A comet that regularly tours the inner Solar System is making its closest approach to Earth since it was discovered, and is expected to become bright enough to be seen with the unaided eye. Our guide will help you find it.
Comet 46P/Wirtanen was first spotted 70 years ago on January 17th, 1948. Though it must originally have come from the outer regions of the Solar System, it was captured by Jupiter’s pull and put into a smaller orbit with a period of 5.4 years. That means it has regularly been observed since, though it usually remains very faint and observable only with big telescopes, even at perihelion, its closest point to the Sun.
However, the current return of Comet Wirtanen is a special one because the comet will pass unusually close to Earth just days after it reaches perihelion on December 12th, 2018. Four days later, on December 16th, 2018, the comet will come to a distance of just 11.68 million km (7.26 million miles) which is around 30 times the distance of the Moon.
For a short time around this date, Comet Wirtanen could become visible to the naked eye. Comets are notoriously unpredictable, but best estimates suggest it could reach magnitude 3, which is the level of a moderately bright star.
It is important to note, however, that this comet’s brightness will be the combined figure for the whole object, and it will be so close that its head, or coma, is likely to be spread out to as much as 2° which is four times the apparent diameter of the Moon in the sky.
The comet might, therefore, not be so easy to spot in the sky with the naked eye alone. Hopefully binoculars will bring it more clearly into view and it should be fairly easy to capture with a short exposure of a few seconds with your camera.
When this post was first published, on November 10, Comet Wirtanen lay in the southern part of the sky, in the constellation of Fornax, and was only visible with telescopes or binoculars at around magnitude 8 or 9. However, it has moved rapidly north since then, brightening as it goes. Observers at mid-northern latitudes have been able to find it more easily with binoculars since the end of November when it entered the constellation of Cetus, the sea monster.
When at its closest, the movement of the comet will be quite obvious from night to night, and even from hour to hour if you check its position against the stars as you view with a small telescope or binoculars. Note that, in our charts, the comet’s position is shown for for 0h UT on each date, so if you are watching in the evening on that date it will have moved.
By chance, when at its closest to us, the comet will be passing between two of the most well known and easy to find star clusters in the sky – the Pleiades, also known as the Seven Sisters, and the Hyades, both in the constellation of Taurus. That will make it easier to find.
Moonlight will brighten the sky background to a degree because the Moon will be a waxing crescent on December 12th and just around First Quarter on the 16th. Fortunately the Moon will be on the opposite side of the sky, and if you observe later in the night, it will have set below the horizon.
It will be fascinating to see just how bright Comet Wirtanen becomes, and also the shape and size of its coma. In early November, the comet was seen to have two stubby tails. They will be pointing pretty much away from us in space during December so they will continue to appear foreshortened.
As an interesting aside, Comet Wirtanen was the original intended target for the European Space Agency’s Rosetta mission. However a delayed launch meant that a new target had to be found, and so it was sent to explore Comet 67P/Churyumov-Gerasimenko instead.
For another realistic appraisal of what you can expect from Comet Wirtanen, see this blog post by my friend Stuart Atkinson.
How to photograph Comet Wirtanen
When it is at its brightest, you should be able to photograph Comet Wirtanen quite easily with a digital camera, as long as you can set it manually. You will need a way to keep the camera steady, preferably using a tripod, and you should get away from streetlights and other light pollution.
Set a reasonably high ISO (film speed), if sky conditions allow, and set the focus on infinity (the little squiggle that looks like figure 8 on its side). Pocket snapshot cameras often show a little mountain symbol to help set focus, but they are less likely to be suitable as it can be fiddly, if possible at all, to take a time exposure, and the lens might not collect enough light.
More experienced astrophotographers will use telephoto lenses or attach their camera bodies to telescopes to record the comet. This is a more specialised operation, beyond the scope of this article. In such instances, they will usually track on the comet over time because its movement against the stars will be evident in just a few minutes.
Related: What’s in this month’s night sky?
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