Amateur Astronomy


I’m looking forward to seeing this on the big screen in this anniversary year. It’s a documentary using entirely 65mm original footage, including large amounts freshly rediscovered and never shown before.They also found tens of thousands of hours of console operator audio.


I saw the green flash!!! :green_heart: … down Dun Laoghaire pier at 6.17 am this morning. Leastways, I’m pretty sure I did. It looked a bit like this in reverse, except it’s more vivid in real life (as the excited voices of the viewers indicate):

I’d jumped in the car on the spur of the moment, as I saw it was clear and happened to check the time of sunrise with just ten minutes to spare. Since it was early on a Sunday morning I parked right beside the pier entrance and the sun rose out of the sea literally as I closed the car door and turned to face east. I’ve never seen a horizon so crystal clear before anywhere in the world, let alone damp old Ireland.

The first glimpse of the Sun was just a single horizontal line of light, slightly detached from the horizon … and it was a brilliant lime green!!! It widened and curved downward until it was obviously the top edge of the Sun’s disc, and as it did so the ethereal cold green colour warmed to the more expected orange. That contrast was the main feature that convinced me I’d seen the real thing. The green phase can’t have lasted more than two or three seconds. Obviously I wasn’t equipped to snap a pic of the main event, but here it is about a minute later looking its usual self:


Btw, the “Sun illusion” is just as powerful as the Moon illusion. It looked like a giant bobbing balloon, not the half-degree-wide tiddler that comes out in the photo.


Mapping the frontier of Physics

Also this


That is pretty cool
I saw a green flash last year on the west coast
Sunsets were pretty spectacular too, thank you dust and pollution!!


Nice links propertyspire!


Neat lunar occultation of Saturn.


Who’s old enough to remember this? …

… Disney’s answer to Star Wars and Close Encounters was released forty years ago this year, in the same month as Paramount’s Star Trek: The Motion Picture.

The studio’s Black Hole concept was hampered by the fact that they couldn’t afford to rent George Lucas’s fancy cameras from Industrial Light & Magic. Well, wait no longer – a virtual camera the size of the Earth has been photographing the real thing for the last twelve years. After many rumours it’s now official – the Event Horizon Telescope’s first results will be presented at press conferences around the world next Wednesday at 1pm. Get ready to see the Milky Way’s supermassive black hole … or, at least, some amorphous blob purporting to be its event horizon. To be honest, I’ve no idea what to expect.


Elon Musk may own a rocket company, but he’s very hazy on some of the basics of astronomy. Witness this extraordinary Twitter exchange in which Musk claims his new Starlink satellites will not be visible enough to interfere with astronomers’ view of the night sky.

You can click the links to see the whole exchange. Suffice it to say that the visibility of the ISS has nothing to do with its having lights (personally I didn’t even know it had any). It has only to do with its position with respect to the Sun and the albedo and orientation of its reflective surfaces.

And while it’s true that the ISS is fairly large, you don’t need to be nearly that size to be visible from the ground. At its typical height, the ISS subtends an angle of about one arcminute, similar in size to the planet Venus at its brightest, and just about the same resolution as the human eye. It’s also the size of a full stop in a newspaper from a distance of a metre. I’m sure you have seen (or can at least imagine) a darkened room with a beam of sunlight coming through a slit in curtains. You can easily see dust motes floating in the air which have a much smaller angular width than the ISS. In a dark room, or the darkness of night, the eye is extremely sensitive to the high contrast of reflected sunlight.

So what about the position of the satellites with respect to the Sun? Here’s a drawing of the Earth with our viewpoint above the equator, at the longitude of Dublin. Imagine it is local midnight so the Sun is directly the other side of the Earth. From here at the equator we’d have to travel 90 degrees in any direction to be able to see the Sun around the limb of the planet.

