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.

Picture of comet 46P
Comet 46P's coma extends more than 45 arcminutes in this picture taken on November 26, 2018.
Gerald Rhemann (Namibia)

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.

Orbit of comet 46P
The orbit of comet 46P/Wirtanen (blue) compared to the orbits of the planets (white). The blue square marks the comet's position on December 16, 2018.

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.

December 2018 track of comet 46P
The track of comet 46P through December 2018. Numbers mark the dates when the comet will appear at those locations.
Sky & Telescope

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.


  1. Oct. 20.11, m1=9.2, Juan Jose Gonzales, Alto Del Castro, Leon, Spain
    Oct. 27.40, m1=8.5, Chris Wyatt, Walcha, NSW Australia
    Oct. 30.41, m1=8.4, Chris Wyatt, Walcha, NSW Australia




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November 27, 2018 at 3:07 am

Excellent article, Joe. You make a good point about keeping "expectations low." Too many webpages and media sources have a tendency to over-hype celestial events, thus causing the general public to lose interest when they fail to put on a good show. By the way, were you friends with Wilma Cherup? She was my cousin and she introduced me to the Amateur Astronomers Association of Pittsburgh 45 years this September. One the best things that ever happened to me.

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November 28, 2018 at 8:22 pm

Hi there! I agree entirely with most of the comments made. My first (and so far the last) sighting of 46P, on 10th Nov, was unspectacular, with 7x50 binoculars on a night in Foxton, New Zealand, (total darkness, excellent seeing, poor transparency), when rain had been forecast and eventually came. It appeared as a faint, big blob. I looked back several times before deciding only a comet could look that weird. It had a faint star at the centre which I later realized was the central condensation. Since then we have had clouds and rain. Such is the lot of the comet watcher. Here's hoping for better in December, when the perihelion is on my birthday. Does God give back-handed gifts, or is he too busy playing dice?

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November 28, 2018 at 9:15 pm

Like and follow the Comet 46P/Wirtanen Facebook page: https://www.facebook.com/46PWirtanen/

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November 30, 2018 at 2:01 pm

Hi there, the skies over New Zealand have not been kind to the astronomer recently, but viewing between clouds (on the 29th Nov 2018 around midnight) I managed to catch an extended view of comet 46P/Wirtanen, first with binoculars and then with a telescope. It was as hard to see visually as ever but this time appeared slightly asymmetrical - I judged towards its Sun side. I frantically searched (back inside at my computer) to confirm, deny or explain away this anomaly. I found an image, apparently from the 26th Nov (probably 27th NZ time) with a similar bulge in this direction. Is this a star behind the very diffuse coma or is 46P/Wirtanen breaking up? I'd love to hear any other views or explanations. Foxton NZ is predicted to be cloudy etc. for the next few days so my observing will be vicarious...
Clear skies, Douglas

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December 1, 2018 at 11:24 am

I spend an hour, last night in the skies of Los Angeles, looking for this comet. I was using my 20x80 Binos
around 10pm, and couldn't find it. I have found many comets before, using these binos, so I'm guessing, the
mag is much greater than a 5. Anyone from the LA area see this comet yet? Happy holidays - docent

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November 30, 2018 at 4:42 pm

The Comet was very easy in 20 x 80 binoculars last night from Ireland, that's 2245 UT on Nov 29. It's a pleasure to scan for a nebulous object and have the object grab your attention once it enters the fov, as opposed to scanning back and forth wondering if you've identified the correct faint fuzzy. I estimate it's on the verge of breaking through mag 5, although I am fortunate to have dark rural skies. Another two weeks of brightening and it seems well on course for clear naked-eye visibility. And no Douglas, I don't think there's any danger of it breaking up.

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December 1, 2018 at 6:32 am

This is very well written and researched article. However, I found what may be an error in the opening sentence: “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.”

On December 16th, the Moon will be waxing gibbous, approaching 58% full. But I wouldn’t call it a full Moon by any means.[1] The moon will definitely be problematic at around the time of the close approach with Earth, December 16th. However, since comet 46P/Wirtanen will be at near peak magnitude for several days beforehand, and the Moon will be setting later each night, early viewing before Earth’s close approach is a good idea to avoid the Moon spoiling things.

You should chart out when the moon will be setting in your corner of the world from about December 8th through the 16th. The earlier the day you choose, the sooner the Moon will set and literally be out of the picture by the time astronomical twilight kicks in and dim things like comets can be seen. Where I am, by December 16th, the Moon will set at 1:02 AM MST (as seen from Prescott, Arizona, USA), so that means a late night of viewing. But on December 12th (periapsis; closest approach of the comet to the Sun), the Moon will have set by a more amenable 10:16 PM and the comet will still be nearly at peak magnitude.



So for example, where I live (Prescott, Arizona, USA), the Moon sets as follows from Dec 10th on. So I’ll plan on watching/photographing the comet over a string of nights going out later and later each night after the Moon is literally out of the picture.

12/8/2018 — moonset at 6:45 PM MST
12/9/2018 — moonset at

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Joe Stieber

December 1, 2018 at 5:27 pm

On 29-Nov-2018 at 9:00 pm EST (30-Nov, 02:00 UT), I saw 46P with 15x56 binoculars from the relatively dark New Jersey Pines in a moon-free sky. Despite thin clouds, it was easy to see with the binoculars. There was no sign of it with unaided eyes. At the time, the comet was about 28 deg altitude.

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December 3, 2018 at 12:40 am

Tonight was the first promising night in Western Pennsylvania since the first week in November. There were scattered clouds from time to time, but the transparency was very good to excellent, otherwise. I spotted Comet Wirtanen just before and after 8 PM (1 hour UT Dec 3, 2018) through my 10 X 50 Binoculars, the 5" f/5 Refractor mounted on the 21" Reflector and the 21" at Wagman Obs. Unfortunately, the worst light pollution here is to the south of the observatory. I'm guessing the brightness to be close to 5 mag. Because of the LP, the coma was about 1/2 degree or slightly less here. This comet is a poor version of Comet Holmes on 2007-2008 and Iras-Araki-Alcock in 1983.

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December 10, 2018 at 12:04 am

I noticed that it was clearing off about 11 PM.  It wasn't totally clear, but I could Cetus easily and I decided to try to find Comet Wirtanen.  There was still a thin patch a cirrus clouds, but they moved off and I finally was able to spot Zee Comet through my 10 X 50 Binoculars from my upstairs bathroom window. More window observing this time of the year.  It's still a fuzzball, probably around 4 to 5 mag with two dim stars about 6 to 7 mag pointing at it in the Eastern corner of Cetus.  Not much to get excited about from my light polluted neighborhood in Shaler, Pa, but it would be a nude-eye object in a dark sky location.  I might head up to Wagman Observatory after midnight.  It depends on the temperature and other conditions, including my own.

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December 7, 2018 at 11:13 am

I’m not an astronomer, but I had a question about a word used in the article: I thought PERIGEE meant closest point to Earth of an object orbiting around Earth (Moon, satellites) Can it also be used to describe the closest point of Earth to any object or planet orbiting the sun?

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