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December 1 The December Sky Northern observers have both their main ‘guides’ on view. Orion attains a respectable altitude by mid-evening, and Ursa Major is well above the north-east horizon, even though the Great Bear does appear to be standing on its tail. Capella in Auriga is very close to the overhead point, which means that Vega is skirting the northern horizon and we have certainly lost the Summer Triangle, though in fact Deneb, like Vega, never actually sets over the British Isles. Cassiopeia is high in the north-west. Pegasus remains visible in the west, though before very long it will start to merge into the evening twilight. Sirius has made its entry, and around midnight Leo appears over the eastern horizon. The Milky Way is at its best, running right across the sky from Cygnus through Cassiopeia, Perseus, Auriga and Gemini down to the horizon. Southern observers lack Ursa Major, but Orion is excellently placed together with the Hunter’s retinue; Achernar is close to the overhead point, and Canopus is also high and Pegasus well above the horizon. Capella is very low in the north, and the Southern Cross also is badly placed during December evenings. The Milky Way is superb, and so too are the Clouds of Magellan; it is sad that there are no comparable Clouds in the northern hemisphere of the sky.
December 2 The Colours of the Stars Many people think ‘that all stars look white’. This is certainly not so. Some are yellowish, some orange, and some very red. An obvious way to show this is to look at Orion, which dominates the evening sky for most of northern hemisphere winter or southern hemisphere summer. The two leading stars are Betelgeux and Rigel, and the most casual glance is enough to show that whereas Betelgeux is orange–red, Rigel is white or slightly bluish. Of course, the colour differences are due to real differences in surface temperature; white heat is hotter than red heat, so that Rigel is much hotter than Betelgeux. However the situation is complicated by the fact that when a star is low over the horizon, its light comes to us after having passed through thick layers of the Earth’s atmosphere and the star seems to flash various colours. Sirius is the best example of this. It is
The Observer’s Year
Future Points of Interest 2005: Occultation of Antares, 16 h, America, W. Africa. 2008: Occultation of Venus, 16 h, Europe. 2010: Mercury at Eastern elongation.
Anniversary 1974: Pioneer 11 flew past Jupiter at 28 840 miles, sending back splendid data. It then went on to encounter Saturn in September 1979. Contact with it was lost in 1996, and it is now on its way out of the Solar System.