An icy green snowball is easily visible in the night sky just in time for the holiday season.
A "Christmas Comet" is coming. The night sky might seem like a static, never-changing place, but on Sunday, Dec. 16, 2018 a 1.2-kilometer-wide comet will pass just 12 million kilometers from Earth. That's about 30 times the distance between Earth and the moon, and about as close as comets ever get, but nowhere close enough to be dangerous. However, it is in the sweet spot to make it appear to be brighter than most comets.
Recently featured on NASA's Astronomy Picture of the Day, Comet 46P/Wirtanen is already the brightest comet in the night sky and is predicted to be bright enough to be seen with the naked eye during the holiday season. Comets rarely get bright enough to see without telescopes, so this is a rare chance to cross comets off your celestial bucket list.
What is the Christmas Comet?
Known to astronomers as 46P/Wirtanen, it's what's called a short-period comet because it enters the solar system to orbit the sun every five and a half years. Compare that to Halley's Comet, which only appears in the solar system every 88 years. First discovered in 1948 by American astronomer Carl Wirtanen at the Lick Observatory on Mount Hamilton near San Jose, California, 46P/Wirtanen is about to make its brightest, closest approach for the next 20 years.
When to see the Christmas Comet
Although it's been visible to those with telescopes and powerful binoculars for a few weeks, 46P/Wirtanen will be both at its brightest, and most easily found, on and around Dec. 16, 2018. That's because on that date 46P/Wirtanen reaches what astronomers call perihelion, when it’s nearest to the sun, so it has the maximum amount of sunlight reflecting from it. It may reach a visual magnitude +4 or maybe even +3, which means that it will be easily visible to the naked eye.
How to find the Christmas Comet in the night sky
Although it won't be made any easier by the strong moonlight in mid-December, it should not be too hard to find 46P/Wirtanen if you know where to look, and when. It will change position in the sky significantly each night because it’s so close to Earth. The easiest way is to use planetarium apps like Sky Guide and Star Walk 2, which contain the exact position of comets, including 46P/Wirtanen. However, it will also help if you know roughly where to look in the night sky.
How to find the Christmas Comet with the naked eye
The key to finding 46P/Wirtanen on the night of Dec. 16 from the northern hemisphere is to locate the constellation of Orion in the southeastern sky. Find the three famous stars of Orion's Belt, and follow a line from the lowest to the highest star, and on to an unmistakably bright red star, Aldebaran in Taurus. Go the same distance again and you'll see the Pleiades star cluster (also called the Seven Sisters), which is so bright you will probably have already noticed it as a bright smudge in the corner of your eye. 46P/Wirtanen will be just below the Pleiades, slightly to the left.
How to find the Christmas Comet on Christmas Night
If you want to find 46P/Wirtanen on Christmas Night, perhaps with a new telescope, it will be higher in the sky very close to the bright star Capella in Auriga. The easiest way is to look on Dec. 23, when 46P/Wirtanen will be just below Capella. A couple of days later, it will have moved slightly to the northeast, but if you've already observed it, it should be much easier for you to find it again.
How best to observe the Christmas Comet
46P/Wirtanen won't be the brightest-ever comet, but with luck, it will be bright enough to see with the naked eye. Of course, there's no guarantee. You can maximize its brightness by observing from a dark sky site where there's no light pollution. However, the easiest way of making sure you see it is to use any pair of binoculars. Your peripheral vision is more sensitive to brightness, so when you do find 46P/Wirtanen, try looking just to the side of it. If you want to make sure you get the best possible view of 46P/Wirtanen, find your local astronomy club using Sky & Telescope's Astronomy Clubs Near Me tool. Amateur astronomers are sure to be out with their telescopes looking for this special Christmas Comet.
How to live-stream the Christmas Comet
The Virtual Telescope Project, a group of remotely accessible robotic telescopes, is hosting a live online observation called "Wirtanen, the Christmas 2018 comet" on Dec. 12 and 16.