but it's a critical point!
So what does this mean?
It means that if you pick your moment, you can find the Moon rising and setting at the points on the horizon where the Sun of 4,500 years ago did, so you can therefore use the Moon as a "proxy" for that ancient Sun and can directly observe how things would have looked when Stonehenge was built.
The only difference is that it's the Moon you're looking at rather than the Sun. Happily, they both appear the same size in the sky and it's less damaging to the eye to stare at the Moon :-)
Back in the late 1990s/early 2000s, Prof. Gordon Freeman of the University of Alberta - who'd spent years doing direct observations on site at Stonehenge of solstices - proposed a remarkable idea.
He suggested there was a secondary solstice axis that ran from Winter Solstice Sunrise to Summer Solstice Sunset and that there was a deliberate sightline through the stones that made use of a notch in the edge of Stone 58 coupled with the edge of Stone 53 on the opposite side of the monument. This combination created a "peephole" of sorts that was directed at Coneybury Hill where the ancient Winter Solstice Sun would have risen.
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| View through the Notch in Stone 58 whose open side is closed by the edge of Stone 53 to frame a very specific spot on Coneybury Hill |
(For more detail on this idea, see the article on this site called "The Secondary Solstice Axis")
On July 21st 2024, the Moon rose very close to the same position that the ancient Winter Solstice Sun once did, so here was an opportunity to attempt a direct observation of Gordon's idea - was his sightline valid? By calculation, it definitely is but a chance to observe the Moon proxying for the Sun along this line was not to be missed.
Unfortunately I wasn't able to gain permission to do the observation from within the monument itself, although if all goes well I might be able to do so during one of the next times things align in the same way, so I had to try and get the data from a position out in the landscape on the projection of the alignment to the NW up towards the Cursus Barrows.
The horizon was perfectly clear and at 21:55:42 BST (20:55:42 UT) the first gleam of the Moon's upper limb appeared behind Stonehenge.
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| First gleam of the Moon - that tiny orange blob just left of centre on the horizon |
By 21:58:52 BST, it had reached half-orb and I was getting excited.
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| 3m 10s after first gleam things are looking promising |
A little under four minutes later, the alignment fell into place.
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| The Moon acting as proxy for the ancient Winter Solstice Sun |
It is extremely difficult to get into exactly the right spot for a shot like this - I don't have a differential GPS system to give me sub-centimeter accuracy, so I had to rely on eyeballing the monument in daylight through binoculars to get into the best position I could, so I may be a couple of metres to the left or the right of the ideal spot.
I'm certainly higher up in elevation than I would be at the monument, so the Moon appears above the horizon from this vantage point whereas it'd be resting on it otherwise.
Nevertheless, this is an excellent result and shows that it's definitely worth repeating the observation from within Stonehenge itself.
I did notice other photographers around - one actually on the visitor path next to Stonehenge - but none of them were in the right spot to get the desired (for the purposes of checking the alignment) photo. I'm sure they got fabulous images of the Full Moonrise with these hoary old stones beneath, and I look forward to seeing them.
My photos are somewhat grainy (old camera), but they're worth far more to me - and potentially to future researchers of the ancient astronomical sightlines at Stonehenge - than a pretty shot of Moonrise.
Well done for spotting this one in the first place Gordon, you are an inspiration.
