At Moonrise on the 19th July 2024, the Moon was very close (within a quarter of a degree) to its maximum southerly declination in its 18.61 regression of the lunar nodes cycle.
I'd booked to go in to Stonehenge on the late evening "Special Access" session, in order to try and improve on my photo from July 9th 2006 of the Moonrise in line with the northeastern side of the Station Stone rectangle.
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| July 9th 2006 Major Lunar Standstill - 18 years and 10 days ago |
(In the mid-1960s, Peter Newham and Gerald Hawkins had independently suggested that the long sides of the Station Stone rectangle were aligned to the southernmost moonrise and northernmost moonset at the lunar standstill - see this article for the background to Newham's work.)
In the intervening 18+ years, English Heritage have placed a marker at the approximate position of where Station Stone 94 once stood (the Station Stone is no longer present, but its stonehole has been found through archaeology). There is a slight inaccuracy in its position (perhaps half a meter off), but it serves as a useful reference.
Moonrise was timed for 20:41 BST (19:41 UT) but the trees of Luxemborough Plantation on the SE horizon delay the Moon's appearance by around 5 minutes.
I set up my camera so that I was in line with the marker for SS94 and the (still present, though slumped) Station Stone 91 on the opposite side of the monument.
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| Initial photo on the alignment from SS94 marker to SS91. Features of other visitors pixellated for privacy. |
I knew I was further to the southwest of my 2006 position directly over SS94 (a spot which I'd then chosen by estimating the centre of the area inscribed by the bank of the North Barrow), so was expecting the Moon's position at appearance to be somewhat different to then, relative to the treeline.
With the camera taking a shot automatically every 15 seconds, I spent the next few minutes over the rope a couple of metres to the NE to try and spot the first gleam of the 97% waxing gibbous Moon over the treetops. Fortunately, there was no cloud cover at the horizon at all - which wasn't the case back in 2006.
And then, at 20:46:55, I spotted it peeping out of Luxemborough Plantation's trees. Rats! It was too far south (to the right) for my camera to have caught it - and would be blocked by the large sarsens of the NE side of the outer circle. I grabbed the tripod and quickly moved the camera to catch the Moonrise.
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| Off-alignment by about 2m to the NE with the Moon's upper limb just appearing. The shot has been contrast enhanced to improve visibility. |
OK, so this is somewhat annoying! First gleam over the true horizon would definitely have been visible from my original spot if there were no trees in the way. We really ought to cut a path through Luxemborough Plantation to restore this sightline.
Back home, I decided that it would a useful exercise to montage these two shots using the treeline as the reference to see what it would have looked like in an ideal world. The fractional parallax introduced by moving 2m NE off-alignment is tiny since the trees are a considerable distance away, so it's worth doing.
Here's the result.
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| Montage of original shot in the alignment position with the off-alignment photo, using the treeline as the registration reference. |
All very well, but how can I tell where the horizon (sans trees) would be? I can guess, but long experience of carrying out observations at Stonehenge has taught me that subtleties in both the shapes of the stones and the horizon profile are important and need to be considered.
Happily, due to the work of David Hoyle (www.standingstones.org) there is an excellent LIDAR/DTM terrain model for the Stonehenge landscape which can be loaded into Stellarium to give a pretty accurate representation of the actual horizon profile.
This is Stellarium's view of 20:46:55 BST on the 19th July 2024:
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| Stellarium view of Moonrise at the same instant as the first appearance of the Moon over the trees. The Archaeolines plug-in is being used to show the arcs of the Major Standstill Moon. |
A further montage, using the Moon size and the hill at 135° azimuth as reference points, allows me to see the whole picture.
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| Final montage with true horizon profile and rising arc of Major Lunar Standstill. My yellow block for SS91 may be a tad short - it was a quick guess for visualisation. |
At first glance, this looks excellent - the Major Lunar Standstill southernmost Moonrise appears as if it will emerge from the intersection point of the tip of Station Stone 91 and the true horizon. I suspect my yellow block is too short, but it was a rough positional indicator I plonked in while doing these montages. It's quite a long stone (3m or so) that's slumped right over and is resting on the earthwork bank.
Remember that the Moon on the 19th July 2024 is not precisely at its southernmost extreme declination, as is evidenced by the fact that Stellarium doesn't have it centred on the green rising arc lines.
However - there's one more factor we need to consider and that's the change in the Obliquity of the Ecliptic.
Earth's rotational axis is presently tilted over at roughly 23.5° to the plane of the Earth's orbit - which is why we have seasons (and indeed solstices that mark the turning points in the year). Back when Stonehenge was built, the tilt was 24°. This additional 0.5° tilt has an impact on the rising azimuths of the Sun and Moon at their extreme north and south limits.
In this specific case, 4,500 years ago when the large sarsens at Stonehenge were erected (or 5,000 years ago for the Station Stones - that's another story!) the Moon would have appeared to rise a further 1° to the right of where we are seeing it in these photos - that's two Moon diameters.
The implication of this is serious.
It means that the northeastern edge (long side) of the Station Stone rectangle is not precisely aligned with the southernmost possible Moonrise position - by something like 1.5° to 2° in azimuth based on these photos.
If we also factor in the potential inaccuracy of the position of the marker for SS94 it gets worse still - we may be looking at an error in alignment of up to 2.5° or 5 lunar diameters!
Newham and Hawkins' suggestion that the Station Stone rectangle's long sides are exactly pointed at the southernmost moonrise (SE) and northernmost moonset (NW) is starting to seem a little off.
We know, as modern astronomers, that the chances of catching the Moon rising or setting exactly at its extremest possible declination north or south of the celestial equator are very slight. The Moon races around its orbit (and hence our sky) really quickly so everything has to come together - orbitally and weatherly - for a precise observation of the extreme to be done. That may only be possible once in a generation.
What all this serves to show is that we need to devote proper resources to researching what is actually seen at Stonehenge during these occasions of rare potential astronomical alignments.
We need to be absolutely sure that if we're going to put modern markers in the ground, that they are accurately positioned which implies confirming earlier work about stonehole positions though new archaeological digs with modern DGPS instead of relying on 1950s surveys and interpretation.
And we need to seriously consider restoring sightlines in key directions by selectively felling some trees that obscure the true horizon. Luxemborough Plantation (SE), Larkhill (NE), Normanton Gorse (SW) and Fargo Wood (NW) will all need attention.
The questions that are raised here deserve further investigation. We have the chance over the next 12 months to refine my observations as we progress though this Major Lunar Standstill season (2025 is the key year).
Or not. In which case, we'll have to wait another 18.61 years for the next opportunity to do so.