Up there at Dublin (marked with a red arrow) we’re only 36.5 degrees from the north pole. So that’s the maximum angle that the Sun gets below our horizon at night. Note, of course, that this is a particular time of year – the equinox, when the equator is in the same plane as our orbit around the Sun. At midsummer we are tilted 23.5 degrees toward the Sun, so the view at midnight looks like this:

You can just make out the British Isles at the top of the globe. From Dublin you’d only have to march 13 degrees north to be looking over the limb of the planet at the Sun. Consequently, that’s the maximum angle that the Sun sinks below our horizon at midsummer.

Now, suppose you are in Dublin and you don’t fancy trekking to the north pole. How else could you peek around the edge of the planet? Stand on Dun Laoghaire pier and look at the Kish lighthouse. It seems to be almost on the horizon. Now go to the top of Dalkey Hill, about 500 ft above sea level. You can see the sea stretching well beyond the Kish to a much further horizon. We’re used to the idea that the horizon gets further away the higher we go. Suppose instead of linear distance, we ask about the angular distance to the horizon. That is, how far around the Earth is the horizon in degrees. (We can easily convert between linear and angular distance if we know the radius of the Earth). This is a very simple geometric problem.

If r is the radius of the Earth, and h is your height above the ground, the angular distance to the horizon is:

Here are the angles in degrees computed for a number of heights above the Earth in multiples of a hundred miles:

This tells us that a satellite sitting above Dublin can “see” the Sun all night in midsummer if it’s at a height of 100 miles. And at a thousand miles height it can “see” the Sun all night from the March to the September equinoxes. But even at a much lower altitude it can see the Sun for some hours after sunset and before dawn.

It follows that if the satellite can “see” the Sun, we can potentially see sunlight reflecting off it. There are, of course, many variables. One of them is the orbital inclination of the satellite – how far above and below the equator does it travel. In the case of the ISS it comes just north of Mizen Head at a height of over 250 miles, so it is an easy target here in Ireland throughout the summer months.

We don’t know what the final orbital parameters for Musk’s Starlink constellation will be, but it will include altitudes both higher and lower than the ISS, and the May 2019 launch had an orbital inclination of 53 degrees, taking the satellites neatly over south Dublin. So are we going to be seeing lots of them? You betcha! I read that Musk has gone off to rethink how the albedo of the satellites might be reduced to make them less of a nuisance.


I had an airfix model of the ship… I can still smell the glue



That’s way cool. Maskelyne was most likely related to the other Nevil Maskelyne, the Astronomer Royal from a century before who was famously the “baddie” in the dispute with John Harrison about his marine longitude clock.

This movie was only fifty years after the first photographic still of a solar eclipse. Back then, nobody knew if the solar corona was attached to the Sun, or was an atmosphere of the Moon, or even smoke in the Earth’s atmosphere. Eventually by adding spectrographs they discovered the previously unknown element helium in the solar atmosphere, as well as unknown emission lines of superheated iron which they called coronium.

Helium was only confirmed on Earth five years before this movie in association with uranium… alpha radiation is helium nuclei. It wasn’t found in quantity until 1903, in US gas wells.


This is the sort of shite that gives science a bad name, imo:


The perennial problem with maps of space is that either the sizes of the celestial bodies or the distances between them can be to scale, but never both. Space is too vast compared to the stuff in it. Maps of the minor bodies in the solar system can make it look like we live among a terrifying swarm of killer asteroids. Well it’s true, we do, but we ourselves are so relatively tiny that collisions with anything of significant size are rare. Nevertheless, the span of geological time is long, and there have been many such impacts in Earth’s history.

This picture is sobering – it shows ten thousand bodies of 10 kilometres or more, and another eight thousand of unknown size. Click on the image for the full scale version. A collision with a ten kilometre asteroid would have planet-wide impact, raining down burning debris and devastating plant and animal life.

But the real reason I wanted to post this map was just the cleverness that went into its composition. It’s not just a pretty picture but a complex visualisation of data mined from multiple sources.

