Sun, Moon and Ruins: a partial eclipse

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

The enigmatic venue behind this post: Reculver Towers and Roman Fort on the north Kent coast (UK), photographed in the early morning. This beautiful site is managed by English Heritage; the image above 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.

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. The imaging system (a Dwarf3) had proprietary solar filters fitted. The two telescopes were fitted with bespoke homemade filters - see here for details.)

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°.

Too much Sun?

What a day: the last of the second heatwave in the UK year (so far) with temperature records having been broken multiple times and the whole thing made far worse by elevated levels of humidity; and what did I choose to do? I chose to stand in the Sun from ten in the morning until gone three in the afternoon.

Crazy? Maybe – but I prefer to believe that it was because I had the welcome chance, with three friends from my local amateur astronomy society, SEKAS, to show people our life-giving local star in all its energetically chaotic glory. The event that gave us this opportunity was my local village fête which, despite the uncomfortable weather, attracted large numbers of individuals, couples and families. Some of these lovely people I knew already, others I was meeting for the first time – either way, there were so many of them interested in what we were doing that the four of us barely stopped talking throughout the day. (The exception to this were the necessary times when we tried to keep hydrated; I was glad of the many bottles of chilled water I’d packed into a ‘cool-bag’ … and my sunscreen and wide-brimmed hat.)

Back in April I had contacted the Village Hall Committee, who support the annual fête’s organisation. Via our local social media page, I floated the idea of offering a safe solar observing experience. I’d half expected that I’d be told that it wasn’t really the sort of thing for a community fair – but the organisers evidently have a wider vision 😊. As a lovely bonus, the person who replied to my email, Tricia, reminded me that she’d stopped to chat one night whilst walking her dog – I was in my front garden with a telescope, naturally – and that I’d offered some advice on buying a telescope. Apparently, she now owns one … excellent!

In the two months between the gift of a south-facing pitch and the fête actually happening I had offers of help and support from fellow members of the South East Kent Astronomy Society – phew! I also tried to prepare the ground a little by posting a few of my own solar images so that people might get an idea of what to expect: “Just to whet your appetites in advance of the Blean Village Fête – at which, clouds permitting, you'll be able to see this sort of thing for yourselves - here's a some images of the Sun taken a few days ago from my garden. We'll have special equipment with us to enable you to do this safely - please, please, please don't look at the Sun directly as you can easily cause damage to your eyes ... and never, ever through binoculars or a telescope as blindness will follow. The whole-disk image was taken using a filter which let's all colours of the rainbow through but reduces the total by 99.999% in order not to damage the camera. Notice the three active regions (aka sunspot groups). Next is a close-up view around one of the sunspots - but this time it's taken using a specialised filter which only permits through one particular wavelength (colour) of light which comes from energised hydrogen. This has the beauty of picking out the chaotic and very energetic surface magnetic field associated with sunspots - just look at those swirls! Do you notice that there are whiter patches to complement the cooler dark sunspots? These are called plages, and they are regions where the surface is hotter than average. The darker loops are prominences that we're effectively observing from overhead. Finally, by over-exposing the disk of the Sun I managed to capture a very impressive prominence: super-hot plasma thrown out into space along the Sun's magnetic field lines. This one is many, many times the size of the Earth.”

Arriving at 10 am, we spent the next hour setting up an array of telescopes and getting their associated bits and pieces powered up. There were three fully automated ‘roboscopes’ in use as part of this array and they attracted a lot of attention. Mostly, this went smoothly – apart from one of my accessories which simply wouldn’t play ball; there’s always one. (Photos by the SEKAS Chairman, Ben Harding, and by the person who has responsibility for the SEKAS observatory site, Ashley Fuggle.)

We tried to introduce the experience by using a simple Newtonian reflecting telescope with its image projected onto a white background. Not only does this serve to illustrate that the Sun’s native colour is off-white and not yellow, but it revealed the sunspots nicely. (I’ve written about the colours of the Sun before if you’d like more background information: here and here.) Also, by placing a piece of card at the focal point of the telescope and noticing that it starts burning very quickly one immediately sees why looking at the Sun without appropriate equipment is a seriously bad idea. Other telescopes revealed crisp images of our white Sun and the day’s active sunspot regions by filtering out the majority of its light before it even enters the telescope itself. These are called white-light filters because they allow through a tiny fraction of all the colours (wavelengths) of the rainbow.

This is a colour image of the Sun taken on the day (by Ashley Fuggle, cropped and augmented by me) using one of the three smart telescopes in use – these impressive ‘roboscopes’, which may be controlled from an app on a smartphone, have a built-in astronomical camera. Each active region (- in this case a sunspot or group of sunspots) is given a numerical designation; for example, the large group centre left is AR-4478. To give you a sense of scale, the blue dot to the left of that region represents the size of the Earth: one could fit 109 ‘earths’ across the diameter of the Sun’s disk.

A highlight of the day for many people was the chance to look at the Sun not in white light but through a filter which eliminated all the wavelengths (colours) of light other than one of the wavelengths emitted by hydrogen, which happens to be a pinkish red. The advantage of this special and highly non-trivial method is that one can observe the flame-like prominences shooting out from the edge of the Sun’s disk. Prominences, the shape of which is influenced by the Sun’s strong magnetic field, are made from the Sun’s matter; in the main, this material falls back into the Sun under the action of its gravitational attraction. Sometimes however, the material has enough energy to break free and to stream out into space as a solar flare – if one of these travels in our direction then, a couple of days later it’ll arrive at our atmosphere and generate the aurora (the Northern and Southern Lights). If it’s powerful enough, we can even see the Northern Lights in the south of England. (Images by Ben Harding, edited by me.)

