Comet 46P/Wirtanen Update…

Image: (c) Rolando Ligustri

It’s never a good idea to poke fate in the eye with a sharp stick, and astronomers know better than anyone how unwise it is to count your chickens before they’ve hatched, but seasoned comet watchers are now feeling reasonably confident that around the end of the year the northern hemisphere will finally – finally! – have a comet in the sky that will be at least bright enough to be easily visible through a pair of binoculars, and might even be visible to the naked eye…

For the past couple of months, to the frustration of their friends and friendly rivals further north, comet observers in the southern hemisphere have been enjoying watching Comet 46P Wirtanen brighten slowly, first through the eyepieces of their telescopes and then through their binoculars. Now, just as the comet prepares to turn sharply in the sky and begin to head north, climbing up into the northern hemisphere’s view, we’ve had the first report of the comet being visible to the unaided eye. If true then that means Wirtanen is behaving itself and not fizzling out or behaving oddly, which in turn suggests that initial predictions putting the comet at between 4th and 3rd magnitude when it slides between the Hyades and Pleiades star clusters a week before Christmas Day might turn out to be accurate after all.

Hmmm. We’ll see…! So many comets in the past (mentioning no names, but comets Kohoutek and ISON I’m looking at you, sniggering at the back there!) have promised us a good show only to let us down in the end. But regardless of how bright it gets in December, from now on it will definitely be worth taking a look for Comet Wirtanen on any clear night.

Before looking at where it is, and what to look for on those aforementioned clear nights, let’s look at some of Wirtanen’s background.

The comet was discovered on January 17th by Carl Wirtanen, and calculations showed it was a small, short period “Jupiter Family” comet. Its period (the length of time it takes to orbit the Sun, its year if you like) is 5.4 Earth years, so it’s a regular visitor to our skies. However, its small size – it is just over 1km across – means it never usually gets that bright in our sky; when it last rounded the Sun in 2013 Comet Wirtanen shone at only magnitude 14.7. However, this time will be different. This time the comet is going to come close to Earth – very close, at least in astronomical and cometary terms. When it makes its closest approach to Earth on Dec 16th Comet Wirtanen will pass us at a distance of only 11.6 million kilometres. That is very close for a cometary fly-past, and newspapers and doom-mongering websites will be making a really big deal about this at the time, but 11.6 million kilometres is roughly 30 times further away than the Moon, so we’re in no danger. The world is not going to end, and there’s no need to give Bruce Willis a ring..!

So does the fact that 46P is coming so close to us – in astronomical terms – mean that we can expect to see something spectacular in the sky?

Sadly not. Although there are lots of exciting graphics doing the rounds online, attist impressions showing 46P with a long, blazing tail and a brilliant head at the time of its close approach, the reality is – of course! – rather different. If we’re lucky, i.e. if it doesn’t fizzle out (always a [possibility with comets, they are notoriously unpredictable) then it might be bright enough to see in the sky with the naked eye in mid-December, but it will not look like a “classic” comet with a long, banner-like tail and a bright star for a head. Instead it will look more like a green puffball in the sky, like a misty round or elongated patch of misty light.

The comet might not actually be visible from a light-polluted town or city; it might only be visible from places with a dark, starry sky unspoiled by the lights of offices, hotels and pubs. But we’ll have to wait and see what it looks like.

Right now Comet Wirtanen is already an impressive object, at least photographically. Long exposure photos taken through telescopes show its coma is now visually larger in the sky than the Full Moon, and growing bigger. As far as its magnitude or brightness is concerned it seems to be around magnitude 5.8 which means it might be visible under a very clear, dark sky to someone with very good eyesight, but I think it’s fair to say it will be a while yet before more than a handful of people can see it without help from binoculars and telescopes.

To see it you’ll need to be out somewhere with a very dark sky and a very low, very open view to the south, because that’s where the comet is – low in the south as darkness deepens. The charts below will show you where to sweep for the comet with your binoculars and where to aim your camera at if you’re trying to take photographs of it. You’re looking for something that looks like a misty, fuzzy blur, like a large out-of-focus star.

Here are those charts…

Hopefully Wirtanen will continue to brighten as it heads north, and by mid-December, when it is passing betweeen the Hyades and Pleiades star clusters, it will be a lovely sight, both to the naked eye and through a camera lens. We’ll have to wait and see.

In the meantime, get out there on any clear night from now on, look to the south, and try to spot the most interesting comet in a long, long time…

Stuart Atkinson.

One thought on “Comet 46P/Wirtanen Update…

  • 9th December 2018 at 7:46 am
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    Up-to- date reports of visual sightings of this comet from Sussex, using small instruments, can be found in the the popastro forum. Regards maf.

    Reply

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