Observing the “pre-Christmas Comet” 46P/Wirtanen in December 2018

Comet 46P/Wirtanen has been called the Christmas Comet for 2018. It’s best seen from the southern hemisphere around December 15. On the weekend of December 14-16 it is in Taurus. This chart shows the sky at 1am.


We haven’t seen a really bright naked-eye comet for many years. The last ones I recall were C/2006 P1 (McNaught) back in 2007, Hale-Bopp and Hyakutake waaay back in 1996. Comet 46P/Wirtanen (pronounced WERE-tuh-nun apparently) is not particularly bright but it’s one of the best we’ve seen for years and it is reportedly (just) visible to the naked eye.

Wirtanen was discovered by Karl Wirtanen from Lick observatory in the USA in 1948. Its elliptical orbit about the Sun takes it out almost as far as Jupiter before it swings in almost as close as Earth’s orbit. This year as it reaches its closest point to the Sun on December 16 and Earth just happens to be in the ringside seat and about as close as we can get – hence the excitement about this comet.

At its closest Wirtanen will be just over 11 million kilometres away (that’s pretty close by astronomical standards!) – far enough away not to be dangerous, but close enough for good viewing. As comets approach the Sun gasses vaporise (OK, sublimate to be technically correct) from their surfaces and form large “comas”, or clouds, surrounding the comet. And here is where it starts to get difficult for viewing. The coma is large and diffuse, and the comet is so close that the coma is spread out over a large area – about as large as the Full Moon appears in the sky. Such a large, diffuse coma makes this comet a little difficult to see.

How do I see Wirtanen?

Viewing 46P/Wirtanen wont be easy but it will be rewarding. It is best placed for viewing over the weekend of December 14-16.

After midnight, and after the Moon has set*, look north and identify the constellation of Taurus, not far from Orion. Using the sky chart above, and a pair of binoculars, slowly scan the sky in the area indicated for the night you are observing. You will need to be in a dark, country site and to let your eyes adapt to the darkness for 15-20 minutes before viewing. It should be just possible to detect it with the binoculars and there are even reports of it being seen with the naked-eye. If you do see it try again the following night to see its motion across the sky.

After this weekend the comet drops towards the northern horizon and is lost to southern hemisphere observers, but it becomes a true Christmas Comet for those in the north.

Wide angle photography with a DSLR camera appears to be a guaranteed way to catch this comet. See the links below.

And make sure to take a break to see the Geminid meteor shower over the weekend as well.

*December 14, Moon sets at 00:18am for Sydney. December 15, Moon sets at 00:51am and December 16 Moon sets at 01:22am


See the following sites for more information:

Sky and Telescope, “Comet 46P/Wirtanen Approaches Earth” 

“A Comet as Big as the Full Moon”

A technical, but informative site: The Comet Wirtanen Observing Campaign

For photos of 46P/Wirtanen see Spaceweather


Update [Dec 20 2018]

While Sydney was clouded out other parts of the country were clear enough to see comet 46P/Wirtanen. I was able to see it from the Blue Mountains, but only with binoculars, and even then it was very faint and at the limit of visibility. It appeared, as expected, as a moon-sized hazy patch. At the time the Moon was still up so the sky was bright. This reduced the contrast between the faint comet and the background sky. Unfortunately, when the Moon set…cloud & rain moved in. My colleague Geoff managed to photograph comet 46P/Wirtanen from near Bathurst. The comet is now moving rapidly towards the northern horizon, and with the Moon near full phase it is no longer observable from much of the southern hemisphere. Once the Moon gets out of the way after the Christmas period it should be nicely visible from the northern hemisphere. Despite all the difficulties it was still arguably the best comet of 2018!

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