All-in-all a lovely, if hot, day for those of us who’d volunteered to offer the experience – and judging by the comments made on the day and on the village’s social media page in the days after, the whole thing was a great success. We’d love to return next year if they’ll have us. In fairness, I ought to share the comment of one person who declared that they’d already seen too much of the Sun and didn’t want to see any more – oh well, it’s not possible to please all of the people all of the time 😉

Attenborough in space

Engaged, inquisitive, sharp-eyed and bright: ‘Attenborough Class’ at Langdon Primary School, about five km northeast of Dover, were a real joy to meet. Attenborough is a mixed Year 1 and 2 class, so the pupils are six/seven years old; this is definitely the youngest group of budding scientists I’ve had the pleasure of meeting. However, any nervousness on my part – and there was plenty! – was dispelled when the first group of pupils came out in the care of their teaching assistant, Clare. They were evidently keen to see something new, and having recently completed a space-themed study topic they also had a pretty good idea of what it was they’d be looking at.

What a delightful village primary school, with a mix of ‘traditional’ and modern buildings quietly set within grass and all-weather playing areas and with mature trees dotted around. My classroom for the morning was a patch of lawn in front of the Headteacher’s office window – the green arrow will point it out for you. (The headteacher, by the way, is my totally wonderful son, Andrew.) The view south from there was over the fence and across the small staff car park. Fortuitously, a large branch had recently been removed from the over-hanging tree so that we had a good view of the Sun’s path – and of the Sun itself as the clouds kindly thinned and cleared.

My first task with each small group was of course to make sure they knew never to look at the Sun without the sort of special equipment I was using. After that I introduced them to my pet robot and its view of the Sun … I had set up a Dwarf3 smart telescope to track the Sun so that I could show them an image on my ’phone. It was easy to point out from the image the string of three significant sunspots visible that day and to talk about what sunspots were. Having laid the foundations, each of them got the opportunity to view the Sun in more detail through the eyepiece of a telescope fitted with an appropriate solar filter. As is the case for first-time observers of all ages, it can be tricky for some to position their eye at the ‘sweet-spot’ near the eyepiece, but with a little coaching from Clare or from me everyone got to see the Sun as they’d not seen it before. Clare, and then class teacher Emma and other staff members also had a look; naturally, who wouldn’t want to have a go.

 

(Images supplied by the school in accordance with their rules on privacy and parental consent. The bald head is my responsibility.)  There’s nothing quite like having the privilege and the pleasure of watching someone see something qualitatively new to their experience, and to know that a seed of curiosity will have lodged in their minds. Germination, growth and fruition will depend on so many things in their young lives, but maybe …

My initial question to whoever was first in each group was the same: what colour is the Sun? It came as a surprise to everyone that it wasn’t the yellow they all used in their artwork: “It’s white!” was invariably the answer I got. Once everyone in the group had seen it we were able to talk briefly about the Sun’s colour and texture; the shell of a chicken’s egg was our working analogy. (For more detail on the nuanced topic of the Sun’s colour please see my earlier post on the subject, here.) In addition, several of the more observant pupils challenged my contention that three sunspots were visible – that’s all we could see from the raw Dwarf’s image – and declared that there were four. Brilliant: objective observation and the courage to defend it. I truly hope they continue in that vein; it bodes well for the future.

This is the view through my Dwarf3, post-processed when I got home to remove the annoying colouration added by the Dwarf’s solar filter and so that I could identify and label the active regions. (I have been meaning to replace their stock filter with something neutral, akin to the filter I made for the other telescope used that morning: a Skywatcher 72ed refractor fitted with a Baader solar film filter – see my earlier post, here, for details.)

As a parting gift I gave each of them a pair of eclipse-viewing glasses and told them about the partial eclipse that will be visible over the UK in August, during their school summer holidays. In fact, there will be two in August 2026 (see here):

• 2nd August; the partial eclipse will begin around 9am BST. It will reach maximum at 10am and end around 11am. At maximum roughly 45% of the Sun will be in shadow.
• 12th August; this partial eclipse will almost be total, with around 90% of the Sun being obscured by the Moon at the maximum. It'll begin at 6.17pm BST, with maximum at 7.13pm.

The glasses came with repeated warnings about not looking directly at the Sun without proper safety equipment. (These were the solar viewing glasses I managed to get at Astrofest earlier in the year, which I wrote about here.) Furthermore, I made the point that if the glasses were damaged in any way they shouldn’t be used. At this point one young person declared that he’d therefore definitely not be showing them to his brother 😉.

I’d love to go back after dark someday and help run – with friends from my local astronomy society – a stargazing evening; maybe next Winter when it gets dark early and there’s a clear sky, so that the pupils can bring their parents/carers with them. However, that’s in the hands of the school. In the meantime, of one thing I am certain: Sir David Attenborough would be proud of them.

