Moon, Sun and Ruins: a partial eclipse

Our venue: Reculver Towers and Roman Fort on the north Kent coast (UK); taken in the early morning. This enigmatic site is managed by English Heritage, this image is taken from their website – here.

Partial eclipses of the Sun by the Moon are not particularly rare. There have, for instance, been five of them visible from the UK in the decade leading up to the most recent, when ~90% of the Sun was occluded by the Moon shortly before sunset on 12th August 2026. Total eclipses, by contrast, are harder to come by unless you’re willing to travel; the last one visible from my spot on the globe was in 1999 – and it was obscured by cloud. C’est la vie.

Most people are aware of the cause of an eclipse: the Moon covers the Sun for a short period of time when viewed from the right place on the Earth’s surface. If they line up perfectly from our perspective then we’ll witness a total eclipse[1], a little bit off-centre and we’ll be able to enjoy a partial eclipse of one degree or another. This relies on two facts: although the Moon is 400 times smaller than the Sun it is also, on average, 400 times closer to us[2]; also, the Moon orbits the Earth in pretty much the same plane as the Earth orbits the Sun. Thus, the sizes are just right and the chances of everything lining up are good. If you’d like a basic animated explanation, try this cartoon from Nasa, and if you want to know why we don’t see one every month, try this one-minute explanation.

Hopefully, these images will illustrate the importance of size. At the top are images from NASA: the August 12th total eclipse as witnessed in Spain and the not-so-total eclipse generated by the largest of the two small Martian moons, Phobos. The latter was taken from the surface of Mars by a camera on the Perseverance rover. See here for details on the Phobos image; for the Moon images see here and here. (For the image of the Martian moon Phobos passing in front of the Sun I am grateful to Andrew and Ben, fellow members of my amateur astronomy society, for reminding me of its existence.) My own image of Callisto crossing the face of its parent planet – look for the dot on Jupiter’s face at the 1 o’clock position – prompts a further illustration of the effect: all four of its larger moons, the Galilean moons – Io, Europa, Ganymede and Callisto – are able to generate a total solar eclipse if viewed from the top of Jupiter’s clouds. This is for the simple reason that the Sun appears tiny at that distance: Jupiter is five times as far away from the Sun as we are on Earth and the Sun therefore appears to be only a fifth of the size. Thus, even the smallest of Jupiter’s Galilean moons, Europa – which is only about 90% the size of our Moon – is easily able to cover the Sun.

That’s probably enough by way of introduction and background, but if you’d like to read more about the Sun then please explore my blog (e.g. here). The intended focus of this post is the deep partial solar eclipse on August 12th when, from my part of the world, the silhouette of the Moon covered approximately 91% of the Sun’s face at its peak. The peak occurred at 19:13 local time and from start to finish the Moon’s transit across the Sun lasted about 110 minutes. The end of the event occurred shortly before sunset (20:10), by which time the apparent circular shape of the Sun’s disk had been distorted by the atmosphere into an oval. Given the timing, it was important to observe the event from a location with an unincumbered horizon. Thankfully, I’m surrounded by nearby coastline having a view across the North Sea towards the setting Sun.

The excellent amateur astronomy society I’m a member of, SEKAS, does a lot of community outreach, which I enjoy, including joint events with English Heritage. On this occasion, the joint venture was to be held at their medieval church and Roman fort site at Reculver on the north Kent coast (southeast England). The site occupies raised ground right on the coast and provided the perfect venue since we could view the Sun through to its setting into the sea’s far horizon. Thus, five SEKAS amateur astronomers had between them ten telescope-based setups of various kinds to allow a safe view of the Sun whilst the English Heritage people made available free solar viewing glasses for those people who’d pre-registered online. (There was a shortage of the ISO 12312-2 safety standard viewers in the days leading up to the event and prices rose to ridiculous levels. My own stock, purchased months beforehand, went as gifts to family, friends and English Heritage. Thankfully, the lovely people from my local Beacon Astronomy Group arrived with a box full, which they handed out as needed. I also saw many people sharing what they had with those around them, which was good to see.)

