Comet 46P/Wirtanen made its closest approach to Earth on Sunday night, 30 times further than the moon, the closest comet since the 1950s. Dubbed “The Christmas Comet” because of its green glow, you may be able to catch a glimpse this week.
Your best opportunity to see 46P/Wirtanen is once the moon sets around 1 a.m. local time. Find the three stars in Orions belt, then to the right, find the orangish star Aldebaran, the eye of Taurus the bull. On Monday, the comet was about 12 degrees to the right, or about the width of your outstretched fist. It will be another 3 degrees up and to the right Tuesday and each night this week.
46P/Wirtanen doesn’t have a pronounced blue tail as many comets do. This tells us that there isn’t there isn’t a lot of dry ice (solid CO2) being outgassed. It does have a diffuse coma, or an envelope of gas and dust ejected as the comet heats up as it nears the sun, giving it a fuzzy smudge of an appearance.
The Christmasey green color of 46P/Wirtanen’s coma comes from diatomic carbon (a pair of carbon atoms bonded together) and cyanide that brings a couple of nitrogen atoms to the mix. Our eyes are most sensitive to green light, but that doesn’t mean the comet will be easy to see.
The brilliant green images you may have seen were photographed with long exposures and/or by combining multiple images. To your eyes, it will look more like a green smudge.
Comets are surround by a coma or envelope of gas and dust formed as ice boils off. As comets make their way closer to the sun, they heat up, and that coma spreads out over a larger area of the sky, making them less of a point of light and much more diffuse. It’s like seeing oncoming headlights on a clear night vs. a foggy one.
A pair of binoculars will help bring a little more light in, clearing up the image a bit to make the comet viewable for rural and suburban observers. This isn’t something you are likely to be able to spot from downtown.