Sky at a Glance December 8 – 15

Photo showing location of Comet 46PWirtanen in the December sky courtesy of Sky and Telescope.

This Week’s Sky at a Glance, December 8 – 15 ~by Curt Nason

The excitement in the sky this week is twofold: a comet and comet (or asteroid) remnants. Comet 46P/Wirtanen has been in the news the past month as a possible naked eye sight in mid-December, with cautionary notes from experienced observers that it might appear large and therefore have a low surface brightness. Under such conditions, light pollution could mask it entirely. I saw Wirtanen from Saints Rest Beach, outside the Irving Nature Park in Saint John, on Wednesday evening. My initial view was with 15×63 binoculars and my first impression was that I could see hints of its green glow that appear in pictures. A trick we use to enhance star colours is to de-focus a telescope or binoculars so that the light falls on a larger number of cone cells in our eye. With the comet’s coma being nearly the apparent size of the Moon, perhaps that was the reason for that impression. Before observing it in my 8-inch telescope I saw it in the 8×50 finder, and I had occasional glimpses naked eye using averted vision (looking off to the side of it to take advantage of the more sensitive rod cells that circle our day-vision cone cells).

Over this week Comet Wirtanen passes rapidly east of the head of Cetus the Whale and into Taurus. Next weekend, when it is closest to Earth at a distance of about 11 million kilometres, it passes between the V-shaped Hyades star cluster and the compact dipper-shaped Pleiades star cluster. It is expected to brighten and look larger than the Moon over that time, which could reduce the surface brightness. Comet maps can be obtained from the Skyhound and Heavens-Above websites, but the most practical one for Wirtanen this week was produced by Bob King for the Sky and Telescope website.

The other highlight this week is the peak of the Geminid meteor shower, which occurs around 9 am Friday, December 14. Thursday night, Friday morning and Friday night are prime time hours. The Moon sets around 10:30 pm Thursday and an hour later on Friday. Find a location away from streetlights, get comfortable in a reclining chair, dress for warmth and use a blanket, and look up toward your clearest and darkest patch of sky. The Geminids progenitor is asteroid 3200 Phaethon, which is likely a dead comet.

This Week in the Solar System

Saturday’s sunrise in Moncton is at 7:48 am and sunset will occur at 4:33 pm, giving 8 hours, 45 minutes of daylight (7:50 am and 4:41 pm in Saint John). Next Saturday the Sun will rise at 7:54 am and set at 4:34 pm, giving 8 hours, 40 minutes of daylight (7:56 am and 4:42 pm in Saint John).

The Moon is a slender crescent this weekend after sunset and is at first quarter next Saturday. Planetary action has shifted to the morning sky with Venus blazing brilliantly and Mercury climbing to its greatest elongation next weekend, rising nearly two hours before the Sun. Jupiter is less than a fist-width lower left of Mercury and closing in. Saturn is too low in the west at sunset for observing, and Mars getting too small to reveal much detail in a telescope.

The William Brydone Jack Astronomy Club meets in the UNB Fredericton Forestry / Earth Sciences building on Tuesday, December 11 at 7 pm. All are welcome.

Questions? Contact Curt Nason.

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