For my part, I took two telescopes [3] as well as a semi-automatic astronomical camera for recording images. I’d also made a pinhole camera from a shoebox … just for fun. For a ‘normal’ outreach event this would have been more than enough. However, apart from English Heritage’s own publicity, BBC local news had listed the event as one of the best ways to observe the partial eclipse. Thus, despite turning up three hours before the event began in order to be able to park and to set up, individuals and family groups arrived soon after wanting to ask questions and to look through the telescopes. In the end, an estimated 1,000 people were spread over the grassy banks around the site and we were constantly busy for almost five hours. It was exhausting, but also huge fun and very rewarding. I spent the whole time skipping between my three telescopes, answering questions and being shown images that people had captured through a telescope eyepiece or with their solar viewing glasses held over their smartphone camera lens. Thankfully, I had my daughter and a grandson there to lend a hand when needed, and my wife rescued me long enough to allow me to eat a sandwich, take a long drink and generally cool down in the shadow of the two towers.

A selection of monochrome stills extracted from my ‘roboscope’. From left to right: the Sun just before the eclipse began with the visible sunspots labelled; a few minutes into the eclipse as the Moon begins to occlude the solar disk; moments before the peak of the eclipse – I missed the actual peak as I was busy helping someone see it through a telescope and I also missed the fact that the ‘roboscope’ exposure length had changed such that everything was over-exposed; the final stage of the eclipse as the Moon’s silhouette moves off to the left. You’ll notice that in the rightmost image the atmospheric distortion has given us an oval disk with surface details like the sunspots blurred due to turbulence. I converted the original images, each of which was derived from a stack of 40 individual frames processed in AffinityPhoto2, from colour to monochrome because the solar filter on my ‘roboscope’ has the annoying property of making the Sun look orange-yellow rather than its natural off-white.

The animated sequence of all the individual images I captured that afternoon is above; it may also be accessed on my YouTube channel, using the link here. Ben Harding, chair of SEKAS and the co-organiser of the event with English Heritage, was interviewed by ITV news – see here and scroll down to the Reculver section. Another video, which is well worth viewing, was recorded by a local multimedia journalist, Q Cummins, Kent reporter for Herne Bay based Hits Radio and Greatest Hits Radio. It captures the response at the peak of the eclipse from the good-natured crowd of people watching the event:


This short video is shared with the kind permission of Q Cummins / Bauer Media, to whom I am grateful.

Best of all were the lovely, inquisitive people – from toddlers to pensioners, both locals and those here from countries in mainland Europe and as far afield as the USA and Japan – who came, talked and observed, and who made the outreach event one of the highlights of my amateur efforts. The following images were taken by my wife or daughter or SEKAS members …

Footnotes:
[1] There is a caveat to this in that the Moon’s orbit is not perfectly circular and so the Earth–Moon distance does vary slightly; if it passes in front of Sun when at its furthest from Earth – when it therefore appears to be a little smaller – we see an annular eclipse. This gives us a narrow ring of sunlight circling the Moon’s silhouette rather than a total eclipse. A full Moon is often given the name ‘Super Moon’ if it coincides with the point in its orbit when it’s closest to the Earth. Either way, the Moon covers only about ½° of the sky – as does the Sun – and can be hidden behind one of your fingernails when held at arm’s length.

[2] Eclipses haven’t always been exactly as we witness them today. Way back in the Cambrian period (~500 million years ago), when the first explosion of life was taking place, the Moon was much closer to the Earth. The Moon would therefore have appeared significantly larger in the sky and would have hidden the Sun more easily. In addition, the Earth was spinning faster: a day lasted only about 21 hours and each year comprised a little over 400 days. The Moon is still moving away from us (at approximately 38 mm/year) and our days are still getting longer (by a couple of milliseconds each century).

[3] For those who like detail: a 150/1200 mm Newtonian with a 32 mm eyepiece on a manually operated equatorial mount; a 72/420 refractor with a 25 mm eyepiece on a Sun-tracking mount; a ‘robotic’ imaging system which tracked the Sun and displayed captured images in real time on my smartphone. Although the perceived brightness of the day doesn’t change noticeably through a partial eclipse because our eye-brain combo compensates, even during a partial eclipse as deep as this one, the temperature did fall noticeably. My objective evidence for this came from the ‘roboscope’: its internal running temperature was 59°C during the early stages of the eclipse, but by the peak this has dropped by 10°.