Winter doesn’t begin until December 21, but now is the time to get acquainted with the principal stars and constellations of the coming months. Several planets also invite our attention, especially when the Moon draws close to them. And despite the seasonal chill, the Geminid meteors are well worth observing around mid-month.
December offers a glimpse of the glacial skies that lie in the season ahead. Of particular note is the Winter Hexagon, which consists of the six brightest stars in each of the six major winter constellations.
To identify the great celestial hexagon, begin by locating Orion above the southeast horizon: The constellation’s main stars form the shape of an hourglass. The brightest star in Orion, Rigel, is situated at the lower right corner of the constellation. Rigel is a blue supergiant whose colour is unmistakable to the naked eye; it will be our starting point for tracing the Hexagon.
The constellation of Taurus is located above Orion. The Hyades, a V-shaped star cluster, forms the raging bull’s head; Aldebaran, a bright orange giant star, represents the fiery eye of Taurus. Despite appearances, Aldebaran isn’t part of the Hyades, but sits about 65 light years away, roughly midway between the Earth and cluster.
Auriga sits to the upper left of Taurus. Its brightest star, Capella, appears single; in reality, it’s a system of four-stars grouped together in two pairs! The first pair consists of two giant stars that measure 10 solar diameters each; the second pair consists of two red dwarfs.
Gemini, the next constellation in the Hexagon, shines below Auriga and to the left of Orion. Gemini symbolizes the twins, Castor and Pollux, which are represented by two bright stars that appear one above the other. The upper one, Castor, actually consists of three binary systems – making a total of six stars altogether! Pollux is brighter than Castor and is, in fact, the brightest star with an exoplanet orbiting it: Pollux b. It was discovered in 2006 and is similar in size to the gaseous giant planets in our solar system.
The Hexagon continues below the twins, toward the constellation, Canis Minor. Its main star, Procyon, is the eighth brightest in the sky, and like many others, it’s a binary system consisting of a white giant orbited by a white dwarf. At a distance of 11.4 light years, it’s also one of the closest stars to Earth.
The last stop in the Winter Hexagon is Canis Major, which contains resplendent Sirius, the brightest star in the sky after the Sun. Seen from the Northern Hemisphere, Sirius never rises very high above the horizon because of its position, 16 degrees below the equator on the celestial sphere. Sirius is also a binary star: Its main component, Sirius A, is a “normal” star (that is, on the main sequence) that shines 26 times brighter than the Sun. Its companion, Sirius B, is 10 000 times fainter and is an extremely hot, very dense, compact star: In fact, it’s a white dwarf – a stellar cadaver – the first star of its kind, discovered in 1862. From here, the Winter Hexagon continues back up to Rigel and completes itself.
The Moon and planets
The month begins with the Moon at last quarter on December 3. From December 4 to 8, the dawn sky provides some beautiful sights: On the morning of the 4th, Jupiter shines above the waning lunar crescent. The following morning, the Moon approaches Mars, and on the 6th, it appears between Mars and Spica, the brightest star in Virgo. On the 7th, the crescent moon sits above dazzling Venus. And finally, on December 8, all five objects are spread out in a line: From lower left to upper right you’ll see a thin lunar crescent followed by Venus, Spica, Mars and Jupiter.
By December 11 the Moon is new; and it reaches first quarter on the 18th. On the evening December 23, a gibbous moon will appear near the bright star Aldebaran, in Taurus. And on Christmas night, the full moon will illuminate the sky.
Saturn has been missing of late. But toward month’s end, the ringed planet reappears in the southeast at dawn, to the lower left of dazzling Venus… though you’ll have to wait until 6:30 to see it well.
December shooting stars
The Geminid meteor shower peaks on December 14, three days after the new moon. The shower is visible after nightfall; but since the radiant is near Castor in Gemini, it appears highest in the sky around 2:00 A.M. From the city, one can only expect to see a few meteors per hour due to light pollution: From the country, however, over 60 shooting stars an hour should be visible when the shower peaks! Though the Geminids are the most prolific shower of the year, they are not as widely observed as the Perseids due to the cold, which hampers even the most courageous observers.
Winter… December 21
The winter solstice occurs on December 21 at 11:48 P.M. Eastern Standard Time. It marks the shortest day of the year, with 8 hours 43 minutes of daylight at the latitude of Montreal; it also marks the longest night, with15 hours 17 minutes of darkness. For starry-sky aficionados, this is the “ideal” night… provided it’s clear, and not too cold, of course!
Clear skies to all!