This 2018 Christmas season, astronomers and star-gazers alike will have a chance at a narrow view that only comes once every 5.4 years. Dubbed with the provisional designation of 46P, the Wirtanen comet (pronounced WERE-tuh-nun) will reach within 7,260,000 mi (11,680,00 km) of Earth, its closest approach for the next 20 years. The 'P' in its designation classifies Wirtanen as a period comet. Period comets are unique in that they are not newcomers; they come back on a periodical basis of at least once every 200 years!

Wirtanen was first discovered in 1948 by American astronomer Carl A. Wirtanen working at the Lick Observatory in California. During a routine survey of the night sky and its motion, the dim comet was unknowingly exposed onto a photographic plate used by the 37-year-old. After two days, the comet was discovered on the plate and work began to determine its motion through several more observations. At the time, the comet carried an orbital period of around 6.7 years. In simpler terms, this meant that the comet took around 6.7 years to complete its journey around the Sun. Through the next few decades, Wirtanen passed near Jupiter's gravitational influence more than once and it's orbital path was altered, bringing it's elliptic orbit closer to Earth. Today, it's orbit is still almost as far reaching as Jupiter. On its closest approach to the Sun, (perihelion) it passes between Earth and Mars. Starting in December, 46P will come into view wrapped in a greenish blur near the constellations of Cetus and Eridanus. As we approach Christmas, it will work its way past the Pleiades star cluster, up towards the Capella star, and on Christmas Day it can be found in the circumpolar region around the North Star (if you're living in the mid to upper northern hemisphere/United States). I'm sure you're wondering; Will I be able to see the Christmas Comet? Well, with a little bit of knowledge, a decent telescope, camera, or pair of binoculars, and a quick drive to a dark site, you could be in luck. In looking for 46P, you will not be looking for a sharp bright object or in a pinpoint area to the North. Instead, it will shine with a hazy or cloud-like appearance, diffused into an area up to two or three times the size of the Moon! There are several different predictions circulating regarding how big and bright it will appear. Most estimates are putting its magnitude, or brightness, at a value of +3. For those not familiar with the magnitude system, the smaller or more negative a number, the brighter it is. The Sun has a magnitude of -27. Sirius, the brightest star in the sky, has a magnitude of -1.5. Wirtanen will dip into the range where a clear, dark sky will be needed to resolve any sort of structure. It will be at its brightest near December 16. There are resources online to determine the exact location on any given night in December/early January if you're hoping to catch a glimpse. Remember to give some time for your eyes to adjust to the darkness, and a pair of binoculars will often do better than a cheap Bushnell telescope. Also make sure to plan around the Moon rise/set times this December, as the Moon will be high and bright in the sky for many of the nights and could hinder your viewing. Here's to clear skies and a "stellar" Christmas gift!



