End off 2018 with plenty to watch in the night sky
Saturn will be difficult to spot without the use of binoculars or a scope this month. Shining at -0.5 magnitude, the ringed planet sits only about a hand's width high in the SW about 30 minutes after sunset, setting about 90 minutes later during the first half of December.
By mid-month, it disappears into the glow of the setting sun and is lost from view, heading towards its superior conjunction with the sun in early January. Look for the faint, barely-visible, waxing, crescent moon to the lower right of Saturn on Dec. 8 and to the planet's upper left on the evening of Dec. 9.
Mars is visible from dusk until around 11:30 p.m. each evening in December. The red planet fades this month from mag. -0.1 to +0.5, as it moves from Aquarius - the Water Bearer into Pisces - the Fish. Look for Mars (as a reddish "star") just above the near-first quarter moon on the evening of the 14th.
Venus has been steadily moving higher in the pre-dawn sky since early November and will reach its peak sunrise altitude (33 degrees) in the pre-dawn sky on Dec.13, when it will sit about 1/3 (33 degrees) the way up the eastern sky about 3 1/2 hours before sunrise. Currently at its peak brightness as a morning apparition, our "morning star" will dim slightly from mag. -4.9 to -4.6 by the end of the month. Venus will be visible in the pre-dawn sky until mid-August 2019.
Having passed through inferior conjunction with the sun on Nov. 27, Mercury appears as a pre-dawn apparition throughout December. Our solar system's smallest and innermost planet should be visible by Dec. 6 at around mag. +0.5. By Dec. 15, Mercury will have reached is greatest western (morning) elongation from the wun (21 degrees), rising in the east about 75 minutes before the sun. Mercury brightens considerably throughout December, from 0.0 on Dec. 8 to -0.5 by Dec. 31.
Like Mercury, Jupiter also had a conjunction with the sun in November (superior conjunction on Nov. 26), and follows Mercury into the pre-dawn sky in December. Moving from Scorpius - the Scorpion into Ophiuchus - the Serpent Bearer by mid-month, Jupiter (mag. -1.7) rises about 1 hour before the sun by Dec 12. On Dec. 21, look for Jupiter in close conjunction with Mercury (just to the upper left of Jupiter) low above the SE horizon, when the two planets will be less then 1 degree apart. Though Jupiter will be fully lit, and Mercury only 3/4 lit, the diminutive planet will actually have a greater surface brightness than its larger sibling. From about mid-month onward, Venus, Mercury and Jupiter will form a straight line (slanted downward to the left) in the eastern, pre-dawn sky (about 1 hour before sunrise), with Venus uppermost, then Mercury (until Dec. 21), followed by Jupiter just above the horizon. After their close conjunction on Dec. 21, Jupiter begins to rise higher in the pre-dawn sky, while Mercury sinks lower towards the horizon. On the morning of Dec 31, look for the reddish star, Antares (in Scorpius), to the lower right of Jupiter.
The Geminid meteors shower (radiant in Gemini - the Twins) peak in the pre-dawn hours of Dec. 14. The Geminids are unique, in that they are one of only two meteor showers that have an asteroid source. The Gems, as they are affectionately called, are associated with the asteroid 3200 Phaethon, a "rock comet", which ejects or outgases dust and rock material from its rocky surface, rather than ice-related dust and vapors like a regular comet. Asteroid Phaethon is also unique, in that it comes closer to the sun than any other named asteroid. The near-first quarter moon sets around midnight. As the Gems will start to intercept the Earth's upper atmosphere about 2 hours after sunset on Dec 13 and if the weather is clear, you may catch sight of numerous "Earth grazers" - bright, long-lasting fireballs streaking across the night sky. Expect to see 120+ meteors/hour. from a dark location in the pre-dawn hours.
As mentioned last month, Comet 46P Wirtanen is expected to reach naked-eye visibility this month. Having reached perihelion (closest approach to the sun) on Dec. 12, 46P should (hopefully) put on fine display when, on Dec. 16, it make its closest approach to Earth (perigee) at approximately 11.7 million kms. Google '46P Wirtanen' to find locator charts and current information.
The winter solstice, the official start to winter here in the northern hemisphere, occurs at 6:23 p.m. on Dec. 21. It also marks our shortest day and longest night of the year.
Until next month, happy holidays, happy new year and clear skies.
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column appears in The Guardian on the first Wednesday of each month. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at firstname.lastname@example.org.
Dec. 7 - New moon
Dec. 12 - Moon at apogee
Dec. 14 - Geminid meteor shower peak; pre-dawn
Dec. 15 - First quarter moon
Dec. 16 - Comet 46P Wirtanen at perigee
Dec. 22 - Full moon
Dec. 24 - Moon at perigee
Dec. 29 - Last quarter moon