Some of us have been anticipating the close approach of comet 46P/Wirtanen for years. The time is nigh! Every 5.4 years, the comet drops into the inner solar system from out past Jupiter and occasionally passes near Earth at the same time it’s at perihelion or closest to the sun. This happy set of circumstances unfolds next month, when the comet reaches perihelion on Dec. 12 just four days before it zips closest to Earth.
Right now, 46P is slowly climbing northward through the stars of Cetus the Whale and visible from mid-northern latitudes starting about 7 p.m. till midnight. I found it in 10×50 and 8×40 binoculars last night both from a dark, rural sky and from my home closer to town. Although its magnitude or brightness stands right at the naked-eye limit of 6, the comet is a very diffuse glow in binoculars and generally still not visible without some kind of optical aid.
But it’s expected to brighten three more magnitudes as it moves northward into into darker, more transparent skies. Throughout, Wirtanen will probably NOT show much of a tail, but it the head of the comet — called the coma — will become increasingly obvious in binoculars as we move into December. By that time, folks living in the country should have no problem just looking up to the left and below the Pleiades star cluster to see it with the naked eye — 46P will look like a ghostly, glowing patch in the sky.
Right now, it’s very difficult to spot from light-polluted locations in binoculars, but if you live in the outer suburbs or have access to a rural sky, it’s relatively easy if you know just where to look. That’s why I’ve included a map that includes with easy, bright stuff like Aldebaran, Orion and the Pleiades to help you get oriented.
For the next week or so, the comet will be moving across Cetus and Eridanus, big, gangly constellations with few bright stars, so it will take just a bit more effort to find. Your key stars / references are the Pleiades, Aldebaran (Taurus’ brightest star) and Menkar, the second brightest star in Cetus and equal in brightness to several of the stars in the Big Dipper. Use them and the other labeled stars to shoot a line to the comet or include the comet as an apex in a triangle. I always find that if make my target part of a simple pattern — line, triangle, square — involving easy-to-see stars, I have a much better chance of spotting it.
Comet Wirtanen brushes closest to Earth on Saturday, Dec. 16 when it will pair up with the Pleiades. We have until about Dec. 13 with little to no interference from a bright moon. After that, the moon gets brighter and higher, diminishing the comet’s brightness and its contrast with the sky. You can also elect to stay up late AFTER moonset, in which case you’ll have through Dec. 19 to see 46P in a dark sky. Starting on the 20th, the moon will be inescapable. To plan your observing nights, check your local time of moonset and moonrise here.
Stop back here for updates on the comet in case it does crazy stuff like go into outburst. Yes, comets have “fits” when they suddenly brighten as fresh ice gets exposed due to solar heating. That ice vaporizes in a quick, carrying off loads of fresh dust that shines brightly in sunlight.
While 46P is no Hale-Bopp, I want you to be excited about seeing the comet and wish you clear skies for a good look. Onward!