The skies are serving up a rich astronomical feast the last month of the year.
To whet your appetite, the perigee full Moon (or “supermoon”) shines on December 3. Your enchanting main course is the Geminids meteor shower, which peaks on the night of December 13-14. As a wonderful side dish, Mars, Jupiter and the Moon group together before sunrise on December 14. For your final course, look for Mercury at dawn on the last days of the year or watch as the Moon occults the star Aldebaran on the evening of December 30.
Supermoon on December 3
The final full Moon of 2017 and the first two of 2018 are called perigee full Moons since they occur within 24 hours of the Moon reaching perigee, the point of its elliptical orbit closest to Earth. The Moon is full on December 3, and it’s the “largest” full Moon of 2017.
Though the term “supermoon” suggests that the lunar disc visibly appears much larger than usual, this isn’t the case. The Moon’s apparent size, which is on average 31 arcminutes (average distance of 384,400 km), increases to 33 arcminutes at perigee (distance of 356,700 km). This difference of just over 5% is too subtle for the naked eye to detect. Tides, however, are sure to be more spectacular, especially if winter storms are raging.
Mid-December conditions are ripe for a bonanza of shooting stars during the Geminids. The meteor shower is set to peak around 1:30 a.m. on the night of December 13-14.
On this date, the constellation Gemini is very high to the south, and the waning Moon rises late at night and doesn’t hamper observations. The Geminids are the most stable and one of the most active meteor showers of the year, more so than the famous Perseids in August. The Geminids are caused by an asteroid rather than a comet, and particles enter our atmosphere at relatively slow speed.
In a very dark, clear sky free of light pollution, you can see a hundred meteors an hour at the peak. Dress warmly and bring along your wish list for the end of the year.
Once winter starts (the solstice is on December 21 at 11:27 a.m.), Mars and Jupiter are very visible to the southeast late at night and at dawn. On the morning of December 14, the crescent Moon forms a nice triangle with the two planets. Mars, recognizable by its orange hue, lies above the much brighter Jupiter. A must-see event after a night under the shooting stars.
Mercury, which is hard to spot in the sky except on rare occasions, is far enough from the Sun’s glow by the end of the month to be easily observable. As of December 21, the tiny planet can be seen just above the southeast horizon at dawn 45 minutes before sunrise. Mercury’s period of visibility runs till early 2018.
Finally, the waxing gibbous Moon occults Aldebaran for about an hour on the evening of December 30. Aldebaran, the brightest star in the constellation Taurus, disappears behind the dark edge of the Moon around 6:28 p.m. in Montreal and reappears on the opposite side around 7:27 p.m. Since the exact times that it disappears and reappears depend on your precise geographic coordinates, make sure you have your eyes on your binoculars or small telescope a few minutes before the times specified. This always fascinating phenomenon plays out literally in the blink of an eye.
Happy December and happy skywatching!