The bodies are all in their actual positions of December 31st, 1999. The trails show their recent trajectories, and quite a large number are named, using clever algorithms to avoid trampling all over the image. This blog post gives an idea of the effort involved. And in general, Eleanor Lutz’s scientific data visualisations are exquisite to look at, as well as super informative.

Apart from the Sun and planets themselves, most of the objects in the picture are nowadays called “small solar system bodies”. The designation of minor planets was changed by the International Planetary Union in 2006 at the same time as Pluto famously lost its planetary status. The term planet is now reserved for bodies which are massive enough to have cleared the region of their orbit of other small bodies (IAU Resolution B5, 2006). Objects which are not planets under this definition, but are still large enough to squish themselves into a spheroidal shape under their own gravity, are called dwarf planets. And all others – the potato shapes, the rubber ducks etc. – are “small solar system bodies”. Inside the orbit of Pluto there is only one dwarf planet, the asteroid Ceres. That said, there are many planetary moons large enough to qualify under the sphericity criterion, but they do not count because they don’t directly orbit the Sun. You can see why there is a bit of disquiet about the arbitrariness of these definitions.

Just to add to the confusion, the term minor planets is still accepted and used by the IAU itself to refer collectively to the dwarfs and SSSBs. Were we to include all the known minor planets in a picture, it would be practically opaque. There are well over three quarters of a million of them, with a quarter million of those still waiting to be catalogued and numbered. Only a few get actual pronounceable names. Traditionally, discoverers get to recommend a name anytime within ten years of discovery. But most “discoverers” are now robotic telescopes and the new minor planets lie among the terabytes of survey data waiting to be analysed.

There’s still room in the minor planet hall of fame, though. How do we get something like asteroid Eileen, visible in the picture just outside Earth’s orbit at the one o’clock position? The NASA JPL Small Body Database tells us. It’s named for Eileen Collins, the first woman to pilot the space shuttle and first woman commander of a space shuttle mission. And Eileen (the asteroid) was discovered in 1986 by the famous asteroid and comet hunting duo of Carolyn and Eugene Shoemaker. Carolyn has her own asteroid, 4446 Carolyn, though it’s not among the four hundred or so that she discovered herself in a career that only started in her fifties! She turns ninety on Monday.


Here’s a longish but very worthwhile video on the chemical conditions for the origin of life and the likelihood of complex life on other planets by Nick Lane, professor of evolutionary biochemistry at UCL. (Actually, it’s almost entirely about life on Earth, but the extraplanetary implications are my excuse for including this fascinating video here).

I’m going to give a spoiler alert, but having watched the whole thing twice I found it so wide-ranging that having a mental overview was very helpful the second time round. The feature common to all life on earth, from the simplest bacteria to the largest animals, is the proton gradient which drives energy flows in the organism. The earliest life must have harnessed a non-biotic free source of this energy. Lane identifies serpentinisation, an interaction between seawater and olivine , an ultramafic mineral today found deep in the upper mantle.

He is refreshingly frank about our almost total lack of knowledge of how things originally got going from non-life to life. But he moves on to a second extraordinary division between simple and complex life on earth which is not to do with big vs. small, or plants vs. animals, or even unicellular vs. multicellular, but prokaryotic vs. eukaryotic life. The latter involves a symbiosis between tiny bacterial cells which produce all the energy, and the gigantic protein factories which are the eukaryotic cells. The mitochondria, those original bacterial cells, were able to shed all their genes other than the ones controlling respiration, once they were inside the protective cell membrane of the host. Thus, eukaryotes produce 100,000 times more energy per gene than bacteria. The lack of intermediates between prokaryotes and eukaryotes imply that the symbiosis is the result of an entirely singular and unique event which makes it effectively “miraculous”, in the sense that there is no process for science to study. There are other reasons to suspect it is extremely low probability, hence the implications for life elsewhere.

(Nick Lane appears to have a number of other “non-dumbed down” youtube vids which I’m really looking forward to watching. Thank god for youtube vs. telly!!!)