Postscript:

A while after posting this I got a lovely thankyou card:

Colours of the Sun (2)

 

Part Two: pretty pictures and a little nerdy stuff

For my own peace of mind I must begin this post as I began Part One with a warning that might seem blindingly (!) obvious to you but which I ought to spell out nevertheless: please never, ever look at the Sun without adequate (certified) protective filters. Even more crucially, make absolutely sure that your binoculars’/telescope’s field of view doesn’t even stray close to the Sun. The filters I used to generate the images shown below removed, at a minimum, 99.999% of the intensity of the sunlight.

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It’s a fascinating time to be imaging the Sun as we’re near the peak of solar activity in its eleven-year cycle. When I started imaging the Sun a few years ago several days could go by with little or no activity; as 2024 progressed it became a rare day when there weren’t several large and/or complex active regions. The Sun’s disc has a diameter of approximately 109 times that of the Earth so in the images below you’ll realise immediately just how extensive these active regions can be.

Shown above is the neutral/white-light imaging setup I use in order to gather images of the photosphere; I introduced it in Part One. With this I can capture the whole of the Sun’s disc within a 3k x 3k (i.e. 9 megapixel) colour image which gives a decent level of detail. Sunspot active regions and the associated faculæ are easily observed. This is not like taking a snapshot using ones phone or handheld camera unfortunately. The issue is one of atmospheric turbulence: the higher the magnification the more distortions a given exposure may suffer from (I uploaded to my YouTube channel a very short video taken through another of my telescopes in order to illustrate this phenomenon.) To get around this I’ll typically collect up to 2000 frames using the software cited in Part 1 then use a free software package called Autostakkert! in order to select the best – usually the least distorted ten or fifteen percent only – before stacking them to create a single optimised image. This will go into another free package called Registax for sharpening and perhaps basic colour balancing before sending it to an image processing package like AffinityPhoto or Photoshop for final ‘polishing’.

Now, as discussed in Part 1, ‘colour’ is a term that needs a bit of thought. When I use my colour astro-camera the frames that emerge have a green tint because the detector chip inside isn’t the standard RGB of a smartphone or other digital camera but it’s a pattern of RGGB (for reasons I’ll not go into here). Correcting for this is trivial, but in truth one could choose almost any colour for the final image simply by selectively altering the colour balance or colour saturation at the image processing stage. A ‘correct’ balance will yield a white photosphere, but if it’s desired then a yellow-coloured Sun is easily possible. I’ve done both, although I’m increasingly tending to the more neutral/natural white.

When it comes to the remotely accessed setup in Grenada, Spain I mentioned towards the close of Part 1  the whole point of the exercise is to observe at specific wavelengths, using in particular H-alpha light. One is therefore using precision narrow-band filters to select out that one colour associated with the emission from excited hydrogen. A colour astrocam is very inefficient for this – there will be nothing at all recorded in the green or blue channels of course – and it’s far better to use a monochrome camera and then add any desired colour digitally during processing. The rig is shown above; it’s a screenshot from the Zoom-enabled session (- details here; my excellent guide and teacher for the experience was Gary Palmer). The setup we used was the one on the left hand side: a Williams Optics 120 mm refractor, with a ZWO-ASI1600MM astrocam, all on an IOPTRON CEM120 mount.

After a couple of false starts due to the weather, my session took place on 4th July. In order to ‘ground’ the images I used my own backyard setup to capture the Sun’s whole disk in white light using my homemade solar filter. It’s shown below, with the active regions identified using the internationally accepted conventions which one can find online. I find the Space Weather website useful in this regard, and you can find images and associated information for any specified date in their archive (see here). Also useful is the NASA Solar Dynamics Observatory, which includes video sequences for particular dates (see here). I’ll share the rest of the story using the images I later generated from all the data that was gathered on the day. Each of these was generated from original data files containing 2000 exposures which were graded by quality and the best stacked and refined using the packages mentioned above.

My backyard image of the photosphere as it appeared on 4th July, collected and processed as described above.

A couple of composite images overlaid onto my white-light image of the photosphere are shown above. I have marked on my image the active region under observation; superimposed on the side are three narrow-band filter images of that same region. Notice the difference in appearance of the sunspots and the area around them depending on which element we’re utilising: hydrogen, sodium and magnesium. The faculæ (regions of higher temperature) show up well in H-alpha light, sunspots less so; the violence of the Sun’s magnetic field changes in an active region can be seen in the contorted surface patterns. Surface granulation shows up well in sodium light, and the sunspots seem increasingly more clearly revealed as one progresses through Na to Mg. Please note again that the colouration is added digitally – hence the variation: I have been learning how to process the narrow-band monochrome such as these for the first time and therefore trying out various methods. During this period, as an additional complication, I began to migrate from using Photoshop to AffinityPhoto. Unfortunately, there are relatively few online tutorials based on the use of AffinityPhoto although this one was useful; the short books by Dave Eagle were particularly helpful.

In the pair of H-alpha images shown above one can readily see prominences reaching out into space. The one on the left is associated with an active region not yet in view from the Earth – i.e. it is being generated around the rim. The Sun rotates once in approximately four weeks at the latitude of these prominences, so this one would have appeared within days. (The Sun is a ball of gas, hence the fluid-like variation of a solar ‘day’ depending on where one is looking between the poles and the equator; see here for more information.) Also apparent in these images are the ‘grass-like’ spicules in the near-surface region of the chromosphere. Each of the two images is itself a composite: one set of 2000 short-duration exposures for the surface and another set with longer exposure times for the spicules and prominences. These two sets were separately processed and then combined digitally to yield the final composite images. The original two layers of the final composite image shown above on the right are reproduced below after stacking the selected subset of 2000 exposures for each of them.

