Celestial News: A comet this way comes
December 6, 2018
Comet Wirtanen, also called comet 46/P, is due to make a very close pass by the Earth in December and might — emphasis on might — become visible to the naked eye around midmonth.
Comet Wirtanen was discovered in 1948 by Carl Wirtanen at California's famous Lick Observatory. It follows a 5.4-year orbit around the sun, going from just inside of Jupiter's orbit to just outside of Earth's orbit. Over time, close encounters with both planets keep the comet's orbit all stirred up, and it never quite follows the same path around the sun twice.
On this pass, Comet Wirtanen will reach perihelion — its closest point to the sun — on Wednesday, Dec. 12, just four days before its closest approach to Earth. On Dec. 16, the comet will pass only 0.077 astronomical units from Earth. That's equivalent to 7.2 million miles, or about 30 times the distance from the Earth to the moon. This puts Comet Wirtanen in the record books as the 10th closest comet to brush Earth since the year 1950 and the 20th closest in recorded history.
At 1 mile in diameter, Comet Wirtanen is relatively small, as comets go, but it is classified as a hyperactive comet. That means that it can become brighter than most comets its size. Because comets are basically dirty snowballs, they are most violently boiling away and brightest when they are closest to the sun. Comet Wirtanen's timing of closest approach to the sun and closest approach to Earth is ideal for creating the best show possible.
Getting a good view of Comet Wirtanen will require finding a dark observing location away from city lights. It also will require dodging the waxing moon, which becomes full Dec. 22 and will temporarily bring comet-watching to a grinding halt.
On Dec. 15, the night before Comet Wirtanen's closest approach to Earth, the moon will set around 12:45 a.m., leaving the comet alone in a nice, dark sky. Facing southwest, look halfway between the Pleiades star cluster and the V-shaped Hyades star cluster that marks the face of Taurus, the bull. The comet won't sport a visible tail because any tail will be aimed almost directly away from Earth at that time. Instead, the comet will look like a round fuzz ball, about as large as a full moon and hopefully bright enough to be visible to the naked eye. Even if it is, it will be very ghostly in appearance. A pair of binoculars will greatly enhance the view.
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On the night of Dec. 16, the bright waxing gibbous moon will set around 2 a.m., technically the morning of Dec. 17. This is the night of closest approach, and the comet should be as bright as it's going to get. Look for it just to the left of the Pleiades star cluster.
Looking ahead, on the night of Christmas Eve, Comet Wirtanen will be positioned very close to the bright star Capella in the constellation of Auriga. Look for it between 6:30 and 7 p.m., before the bright moon rises. The comet could remain faintly visible to the unaided eye until well after New Year's Day. During January, Comet Wirtanen will be far enough north that it will remain visible all night long, as it circles the north celestial pole.
While you are out comet-watching, also keep an eye out for bright shooting stars. The annual Geminid meteor shower peaks on the night of Dec. 14.
Jimmy Westlake's Celestial News column appears monthly in the Steamboat Pilot & Today. Check out Westlake's new 2019 Cosmic Calendar of sky events on his website at westlake.com. It features twelve of his best astro-photos and a day-by-day listing of cool celestial events that you and your family can enjoy watching all year.