I’ve listened to some stuff recently about the importance of plate tectonics to life on Earth, and the fact that we don’t see it on Mars or Venus. For some bonus listening, here’s a BBC podcast about the extraordinary hypothesis that life may have originally given rise to plate tectonics!


Astronomy is both science and art. This piece of art is a composite of four images taken from different heights during Juno’s 20th science pass of Jupiter on May 29th. Juno is on a long elliptical orbit of Jupiter and manages to swoop in close by flying over the poles, avoiding the equatorial radiation belts that would fry it.


After the Sun and Moon, Venus is the brightest celestial body in the sky. At least it is when it’s in the right position for viewing, at maximum elongation from the Sun. But being an inferior planet, it spends a fair bit of time close to the Sun. Right now it’s about to disappear behind it, and it won’t be conspicuous in our skies again for two months:

But something else has just disappeared behind Venus. Is it a bird? Is it a plane? Is it Nibiru? Nope, it’s the Tesla Roadster on its way back from Mars. You can track it at



If you’ve ever watched shooting stars, you probably know that what you are seeing is interplanetary debris burning up in Earth’s atmosphere. The Perseid meteor shower coming up next weekend is a particular concentration of debris left behind from previous orbits of comet Swift-Tuttle. This comet passes quite close to Earth on each 133 year orbit, which makes its debris trail quite reliable. An actual collision with the comet has a low probability but can certainly not be ruled out into the far future. You don’t really want to think about such an impact, which would be thirty times as energetic as the one that wiped out the dinosaurs 65 million years ago.

But did you know that the earth sweeps up not just interplanetary but interstellar debris as well? Right now we are passing through a somewhat denser-than-average patch of the Milky Way which we call the local interstellar cloud. Grains of iron from this cloud have been found on the snows of Antarctica. The southern continent is a favourite hunting ground for meteorites because of the pristine conditions and the way ice movements and wind erosion combine to expose fallen objects. It’s where the most famous Martian meteorite was found, conjectured to contain fossil evidence of life.

You might wonder how we can tell a speck of iron came from an interstellar cloud, rather than a metallic asteroid fragment, or even a terrestrial source. This particular iron is an isotope called iron-60. It’s made in supernova explosions and scattered in space. It has a half life of 2.6 million years, so any of it swept up in the gas cloud that formed the Sun and planets has decayed eons ago. It’s been found in ocean sediment cores, but recently it has been found in fresh Antarctic snow that fell only 20-30 years ago. Clever analysis can be used to rule out alternative origins such as nuclear weapons or cosmic rays.

Back to next week’s Perseids: the best night is probably Sunday night after midnight (Aug 12th/13th) – a tad unfortunate for those with work in the morning. Indeed, owing to an interfering moon, the best time is closer to moonset around 3am. Weather and sleep pattern allowing, I’ll probably give it a look as I had a very good experience last year, with a peak ZHR that I estimated at over sixty per hour. I watched from Brittas Bay south beach carpark, which is a handy half hour jaunt from Dublin, and a reasonable dark sky location. (Note: carpark not open overnight).

Meteor watching can involve lots of neck craning so if you’re really serious bring something to recline on and other stuff to interest you between meteors such as binoculars, starcharts etc. For instance it’s a great time of year to see a whole other galaxy with your naked eye, as Andromeda will be high up near Perseus. But don’t have unrealistic expectations – it may be composed of half a trillion stars, but it’s still a barely visible fuzzy blob. I still think there’s a sense of awe in live skywatching even though you can get far better pics on the web.


It’s five years this week since Rosetta became the first spacecraft to orbit a comet, and began its two year close-up mission around 67P/Churyumov–Gerasimenko. Christian Stangl combines image sequences into this stunning tribute video, which you’ll want to watch on full screen:


Milky way stitch of several photos, 100mb in size. Took it a few days ago in a truly dark sky location in Australia


Blindjustice, I am speechless! If I didn’t know better I’d think you were winding us up. That pic is beyond stunning. I just spent a couple of hours poring over it, but it would take way longer to look at all its interesting details.