Although filaments – a prominence as seen from above – may also be discerned in the coloured images above, particularly the one on the left, I’ve inserted an image below in which I have accentuated them during processing.

Given the novelty of all this I confess that it took a long while to get to grips with the remotely collected data files I was sent by my guide Dave Palmer. However, in the process I’ve learnt a lot, enjoyed myself and am pleased with what I have to show for all the effort. I hope you like the result.

Colours of the Sun (1)

Part One: a bit of light science

For my own peace of mind I must begin this post with a warning that might seem blindingly (!) obvious to you but which I ought to spell out nevertheless: please never, ever look at the Sun without adequate (certified) protective filters. Even more crucially, make absolutely sure that your binoculars’/telescope’s field of view doesn’t even stray close to the Sun. The filters I used to generate the images shown below removed, at a minimum, 99.999% of the intensity of the sunlight.

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What colour is the Sun?

Red and orange, as it rises or sets? Yellow, when it’s low in the sky – and in pretty much every child’s painting? What about in the middle of the day when it’s too bright to stare at, and what does it look like to the astronauts on the International Space Station? Just what is the true colour of the Sun?

It depends …

What we perceive is an admixture of the light that the Sun emits (all the various colours, or wavelengths, it generates), the effect of whatever that light travels through and finally our ability to detect it. Our eyes – actually, our eye and brain in combination and assuming the absence of colour blindness – see reds, oranges and yellows when the Sun is nearer the horizon and its light is therefore passing through a lot of Earth’s atmosphere. The shorter wavelengths of light, towards the blue-violet end of the rainbow, have been preferentially spread out by process called Rayleigh Scattering. This describes the scattering, or spreading, of light by particles smaller than the wavelength of the light: the scattering increases as the wavelength of light shortens. Thus, the blue-violet end of the rainbow spectrum is scattered widely, giving us a blue-coloured sky, whilst the redder colours, with longer wavelengths, are scattered less and are therefore more likely to reach us from the Sun’s direction.

If we can reduce the intensity of the sunlight to safe levels when the Sun is high in the sky and thus passing through much less atmosphere, we’d see the Sun as a whitish disk. Our friendly local astronauts, free of our atmosphere altogether, would also tell us that it looks white. This is because the Sun is emitting light across an incredibly wide range of wavelengths. Although its emission is most intense at wavelengths corresponding to green light, the presence of wavelengths to either side add together to give us white (see here and the figure below for a little more information). Ben Harding, a long-time member of a local amateur astronomy society, SEKAS, and a source of considerable useful advice, pointed out to me an excellent way to demonstrate the Sun's colour when it's high in the sky and therefore not going through a lot of atmosphere. At the many public events SEKAS participates in he projects a suitably attenuated image of the Sun from a telescope onto card in order to show everyone that it is indeed white - see image below, on the left. This image is used with his kind permission. It turns out that we can also get a hint of this from our Earth-bound vantage point even without equipment. Light summer clouds appear as near-white in colour due to a process called Mie Scattering which describes the scatter of light from spheres of a size comparable to the wavelength (colour) of the light – microscopic droplets of water in a cloud fit the bill. In this case, there is no significant variation in the strength of the scattering process with the wavelength of light: thus, all colours are affected approximately equally and we therefore get an idea of the actual colour of the Sun, white.

Solar projection revealing the Sun's true midday colour. On the right is a graph showing the intensity of light emitted by the Sun across the visible wavelengths. It approximates to something called Black Body Radiation, with a peak intensity in the green part of the visible colours but significant intensity to either side and out towards ultra-violet and x-rays and towards the far infra-red; see here. Note the sharp spikes in intensity at specific wavelengths; these tell us about the chemical make-up of the Sun; we’ll return to them below.

This is my first ever image of the Sun. It was generated from a stack of frames collected by my entry-level astrocam (Altair Astro gpcam2 290c) and my first ‘proper’ telescope bought post-retirement (Skywatcher 150PL on an EQ3/2 mount with retro-fitted drive motors; see here for the full story) and a homemade solar filter (see here). My field of view was limited to 0.27º x 0.15º so I only captured a portion of the Sun’s disk which has an apparent diameter of about ½º. However, it’s enough to illustrate both the colour of the Sun in visible light and the granulation – the result of thousands and thousands of convection currents, like the swirl of cold milk poured slowly into hot coffee. In fact, the Sun’s surface is reminiscent of the shell of a chicken’s egg. There are lots more images in Part 2 of this post.

This is my current setup for imaging the whole disc of the Sun. The telescope is an AA Starwave 80edr fitted with a Baader 2x Barlow lens and an AA533c camera cooled to -10°C; the mount is a Skywatcher HEQ5, controlled from a laptop via ASCOM/EQMOD using Carte du Ciel and SharpCap (URLs for the software are in the appendix to an earlier post, here, together with installation notes).

