On December 16th, the comet will pass within 12 million kilometers of Earth and should brighten to about 3rd magnitude, though the diffuse coma and a nearly full Moon will make observations difficult.

Typically during the course of a year about a dozen comets will come within the range of amateur telescopes. Most quietly come and go with little fanfare, but during the upcoming weeks one rather small comet will be making an unusually close approach to the Earth. On December 16th, Comet 46P/Wirtanen (pronounced WERE-tuh-nun) will pass just over 11 million kilometers (7 million miles) from Earth.

Discovery and history

It was on January 15, 1948 that Carl Alvar Wirtanen, a 37-year-old senior observing assistant at the Lick Observatory in California, detected the faint image of a 17th magnitude comet on a photographic plate.

Initially, astronomers determined 46P/Wirtanen to have an orbital period of roughly 6.7 years. On occasion, however, 46P will pass close to Jupiter and its potent gravitational field, which can perturb the comet’s orbit. In April 1972 and again in February 1984, the comet made close approaches to Jupiter, ultimately shortening its orbital period from 6.7 to 5.5 years and pushing the comet’s perihelion distance some 82 million kilometers (51 million miles) closer to the Sun, to a point just over 8 million kilometers (5 million miles) outside of Earth’s orbit.

Thus, the stage was set for this year’s very close approach to the Earth. Ranked in terms of distance from Earth, this will be the 20th closest approach of a comet dating as far back as the ninth century A.D., and the tenth closest approach since 1950. The minimum distance between 46P and the Earth (perigee) will be 11,586,350 kilometers (7,199,427 miles), occurring December 16, 2018, at 13:06 Universal Time, a little less than four days after the comet passes its minimum distance from the Sun of 187 million kilometers (98.1 million miles).

Where to find it and viewing prospects

As December opens, 46P will be near the border between the constellations Cetus and Eridanus at a declination near –20°. By New Year’s Eve, it will have rocketed on a north-northeast trajectory to a declination of +56° into the constellation Lynx. For most mid-northern latitude locations, it will become circumpolar on the day after Christmas.

Along the way, on the evening of December 15th it can be conveniently found passing almost midway between the Hyades and Pleiades star clusters. On December 21st, it will from a triangle with the lowest two stars that make up “The Kids” asterism in Auriga: 10 Eta Aurigae and 8 Zeta Aurigae. And on the nights of the 22nd and 23rd, it will pass within a few degrees to the south and east of the brilliant yellow-white star Capella.

A number of different predictions have been made regarding the brightness of 46P as it passes closest to Earth in mid-December. The Smithsonian Astrophysical Observatory’s Minor Planet Center forecasts a peak magnitude of +8. However I believe, based on observations of 46P through late October, that the forecast of Japanese comet expert Seiichi Yoshida will be much closer to the truth,1 indicating a magnitude of no fainter than +4 during December and peaking near +3 at the December 16th perigee.

I myself have examined data collected from two previous favorable apparitions of 46P in 1986 and 1997. Extrapolating from the comet’s average distance and magnitude during those apparitions puts the comet’s peak brightness near +3 in mid-December, in agreement with Yoshida’s estimates. I also expect 46P’s coma to rapidly swell during early December, reaching an apparent angular diameter conservatively somewhere between 1° and 1.5° — two to three times the apparent diameter of the Moon — on the night it is closest to Earth.

(Be sure to download Sky & Telescope's black-and-white finder chart, perfect for taking to the telescope. For those not blessed with dark skies or clear weather, the Virtual Telescope Project will stream live images of 46P from their robotic telescopes on December 12th and 17th.)

Keep your expectations low

But as compelling as this all may sound, I now must temper any excitement by providing a very important disclaimer.

At the beginning of December, many people with binoculars and small telescopes will no doubt attempt to follow the path of 46P/Wirtanen across the night sky. But actually seeing it will strongly depend on your observing site. From locations that are plagued by light pollution, I bet that sighting this comet is going to prove to be a difficult to near-impossible task. And even for those who are blessed with dark and starry skies, finding the comet could prove to be a bit of a challenge. This is because the comet will be unusually large in angular size, as well as appearing very diffuse . . . almost ghostly. Indeed, many with little observing experience will sharply question the predictions for a third or fourth magnitude object. But remember, you’re not looking for a sharp star-like object, but rather something which is spreading its light out over a relatively large area.

In fact, under a completely dark sky, free of light pollution, perhaps the best instruments for locating the comet will be your own two eyes, especially if you use averted vision.

Most who ultimately locate it in their binoculars or telescopes will, I believe, typically describe it as a nearly circular cloud, comparable to or rivaling the Moon in angular size and appearing a bit brighter and more condensed near the center. Some photographs might show a slight elongation of the comet’s coma, but hardly the kind of tail or appendage exhibited by other, larger comets that attain 3rd or 4th magnitude.

Speaking of the Moon, it will become an increasing nuisance during the middle of December, lighting up the sky during the first part of the night and seriously interfering with observations of the comet. The Moon will set later in the night, leaving the sky dark during the predawn hours, but as it approaches full phase on December 22nd, the amount of time between moonset and the first light of dawn will get noticeably shorter.

After Full Moon, dark sky opportunities open up in the evening sky. From mid-northern latitudes on Christmas Eve, there will be a 46-minute window of darkness between the end of evening twilight and moonrise. A week later, 46P will be visible most of the night without any lunar interference, and about a half hour after the start of the New Year it will stand more than 70° above the northern horizon, and probably hover at around 4th magnitude.

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