As winter gradually settles in, the naked-eye planets from Mercury to Jupiter present a celestial banquet for the eyes; and in preparation for the festive season, the Moon occults Aldebaran while the Geminid meteors light the mid-month sky.
Mercury, Venus and Mars at dusk
The elusive planet Mercury is relatively easy to spot during the first half of the month. It’s normally lost in the Sun’s glare, but the gap between the two reaches a maximum (greatest elongation) on December 11, resulting a favourable evening apparition.
The best period for spotting Mercury is between the 5th and 15th. Using binoculars, begin scanning the sky about 30 minutes after sunset, 5 degrees above the southwest horizon. The planet will appear like a tiny star-like point shining against the deepening glow of twilight: Of course, a clear horizon is absolutely essential. On December 7, Mercury will pass about ⅛ degree below λ Sagittarii (Kaus Borealis) the 2.8 magnitude star representing the tip of “the lid” in “the teapot” of Sagittarius: If you follow the tiny planet from December 6 to 8, you’ll experience just how quickly it moves with respect to the background stars.
After the 15th, Mercury begins rapidly approaching the Sun once again and disappears in the glare of sunset around the 22nd. The furtive planet is between the Earth and Sun (inferior conjunction) on December 28.
Throughout December, Venus, second of the inner planets, is impossible to miss shining as the brilliant Evening Star high above the west-southwest horizon: It remains a fixture in the evening sky and by mid-month sets more than 3½ hours after the Sun! The dazzling planet begins December among the stars of Sagittarius; it moves into Capricornus on the 6th and quickly crosses the constellation, winding up in Aquarius on the 31st.
The waxing crescent moon will appear to the right of Venus on December 2, and above it the following evening. As the sky darkens, be sure to notice the soft glow of earthshine illuminating the otherwise dark portion of the lunar surface; it provides an added dimension to this beautiful twilight scene.
Mars continues to slowly fade as its distance from Earth gradually increases: This December it wanes by ¼ of a magnitude from 0.60 to 0.85. However, since Mars spends the month in a region of sky devoid of bright stars (starting in Capricornus and moving into Aquarius on the 15th) it manages to stand out nonetheless. Though currently too small to be of interest in a telescope, the Red Planet’s tell-tale colour makes it an interesting naked-eye object.
Begin looking for Mars around mid-twilight, 45 minutes after sunset: You’ll spot it shining some 30 degrees above the south-southwest horizon. Throughout December, Venus plays “catch-up” with the Red Planet: As the month begins, Venus is in Sagittarius, some 24 degrees to the lower right of Mars, but by month’s end, the two will shine among the stars of Aquarius and will be 12 degrees apart, on their way to an early-February conjunction. A waxing crescent moon will appear near the Red Planet on the evenings of December 4 and 5.
Jupiter at dawn
Jupiter rises progressively earlier as the month goes by: At the beginning of December, it emerges above the eastern horizon around 2:30 a.m., and rises around 1:00 by month’s end. In any case, the giant planet is well placed and easy to spot shining high in the southeast among the stars of Virgo at dawn. Jupiter’s cloud bands are fascinating to see in a small telescope, but you can even see the planet’s four Galilean moons with an ordinary pair of binoculars.
On the morning of December 5, the four moons—Io, Europa, Ganymede and Callisto—will form a line, extending out from the planet’s lower left, in that exact order: This is actually the order of their orbital distance from Jupiter. Then on December 29, the four Galilean moons will again align in the same order, but this time extending from the planet’s upper right. The best time to observe these alignments will be around 6:00 a.m., an hour-and-a-half before sunrise. Meanwhile, our Moon will appear as a waning crescent shining above Jupiter on the morning of December 22.
One last occultation of Aldebaran for 2016
Lunar occultations occur when the Moon passes in front of a celestial object (when that object is the Sun, it’s called it a solar eclipse). We are currently in the midst of a series of 49 lunar occultations of Aldebaran, the brightest star in Taurus, running from January 29, 2015 to September 3, 2018; of these, 7 could be seen in their entirety from the Montreal area.
The last lunar occultation of Aldebaran this year takes place on the night of December 12. Seen from Montreal, Aldebaran will disappear behind a nearly-full moon around 11:13 p.m. and will reappear on the other side about 00:27 a.m. Note that the exact timing is sensitive to the observer’s geographical location; to avoid missing the event, prepare to observe it a few minutes in advance. All that’s needed is a clear sky: Though binoculars or a small telescope aren’t necessary, they help reveal the colour contrast between the orange giant star and the bright lunar limb. If for any reason you miss this occultation, take heart: Two more will be visible in 2017, but you’ll need to wait until next November and December to observe them.
The Geminid meteors
The Geminid meteor shower occurs each year during the night of December 14 to 15, and while the radiant (the point in Gemini from which the meteors appear to emanate) is well up in the southeast around mid-evening, this year the nearly-full moon will also occupy the constellation—not a very promising situation. However, since the Geminids are one of the brightest and most prolific annual showers, they are worth mentioning. If you happen to be out for a stroll, be on the lookout for a potential bright fireball or two… and be sure to bundle up!
Happy holidays and… Clear skies!