Having made a start at describing the colours of the Sun according to our eyesight we now need to dig a little deeper. We tend to assume that we’re looking at the surface of the Sun, but it turns out that things are not entirely straightforward in that regard. Remember, the Sun is a huge ball of gas; it’s a pretty good sphere, which one might expect since both the gravitational force pulling it in and the pressure trying to push it out are both acting in all directions uniformly. However, what we think of as the surface of that sphere is in fact the deepest of the three principal outer layers of the Sun: the Photosphere  (It’s called that because it’s where we perceive most of the Sun’s light to come from: hence, ‘sphere of light’.) The photosphere is at about 4000-6000°C depending on altitude. This is what we see if we look at the Sun through a neutral, or white-light, safety filter. It’s also the layer in which we can see sunspots (patches with a slightly lower temperature which, as a result, appear dark) and faculæ (typically, nearby areas of slightly raised temperature which therefore look brighter).

Above the photosphere is the Chromosphere – literally, sphere of colour. Its temperature rises with altitude to over 8000°C and it glows with a red colour derived from the hydrogen plasma that makes it up. We don’t normally see the red because the chromosphere is of much lower density than the photosphere and the intensity of the light generated is therefore swamped by the denser photosphere. It is however possible to see it when the brighter photosphere is blocked out at the peak of a total solar eclipse. The chromosphere is the layer of the Sun’s atmosphere associated with giant prominences that may extend thousands of kilometres into space, and filaments (essentially, prominences seen from above) and spicules. The latter are relatively short spikes of red-glowing hydrogen that can give the Sun a sort of ‘grassy’ look. There’ll be several of my images in Part 2. Further out still we get to the Corona, but I’m not going to dwell on that in this post.

So far, we’ve established that the colour we see depends on the temperatures present in the Sun’s outer layers (Black Body Radiation),what the sunlight has travelled through (e.g. Earth’s atmosphere) and the workings of our eyes (or our cameras). We’ve also had a hint that the chemical make-up of the Sun has an influence (e.g. the red colour of the chromosphere being due to hydrogen). The additional important factor arises from the fact that, whilst the Sun is mostly composed of hydrogen (H) and some helium (He), there are many other elements present in small quantities. We can use them to look at the Sun using particular wavelengths/colours of light – these are the emission lines uniquely associated with each given chemical element. For example, if we put energy into a hydrogen atom – and there’s no shortage of energy in the Sun – it enters what’s called an excited state. The natural next step is for that atom to shed the excess energy, typically by emitting a packet of light (a photon); the beauty of that process is that every element has its own ‘fingerprint’ of emitted photon wavelengths or colours. Thus, a principal colour emitted by hydrogen is red; it’s often labelled as Hα, or H-alpha, and has a wavelength of 656 nm (nanometres, 10^-9 m). If we observe the Sun through a filter that transmits only this wavelength of light then we’ll see a red Sun. Moreover, by picking up that one particular wavelength alone we’re effectively looking specifically at how hydrogen, to the exclusion of all other chemical elements, is behaving within this solar environment.

We can track through the ‘rainbow colours’ by using filters that single out the wavelengths emitted by other elements present on the Sun. Sodium, Na, emits light primarily at 589 nm, which is yellow; magnesium, Mg, emits photons at 517 nm and that’s a green light; calcium, Ca, emits blue light at 393/396 nm. For each colour (or wavelength of light) we are focusing on a single element and thus on a slightly different aspect of the Sun’s behaviour; in effect, we can use these selected elements as an internal solar probe in order to complement observations in ‘white’ light (i.e. all the wavelengths together – our view of the photosphere). There is a caveat: the effect of Rayleigh scattering (see above) is to scatter the shorter wavelengths preferentially and getting good Earth-bound images using the blue light from Ca is therefore hampered. As a consequence I show no Ca-light images in this post.

It is in fact possible, with the right equipment either on Earth or on a satellite, to focus on the light emitted from excited atoms of each of the elements in the periodic table up to iron, Fe. (There is insufficient energy in the Sun to make anything heavier than iron – that requires the sorts of energies associated with a supernova explosion. Remember that the next time you look at gold jewellery; it exists because earlier generations of giant stars have died 😉). There is additional information here, here and here.

For we amateurs, however, the bright emission wavelengths of hydrogen, sodium, magnesium and calcium are accessible. The cost of the kit is, however, a very long way beyond my budget. This is particularly true for those wishing to collect images using the light from hydrogen-alpha emission: thousands of pounds. Hence my own ‘white light’ setup which depends only on reducing all the incoming wavelengths to leave only one part in 100,000; I can image only the photosphere: sunspots, faculæ and granulation. Although I’m mostly content with this state-of-affairs I did treat myself to a relatively inexpensive gift during the Summer: I paid for 90-minutes of guided access to a solar imaging setup in Spain. The details are here. I recommend it to anyone with an interest in this area. 

I’ll share some pretty pictures in Part 2 of this post, along with a bit of nerdy stuff on image processing. Until then, keep looking up (safely) …

To Capture a Sunspot: solar filters

Occasionally I post on social media images captured using one of my telescopes in conjunction with a high framerate astro-camera. The image posted most recently was of the Sun, specifically of sunspots; I have collected a lot of these images in the past few years. In part this is because we’re nearing a period of maximum solar activity in the Sun’s eleven-year cycle and there’s simply more to see, but daytime astronomy also affords benefits when trying out new bits and pieces – it’s so much easier to learn how to handle new equipment in daylight. My recent foray was a case in point; I had bought a second-hand 80 mm refractor (see endnote [1] for more information and advice) and was keen to test its features after weeks of poor weather, ill-health and general busyness. This is definitely not a post on expensive astronomical equipment however, almost the opposite in fact: my aim is to share with you how I capture images of the Sun safely without spending a lot of additional money. It is a direct response to the questions posed to me by a member of a local amateur astronomy club: “Did you buy a solar filter cap or make one? What solar film did you use?”

