I had one of those nights last night, and as I browsed the internet insomniacally, I came across some striking pre-dawn photographs of Comet Tsuchinshan-ATLAS taken by one of my cousins in Hawaii. I checked and it seemed possible that I might be able to see it from the Torenvalk watchtower six km away from us, so got up at 6 and drove over.
Unfortunately the light pollution in our part of Belgium, one of the most densely populated parts of Europe, is too great. I got a nice photo of the Moon with Earthshine, and Regulus visible below it, but that’s all I was getting. The comet is lost in the haze on the horizon; the diffuse streak across the picture is a light beam from a streetlight, and the small streak on the left is an aeroplane trail.
You may not be familiar with the magnitude scale of brightness in astronomy. The human eye responds logarithmically to light, so a first magnitude star is 2.512 times brighter than a second magnitude star, and a second magnitude star is 2.512 times brighter than a third magnitude star. (And a first magnitude star is exactly 100 times brighter than a sixth magnitude star, because 2.512 is the fifth root of 100.) Most people can see stars down to sixth magnitude in a nice dark sky far from any other light source, once their eyes have adjusted.
The earliest record we have of a classification of the magnitudes of stars is by Ptolemy in about 150 AD. His judge-it-by-eye measurements turn out to be pretty robust when compared with modern scientific measurements, and when an nineteenth-century astronomer called Norman Pogson proposed the ratio of 2.512 because it fitted Ptolemy’s classification rather well.
The brightest star in the night sky is Sirius, at magnitude -1.46 (the lower the magnitude, the brighter the star). Venus can get as bright as -4.9. The Full Moon is -12.7 and the Sun around -26.8. Those are fairly meaningless numbers; I find it easier to remember that Arcturus and Vega are almost exactly magnitude 0.0, Aldebaran and Spica around magnitude 1, and the six brighter Big Dipper stars between 1.8 and 2.4, with Mizar (the middle star of the Big Dipper’s handle) dead on 2.0 (though in fact it’s a much more complex system than appears to the naked eye).
This morning, Comet Tsuchinshan-ATLAS’s brightness was 2.6 according to the most optimistic sources, and when I arrived at the observation tower, I could clearly see Regulus, which at magnitude 1.4 is three times brighter, and indeed the Big Dipper. But as dawn arrived, the Big Dipper was long gone, Regulus faded into the surrounding sky, and I could see that the sky where the comet should have been was even brighter, so I came home and went back to bed.
Comet Tsuchinshan-ATLAS is predicted to be really spectacular in the evenings of the second week of October, starting from Wednesday 9th through the weekend. That’s something to cheer us up in the Northern Hemisphere as the evenings start to draw in. Keep an eye out for it!
It’s the classic Monty Python question; I mean what has it done for us? We all have a vague notion that it gives us the tides, but how else can a ball of rock in space help us here on Earth?
The astronomer Maggie Aderin-Pocock has been one of the presenters of the BBC astronomy TV programme The Sky at Night for ten years, following in the footsteps of Patrick Moore. I’m afraid it’s generally on too late for me to watch, but I read this book with much interest, having read Patrick Moore’s classic Guide to the Moon forty years ago.
Moving with the times, it’s a very approachable combination of autobiography, science and culture, with the second quarter of the book looking at the history of lunar observation and at literature inspired by the moon. There’s not much about the Apollo landings – you can find plenty of information about them elsewhere – but there’s a lot about the research findings of what is on and inside the Moon.
But the guts of the book are to explore the effect that the moon has on us – both culturally and scientifically. Aderin-Pocock’s approach is that curiosity about the moon is a gateway drug that may lead readers into more research on science. It’s tightly and breezily written, and recommended. You can get it here.
Thursday this week marks the 2033rd birthday of the Roman Emperor Claudius, who reigned from AD 41 to AD 54. (The calculation is a bit unfamiliar – there is no Year 0; he was born in 10 BC, so his ninth birthday was in 1 BC, his tenth birthday in AD 1, and his 2033rd birthday is 2023 years after his tenth birthday.)
My good friend Professor G has serious doubts that Claudius was actually born on 1 August, 10 BC, but she also makes a good argument as to why he would have wanted to celebrate his birthday on that day. Anyway once he had become Emperor, six months before his 50th birthday, it was too late to change the official narrative.
The historian Cassius Dio records that in the run-up to Claudius’ 54th birthday on 1 August 45 AD, he took the following preventative action:
Since there was to be an eclipse of the sun on his birthday, he feared that there might be some disturbance in consequence, inasmuch as some other portents had already occurred; he therefore issued a proclamation in which he stated not only the fact that there was to be an eclipse, and when, and for how long, but also the reasons for which this was bound to happen.
So, it’s fascinating that a) Claudius thought his birthday was a big deal for his loyal subjects (he may or may not have been correct), and that b) he was completely certain that there would be a solar eclipse on his birthday. The stakes were pretty high – if he had issued a proclamation about an eclipse that didn’t happen, he would have looked really stupid.
What’s really interesting is that Cassius Dio then gives the reason for a solar eclipse, saying that he is quoting Claudius’ decree:
These reasons I will now give. The moon, which revolves in its orbit below the sun (or so it is believed), either directly below it or perhaps with Mercury and Venus intervening, has a longitudinal motion, just as the sun has, and a vertical motion, as the other perhaps likewise has, but it has also a latitudinal motion such as the sun never shows under any conditions.
