The Night Sky in December
The year’s final month plays host to the darkest skies, courtesy of the Northern Hemisphere’s winter solstice on December 21, the date when the Sun’s apparent position against the background stars reaches its most southerly point.
The length of night is long, too, affording a view of nearly four complete season’s worth of stars. Early evening look west and you’ll see the now misplaced Summer Triangle, a large pattern formed from bright stars Vega, Deneb and Altair. Southernmost Altair, sets around 9pm mid-month, giving the out-of-season triangle some closure.
Looking south early evening, the stars of autumn are on show marked by two additional geometric patterns, the Great Square of Pegasus and, below it, the dimmer Circlet asterism in Pisces; a ring of faint stars. Bright Jupiter currently sits south-east (below left, as seen from the UK) of the Circlet, a useful location guide. If you can see it, the Great Square is directly north, square sides a little over twice the width of the Circlet.
Rising in the east early evening is the winter constellation of Orion the Hunter, a truly resplendent pattern, easily identified because of the three similar brightness stars forming the Hunter’s belt. Positioned in the middle of Orion, they appear equally spaced in a straight line. If you have binoculars, take a look at the faint line of stars hanging down from the Belt. Known as Orion’s Sword, this contains the beautiful glowing nebula M42, the closest starforming region to Earth.
Follow the belt line up and right to arrive at bright and orange-hued Aldebaran, the brightest star in Taurus the Bull. This is currently rivalled by the nearby bright planet Mars also showing a hint of orange, although salmon-pink is probably a more apt description of its visual colour.
Mars currently appears to be drifting west (retrograde) against the background stars. It’ll keep moving in this direction until January 12 when, due to the orbital speed difference between Earth and Mars, it will appear to change direction, thereafter heading east (prograde).
As the night progresses Orion gets higher, reaching highest altitude when due south around midnight mid-month and 11pm by the end of December. Follow Orion’s belt away from Aldebaran and you’ll eventually arrive at Sirius, the brightest star in Canis Major the Great Dog, and the brightest night time star. The Winter Triangle is formed from the three stars Sirius, Procyon in Canis Minor the Little Dog and Betelgeuse in Orion.
From the UK, Sirius flickers, sometimes flashing bright vibrant colours. Two effects cause this. Atmospheric turbulence causes micro-refraction, rapidly deviating a star’s light by a small amount to cause the flicker. The colours are a result of atmospheric dispersion, an effect where the atmosphere acts like a prism spreading starlight into its component colours. The effect gets more severe with lower altitude. Combined they produce the effect seen with Sirius.
Sirius is a close neighbour of the Sun, just 8.6 lightyears distant. Also known as the “Dog Star”, Sirius has a white dwarf companion, Sirius B, which is nicknamed the “Pup”. This ultra-dense star is in a mutual 50-year orbit with Sirius A. The Pup is physically nearly as large as the Earth but contains 1.02 times the mass of the Sun.
The apparent separation of the pair waxes and wanes over the 50-year orbital period and currently they are at their widest. With an apparent separation slightly smaller than the current apparent diameter of Mars, and considering how bright Sirius A appears, the Pup is challenging to see!
Finally, if you are something of a night owl and stay up until the early hours of the morning, you will see the stars of spring led by Leo the Lion making an appearance, completing the seasonal cycle of the constellations in a single night.