The very first vaguely successful image of the Sun I managed to get. It was taken using my first telescope and an entry-level astro-camera and the combination of high magnification and small camera detector size meant that I captured only a small segment of the Sun’s disc. However, it remains in the slideshow of background images on my PC because it gives an impression of size and of the Sun’s neutral colour. Look closely and you’ll also see the ever-present convection cells as the Sun’s near-surface rises and falls. I’ve put a few more details into endnote [2].

This brings up to the matter of solar filters: how to reduce the amount of light entering our telescope to a level that will neither burn our eyeballs nor fry our camera’s detector chip. I should note at the outset that I am not discussing in this post the more dramatic phenomena associated with the Sun’s surface – no prominences or flares etc. of the sort shown in the images here. These require highly specialised (and expensive) equipment which limits the light entering the telescope to a specific wavelength only. The bits and pieces I describe here will allow all wavelengths (i.e. all colours) to pass through, but at very low intensity. Indeed, the solar safety filter material I use removes 99.999% of the light; which means that only one part in 100,000 reaches the telescope and your eye or camera.

Hopefully, the following series of images will explain it all …

Shown above is the setup in my garden I used to capture the image in question, a closer view of the solar filter fitted to the front of my telescope and an inset image of the filter’s rear face. Notice that there is a second solar filter fitted to the smaller finderscope, which is used to help find the ‘target’, shown to the upper right of the central image. The orange-coloured filter holder is a lid from an old food container (I think it was bought full of dry-roasted nuts!) with its central part removed using a hacksaw and the edges smoothed using sandpaper. The diameter of cut-out disk is as close as I could get it to the diameter of the telescope tube. The lid rim’s internal diameter has been reduced slightly using a strip of material cut from some anti-slip matting, fixed in place using double-sided tape. The final filter assembly was a snug fit over the front of the telescope.

The essence of the whole DIY project is to find a tube that has some rigidity (enough to hold its shape when picked up) and has an internal diameter just larger than the outside diameter of the telescope in question. In the example above I have used part of the thick cardboard tube in which one might find a bottle of a certain single malt whisky: it just happened to fit nicely onto the 72 mm refractor I now use for observational astronomy and in my visits to primary schools etc. I buy high-quality solar film in A4 sized sheets since it’s a cost-effective way of fabricating several filter assemblies; it’s available from many stockists but I happen to use this one. You’ll also need some epoxy resin adhesive. (In passing, I note that there are pre-made solar filter assemblies also available to buy; a quick scan online suggests that they retail for about £50 and upward each.)

Having found a suitable tube and cut it to an appropriate length, the key next step is to attach the solar safety film in such a way that it is not creased or scratched. I have found that a thoroughly clean sheet of glass is a great help; I keep an unused glass shelf for all such work but a smooth and flat ceramic tile might serve, as would a kitchen worktop if you can find a section that’ll not be needed for a day. Leaving the protective backing sheet on the foil, place it on the glass surface, foil upwards. Now mix enough epoxy resin to be able to run a thin thread around the end of the tube that’s going to take the solar safety filter; try not to get any epoxy on the tube’s inner surface, although small amounts aren’t critical. Carefully lower the end with the epoxy onto the foil sheet, avoiding any twisting or sliding motion: the foil should have remained flat against its backing sheet and the glass. There’s probably no need unless your tube is exceptionally light, but you could gently lay something like a small hardback book across the top in order to apply even downward pressure if you wish. Now walk away and leave the epoxy to set. Once all that’s done you can cut away the remaining foil with a pair of scissors and store it for another day.

Small adjustments are probably needed to ensure the filter assembly properly fits the end of the telescope. For reasons rooted only in habit, I tend to do this as a final step despite the fact that it’s probably wisest to get all this out of the way before attaching the safety film. There are all sorts of ways of achieving this, depending on how many millimetres larger the tube’s diameter is than the telescope. It may require only a layer of tape to the inside of the tube (- the end away from the foil of course as one doesn’t want to risk damaging the safety film if anything comes adrift). A layer or two of anti-slip matting fixed using double-sided adhesive tape works well, but it’s easy to find self-adhesive strips of felt or neoprene online and these can also be very useful. Remember, you are aiming for a fit snug enough that nothing’s going to fall off accidentally but not so tight that filter-destroying force is needed in order to slide it onto the telescope tube.

Here’s a selection of the filters made thus far for telescopes of diameters from 50 mm (finderscopes and guidescope) to 150 mm in the case of my Newtonian reflector (- in that case, I used the ring from a cake baking tin that had lost its base to rust); I’ve also made filters for both my grandsons’ telescopes. All this was from two (or three?) sheets of solar safety film. Also shown in the image is a scrap of anti-slip matting and lengths of self-adhesive neoprene and of felt.

