As we come to the end of December, comet 46P/Wirtanen is fading. Comets can be unpredictable , but this one came pretty close to expectations, missing its brightness prediction by just one magnitude. Instead of an easy magnitude 3 (one level below the Big Dipper stars), it reached a dimmer magnitude 4. If you lived in or near a large city, 46P proved challenging. But from outer suburbs, it was visible in binoculars and from dark skies, you could easily spot it with the naked eye. I saw it routinely many nights because — incredibly — we had a week of clear weather when the comet was at its best at mid-month.
If I could have wished for one thing it would have been a brighter, more obvious tail. That’s where much of a comet’s beauty lies. Wirtanen managed only a short, faint stub that required a good-sized telescope to see. Had we been able to view the comet from the side, the tail would have been more obvious, but as it was, 46P was best and brightest when it was at opposition to the sun with its tail pointing away from us and mostly hidden behind the fuzzy coma.
Nature is what it is. Certainly, the comet had its own sort of ghostly charm. And still does. You can still see it in binoculars as it crawls across the long, skinny constellation of the Lynx and from there into Ursa Major the Great Bear. The bright, twinkly star Capella, halfway up in the northeastern sky at nightfall, will help point the way. 46P lies about two fists to its lower left. Use the map provided, start at Capella and star-hop to the northeast until you land on a fuzzy patch.
Two nights ago (Dec. 28), I easily spotted 46P in a dark sky but noticed that it had faded to about magnitude 5. Although difficult to see with the naked eye, I saw it easily in binoculars as a soft, glowing patch of light. For the next 10 nights or so, without a moon to muss the view, the comet should remain visible in 40-50mm binoculars from reasonably dark skies. Once the moon returns, most casual observers will let go of Wirtanen and move onto other things. Even as you do, know that 46P/Wirtanen will hang out in the Great Bear all the way through early March, fading all the while.
Winter’s a wonderful season for early morning skywatching because of the late hour of sunrise. I can get up a little before 7 and still see the brighter stars and of course the planets, Venus in particular. Tomorrow morning, the last day of the 2018, we have a great lineup at dawn that includes two stars, three planets and the waning crescent moon. Let’s go down the line from highest to lower: Spica, Virgo’s brightest star, followed by the moon, Venus, Jupiter and Mercury. We’ll throw in Antares (Scorpius’ brightest star) even if it’s out of line just because we’re feeling magnanimous at year’s end.
The map depicts the sky about an hour before sunrise. For the best view find a location with an open view to the east-southeast. Mercury will be quite low at map time, so you may want to wait a bit till it rises higher, about 15-20 minutes later. All the others should be easy to see.