December Night Sky

December is a time of transition for the night sky, with even Cygnus, that last element of the summer sky, disappearing in the West during the evening, as Orion, the core of the winter sky, rises in the East. This endless cycle of comings and goings over the year is, for me, one of the great pleasures of astronomy.

Two particular things I’ll be watching out for this month. The Geminid meteor shower has been increasingly bright in recent years, as the earth’s orbit and the denser parts of the meteor stream have moved closer together. Geminids tend to be bright, but slow by meteor standards. The maximum this year will be on the nights of 13 and 14 December (just after new moon, so no interference from that source). As always you are more likely to see them after midnight as the night sky where you are is then facing the direction of motion of the earth in its orbit. It’s a bit like looking out of the windscreen, rather than side or back windows, as a car drives through a snowstorm! You don’t need any optical aids – just patience and warm clothing. At the peak you should see about one a minute. Ideally you should look east (if it is evening) towards Gemini, the constellation from which the meteors seem to radiate, but it is not essential.

Another event of interest is on 23 December when the moon will occult (ie go in front of) Aldebaran, the brightest star in Taurus ( Aldebaran is seen as being the bull’s eye, and this is certainly one of the constellations where the image the ancients associated with it is easy enough to see). Aldebaran is the brightest star that the moon can occult in this way. It is an orange/red giant, about 100 times as bright and wide as the sun, though only about twice as massive. At 65 light years away it is also one of our closer giant neighbours. The occultation starts at about 6.08 pm as the dark limb of the almost full moon moves in front of the star. It will reappear on the bright western side about an hour later – this is the time it takes the moon to move by its own diameter in its orbit around the earth.

I wondered if there is any chance of seeing particularly the beginning of the occultation as a gradual process rather than instantaneous winking outof the star’s light. All stars look like points of light even in the largest telescopes, but of course they are tiny discs, but too distant to see as such, except using special techniques with the largest professional scopes. Aldebaran being a quite close giant has a relatively large apparent diameter , amounting to 0.02 seconds of arc – or about 1/100,000 the apparent diameter of the moon. I calculate that it will take the moon just 1/20 of a second to move across the star’s disc. You will need to be pretty awake for this to seem anything but instantaneous! In theory there will be a second chance to test this an hour later as Aldebaran reappears on the moon’s bright limb, but it will be hard to know precisely where to look, and it will there before you know it!