Now for some images …

The above are some of the whole-disc images of the Sun I have captured prior to the one shown at the top of this post. The numbers and the sizes of sunspots vary enormously. We’re approaching the peak of the current 11-year solar activity cycle (expected in early 2024) so it’s unlikely you’ll look at the Sun at present without seeing any. Given that one could fit 109 Earths across the diameter of the Sun you’ll not be surprised that the largest sunspot in the image top left is several times the Earth’s diameter. Each sunspot cluster is given a unique identifying number – often preceded by ‘AR’ for active region; you can look this up here. Sunspots themselves are regions associated with the Sun’s magnetic field as it protrudes from the surface. Their dark appearance comes from the fact that they may be 2000°C cooler than their surroundings. Look closely and you’ll also notice brighter regions: those around the darker sunspots are called plages whereas the lighter patches often seen most easily near the edge of the solar disk are faculæ; these are associated with hotter regions in our field of view.

If we take a closer look you’ll see that sunspots have a central dark region, the umbra, and a less dark surrounding area, the penumbra where temperatures are at an intermediate level between the umbra and the surrounding surface.

Even the smallest scraps of solar film left over from making a telescope filter can have their uses. I captured this sequence of shots of a partial solar eclipse using my phone with a piece of safety film covering the phone’s camera lenses. The quality is what you’d expect from a handheld phone in a car park several miles from my house, but it was a fun thing to try. However, see below …

This is a better view. It’s another partial eclipse, this time captured using one of my telescopes and astro-cameras. You may be able to discern the silhouette of some of the Moon’s mountain ranges as it clipped the Sun. We are extraordinarily fortunate in the fact that the Sun and the Moon both appear to us on Earth as discs that are about ½° wide – which is why the Moon can cover the Sun when suitably aligned. (By the way, if you hold your little finger out at arm’s length the fingernail end covers about 1° of the sky so it’ll easily cover the Moon; see here.)

Happy sunspot hunting 😊

1700 words + endnotes

Endnotes
[1] For an overview of telescope types and what to consider and look for when buying try these web sites: here and here. I hasten to add that, like other second hand astronomy items, I bought the telescope mentioned in my opening paragraph from someone I knew to be trustworthy; one has to be careful.

Starting out in astronomy need not be prohibitively expensive – getting into photography, or off-road cycling, or many forms of sport, … might be comparable. However, amateur astronomers often talk in terms of ‘falling down the rabbit hole’: if you get hooked by the hobby you’ll find that there’s a never-ending series of spending opportunities ;-) My advice is to think about what it you most want to do/observe and start your search from there, being aware that as your aspirations evolve you may want to upgrade. The above links are only two of a multitude of places to get advice; read them in order to get an overview, but there’s a huge benefit to be had if you can try things out and talk to experienced people face-to-face. My suggestion is that you join your local amateur astronomy society. I’ve had loads of support from the lovely people here and also here and here. Most societies have websites and/or social media groups and you’ll find members only too keen to answer questions and offer informed advice. See here or here for a list containing many such societies in the UK. (Please note that these lists are not completely up to date, but they’ll get you started.) Once you have some equipment of your own you’ll find another slew of websites and helpful social media groups and online videos dedicated to users of similar kit.

[2] We all know that the Sun is both large and massive, and that it’s hot. In terms of size, the diameter at its equator is about 109 times that of our beautiful Earth; it represents 99.8% of the mass in our entire solar system. Its core temperature, which is where the fusion reactions occur that generate its output, has a temperature of about 15 million °C whereas the Sun’s surface temperature is about 5,500°C. As one rises into the corona (its outer atmosphere) the temperature rises again to about two million °C. (See here and here. Thus, what we perceive from Earth is the ‘cooler’ surface, referred to as the photosphere. In fact, the colour we see is strongly affected by the Earth’s atmosphere and by the limitations of our eyes: light from the blue end of the spectrum is preferentially scattered as the mix of wavelengths from the Sun passes through – this gives us our blue sky and leaves the Sun appearing yellow-orange-red as it nears the horizon but blindingly white when it’s high in the sky (- don’t look!). Our eyes fail to give us its intrinsic colour; if we could look at it through protective glasses from a space station our eyes would perceive the Sun as a white disc.

One of the simple calculations I used to set for students in their foundation year was to use something called Wein’s Law in order to estimate the temperature of the Sun’s photosphere. All that’s needed is the wavelength of light at the peak of the Sun’s emission, which we approximate to the wavelength of green-yellow light. If you’re that way inclined, try it out using the link above. The same formula may be used to estimate the surface temperature of other stars, or indeed the temperature within a furnace – the physics is identical. See also Video 13 in my lockdown series ‘Physics in the House’.

♪ The Sun has got its spots on … ♫

It has been a whole year since I last posted. I’m sure that this is in part one of many diverse legacies of SARS-COV2, but it’s also related to the fact that my primary focus for the blog was always to reflect on living life as a scientist.* I have in the meantime continued to write about topics in science elsewhere: using my u3a local branch’s social media group example. However, the point is that I intended only to write on this blog as and when my week involved me in rolling up my metaphorical scientists’ sleeves and getting directly involved. You can perhaps see where this is headed …

Last week I visited Year 5 of The Churchill School; one of the two classes is taught by my talented daughter-in-law, and it was she who invited me. I’ve been to the school many times before, but less often as a visiting scientist (e.g. here). I freely confess that school classrooms unnerve me. I (mostly) loved school when I was part of the whole thing as a child and I have the deepest of respect for excellent teachers like my daughter-in-law. However, nowadays, the thought of standing in front of a class of children – let alone two 30+ classes combined – with the aim of leading them into some new knowledge and understanding renders me a little weak at the knees. It was truly lovely, then, to get a warm welcome by the four staff members involved – but it was the eagerness of the 9/10 year-olds that completely blew me away.