Please, please post some details of how you did it – camera, lens, mount, exposure time, software (and anything else relevant). You were using a tracking mount, right? Even under darkest skies there’s no way that without one you got such trail-free star images which, by my reckoning, go down to 11th or 12th magnitude. Certainly, 10th magnitude stars are crystal clear. Oh, and time of day would be nice so I can work out how high in the sky the view was.

At very first glance I thought that was the Moon at the bottom! … it’s Jupiter of course. And Saturn is at top right, which frames the picture beautifully. The constellation of Sagittarius roughly occupies the top right quadrant, Scorpius the bottom left quadrant.

Interestingly, someone just posted a picture of Jupiter recently on and the “top” (from a Northern hemisphere chauvinist p.o.v) of Scorpius was not far above the horizon. It gives me an idea of the difference between the current northern European and Australian views of the sky. It also reminds me why we can never get that magnificant view of the galactic centre in Sagittarius. The rectangle I drew in red never gets much more than 10 degrees above the horizon from Ireland, and you are heading even further toward the southern pole as you go left across your picture. I don’t know if it’s intentional, but your photo is very nearly centred on the galactic centre.

Like I said, it would take hours to go over all the interesting detail in this photo. Here’s a full res zoom of just that highlighted red rectangle, in which I’ve done some more highlighting:

Inside those coloured rectangles are:

  • Red – the Lagoon Nebula (M8)
  • Green – the Triffid Nebula (M20)
  • Blue – open cluster M21
  • Yellow – the star, 4 Sagittarii

The Lagoon is an emission nebula, and you can clearly see the red-pink Hydrogen alpha glow in your picture. It’s a so-called H II region about a hundred light years across, in which star formation is actively taking place. The glow is ionised hydrogen gas excited by the intense ultraviolet light of all those new young stars. Somewhat paradoxically, stars can only form in some of the coldest places in the universe – inside molecular clouds that provide a shield from ionising radiation and get down to a chilly 10 K ( -263 degrees Celsius). That’s the sort of temperature needed for the gas pressure to be low enough to be able to collapse under gravity. The Lagoon nebula is home to lots of Bok globules, extra-dark knots of gas and dust within which infrared photos can detect warm protostars forming.

The Triffid is another star-forming region but, by contrast, is a reflection nebula. That is, it’s the result of starlight reflected off gas clouds in the background. It’s dimmer than an emission nebula, so your photo shows less nebulosity than for the Lagoon. It does have emission regions too, such as the pink bit below centre-left of the green rectangle. The bright star in the centre of that is HD 164492, a monster which is twenty times the mass of the Sun and tens of thousands of times as luminous. But if you could zoom in on just that region alone, there would be over 3,000 other young stars surrounding it.

M21 is an open star cluster. That’s a region where the stars have done their forming, the stellar winds have blown away much of the original surrounding gas and dust, and the stars are starting to drift apart to go their separate ways. But M21 is still young – it formed around the time that humans and chimps were diverging on the evolutionary tree, six million years ago. And some of the stars are way younger than that – mere hundreds of thousands of years. They are so young that they haven’t yet “ignited”. They are hot from the gravitational compression of the gas that formed them but haven’t started the core fusion processes that will keep them stably burning for millions of years to come.

I highlighted one individual star in the yellow rectangle, 4 Sagittarii. It’s a bit of an oddity due to its high rotational velocity of 150 km/sec. By contrast, the Earth spins at less than half a km per sec at its equator, and the Sun at three quarters of a km/sec. Of course, the Earth is much smaller than the Sun, so its angular frequency is 25 times that of the Sun in spite of the lower absolute speed. By the same token, we don’t know how often 4 Sagittarii is rotating because we can’t tell its radius. But there’s evidence that it’s a binary star with an unseen companion. Maybe there is mass exchange between the two components which is causing it to spin up.

Anyway … all that is going on just in one little patch of your photo. Truly awesome! Thanks for posting.