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!