”Longitudinal motion” means the progress of the Sun and Moon around the ecliptic as their daily and monthly regular apparent movements. “Vertical motion” means varying distance from the earth, and it’s interesting that it’s taken for granted that the Moon’s distance varies but Claudius/Cassius Dio is not as sure about the Sun. And “latitudinal motion” means the Moon’s divergence from the ecliptic due to the inclination of its orbit; since the ecliptic is by definition the Sun’s path, the Sun doesn’t do this.
When, therefore, the moon gets in a direct line with the sun over our heads and passes under its blazing orb, it obscures the rays from that body that extend toward the earth. To some of the earth’s inhabitants this obscuration lasts for a longer and to others for a shorter time, whereas to still others it does not occur for even the briefest moment. For since the sun always has a light of its own. it is never deprived of it; and consequently to all those between whom and the sun the moon does not pass, so as to throw a shadow over it, it always appears entire. This, then, is what happens to the sun, and it was made public by Claudius at that time.
It’s not a bad summary of the mechanisms, and I am inclined to think that Cassius Dio is directly quoting from the imperial proclamation. Claudius, who was a polymath, was au fait with astronomy. Cassius Dio then goes on to explain lunar eclipses, and I find his vocabulary just a bit different, as if he was writing it himself rather than quoting a century-old document. It too is not a bad summary.
But now that I have once touched upon this subject, it may not be out of place to give the explanation of a lunar eclipse also. Whenever, then, the moon is directly opposite the sun (for it is eclipsed only at full moon, just as the sun is eclipsed at the time of new moon) and runs into the cone-shaped shadow of the earth, a thing that happens whenever it passes through the mean point in its latitudinal motion, it is then deprived of the son’s light and appears by itself just as it really is. Such is the explanation of these phenomena.
We tend to forget that the ancient Romans and Greeks had a very good understanding of the movements of the Sun and Moon relative to the earth, but is is well expressed here. and of course people had been predicting eclipses for thousands of years at this stage.
Solar eclipses are more difficult to calculate than lunar eclipses. They do repeat fairly reliably after 18 years, 10 or 11 days (depending on how many leap years are involved) and 7 hours. From a fixed point on the Earth’s surface, such as, for instance, Rome, this means that if you hang around long enough after one eclipse, you’ll see another 54 years and 32 or 33 days later (again depending on the number of leap years in the interim).
As it happens, just a month before Claudius was born, on 30 June, 10 BC, there was a solar eclipse which was nearly total from Rome and most of Europe. If he ever looked up the details of past eclipses (and I’m sure he did) and knew about the 54 years and 32-33 days cycle (and again, I’m sure he did), he’d have been very aware that his 54th birthday was certain to see a repeat eclipse.
Of course, I think it’s also significant that we only know about this from Cassius Dio, who was working from archive documents a century later. My suspicion is that Claudius and Cassius Dio were both much more interested in eclipses than the average inhabitant of the Roman Empire, and that the average provincial governor probably filed Claudius’ proclamation with the latest imperial directive about corn tariffs, so that the mass of the population never heard about it.
But it’s interesting all the same.
Path of the 1 August, AD 45 eclipse from EclipseWise. It is a bit incongruous to see modern (well, pre-2005) political boundaries on the map!Path of the 28 June, 10 BC eclipse from EclipseWise. (Computers cannot cope with the fact that there is no Year Zero in our calendar, so 10 BC is shown as the year -9.)
I hope you can see a small point of light high in the sky here? It is the planet Jupiter, taken from Het Torenvalk, our local nature lookout point, at 5pm this afternoon, half an hour after sunset.
This month is a great opportunity to see all five of the classical planets in the early evening. Jupiter is bright and high a bit east of south; Mars also bright, red and glowering in the east. Saturn is a bit dimmer, but still brighter than most other things in the sky, a bit west of Jupiter.
For Venus and especially Mercury, you’ll have to be lucky with clear horizons and clear skies in the 30-60 minutes after sunset, in the southwest. This evening both had dipped below the treeline at Het Torenvalk by the time it was dark enough for either to be visible. But as the month wears on they will become less difficult to find. On Christmas Eve, 24 December, the crescent moon will be very close to both of them – so if that is a clear evening where you are, pop outside a bit after sunset and have a look.
(Also NB despite summer brightness there will be a better view in the Southern Hemisphere as the planets are at a better angle to the horizon.)
(Also a good astronomy app will help. I’ve been using the free versions of SkyView, Night Sky and Sky Guide, but there may well be better options out there.)
I got up at stupid o’clock this morning to look at the array of planets in the morning sky just before dawn. For the last few days, Venus, Mars, Jupiter and Saturn have been visible all in a row, in that order, around 4 am, and some lucky observers may have been able to catch Mercury in front of the queue a few days ago. (We couldn’t – the weather was bad here over the weekend and Mercury was probably too low on the Belgian horizon anyway.) Street lighting around here is generally bad, but I went to the top of the tower of the Torenvalk park to try and rise above it.
Today’s phone cameras teeter on the edge of being able to do astrophotography with no extra equipment. In this picture I manage to capture Jupiter at the top right, and if you look closely you can see Mars about a third of the way over, just above the two streetlights. Venus was just above the cloud on the left, clear and bright to the naked eye, but drowned out by the dawn in the photograph.
But this was actually the second photo I took. First time round, I forgot to switch off the flash; with the unexpected result that illuminating the railing in front of me dampened the effect of the dawn and made Venus visible in the photo after all. Jupiter is still there, but this time it’s Mars that disappears into the background.
Saturn was way too far over to the right / south to fit onto the field of view, and using the panorama mode on the phone camera to try and capture all four planets just gave me a long black rectangle.
But I felt a bit of solidarity with the Babylonians tracking the movements of Ishtar, Nergal, Marduk and Nunurta four thousand years ago.