I turned up while they were still in a school assembly so that I could get everything set up in peace. I had brought with me my newly acquired second-hand 72 mm refractor, the lightest of my two equatorial mounts and a tray of bits and pieces which included a homemade solar filter. The neutral filter material, which removes 99.999% of the incident light, is of the highest standard, but I can’t say the same thing about the cardboard and duct tape assembly that holds it in place in front of the telescope’s objective lens. In order for the equatorial mount’s tiny battery-driven motors to keep the telescope pointing towards the Sun over time, it has to be tilted to the correct latitude, levelled and compass-aligned. The telescope itself also needs to be balanced (both front to back and side to side). Once the Sun had been located and its image focused I have to say that I was more than relieved to see several sunspot clusters clearly visible; passing clouds were, as ever, less predictable. (By the way, the images taken at the school were carefully checked by them before they were released for my use; faces have been covered/blurred as necessary.)

I was given a generous introduction inside the school before we all trooped out to the school’s expansive playing field and the children sat in a semi-circle on the summer-dry grass. I kept my own introduction to the telescope very simple; no-one needed to know about focal lengths and lenses in order to observe the Sun. What did need to be shared at the outset though was the standard but ever-vital warning not to look directly at the Sun for fear of damaging the eyes. In this instance it was important to go much further, making the point that looking at the Sun through binoculars or a telescope is likely to cause blindness without the appropriate safety measures in place. You also have now been warned 😉

I had given them a rough idea where the sunspot clusters were by describing the Sun’s disk as a clock face: two and eight o’clock got them to the most obvious, although it was a busy day on the Sun and there was a lot to choose from. This is an image captured by a talented amateur astronomer, Roger Hyman ( find him here or here) on that same day; the image is used with his permission. The orientation isn’t exactly as we saw it – the angle of the camera he was using and other factors will affect that, but it’s close enough. All clusters of sunspots are given a unique identifying number and on Friday 15th July 2022 they were, beginning at the 2:30 position and moving clockwise: 3057, 3056, 3055 and 3053 (see here).

In between the slowly drifting clouds, everyone got a chance to look through the telescope – even the adults. All but a handful said they’d seen the sunspots: some after a bit of coaching in terms of how to use an eyepiece. It took a while, and given the need for a lot of patience they did astonishingly well.

Every time a cloud came over we used it as an opportunity for questions, which came thick and fast. Indeed, the flow of questions continued when everyone had had the opportunity to take a peek and we had moved back inside the building. These were exceptionally wide-ranging, and varied from the scientifically and philosophically challenging to the more speculative ‘what if’ (the Sun disappeared, you went into a black hole, …) and ‘would you’ (like to go into space, live on Mars, …) type of question. All of them deserved to be taken seriously and given the best response I could muster. I can only say that I was genuinely impressed; it was uplifting to see so much evidence of the potential on display within that classroom. A few of them were obviously highly clued up, and several made a point of telling me that they wanted to be a scientist. Perhaps what I should have said in response, but for some reason didn’t, is that in a sense they already were – just as much as I was at their age.

I was at the school for almost 2½ hours in total, although it felt far briefer than that.

I promised this blog post in order to provide a reminder of what we did, and to offer a little information in a more coherent way than I fear I managed on the day. Perhaps one might start by saying that sunspots are associated with a localised increase in the Sun’s magnetic field. They appear darker than their surroundings simply because they are a little cooler, with their central region (the umbra) at about 3000-4000ºC compared to the average of 6000ºC. Their sizes vary a great deal, with the largest being several times the diameter of the Earth. (If you are able to see the above image of the Sun on a reasonably large screen then you might notice the small blue dot I inserted to the left hand side: that is approximately the size of the Earth on the same scale.) Sunspots can last for days, but eventually disappear. The number of sunspots visible at any one time varies over the eleven years of the solar activity cycle; we’ll be at the next maximum in this cycle in 2024. You can find lots more information here, here and here; a recent BBC ‘Sky at Night’ programme covered some of this ground also.

Perhaps a fitting way to close this post would be to offer a couple of suggestions for useful astronomy apps and to share a small number of my own images. The first app I downloaded when picking up after retiring the hobby of my youth (from Android Play Store) was ‘Sky Map’, which I still use as a simple interactive guide to the sky; for a little more sophistication one might go for ‘Stellarium’. The choice is yours. In addition, and especially if you want to pursue this further, I’d recommend joining your nearest amateur astronomy group. Unfortunately, the one I’m in – Ashford Amateur Astronomy Society – is an hour’s drive from my home; maybe there’s one closer to where you live.

The above were taken at different dates, through different telescopes and with different cameras. The result is that we have a fairly recent image of the whole disc of the Sun together with a more highly magnified image of one part of the solar disc, and a close-up of two particular sunspot clusters taken in 2021 which bring out a bit more detail in terms of their structure. You’ll notice that each sunspot comprises a dark central region (the umbra) and a somewhat bright outer region (the penumbra); as you might anticipate, the penumbra has a temperature which is intermediate between the umbra’s and the Sun’s average surface temperature.

Happy observing.


* I retired from my paid employment as an academic and multidisciplinary materials research team leader about seven years ago, but I’m still a scientist – just as I was as far back into childhood as I can remember; I’ve covered this ground in earlier posts (e.g. here).