StarWatch for the greater Lehigh Valley
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DECEMBER  2018

DECEMBER STAR MAP | MOON PHASE CALENDAR | STARWATCH INDEX | NIGHT SKY NOTEBOOK

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1163    DECEMBER 2, 2018:   Ready for Comet Wirtanen?
I make it a regular routine to go outside at least once an evening to check the sky. Last Friday, November 30, I saw Orion the Hunter, probably for the first time in several weeks. It startled me because I was actually expecting it to be overcast; but there it was, three belt stars, shoulder and knee stars, all sitting pretty in the south. The time was about midnight. There were gossamer patches of hazy sky, and the vapors from my exhalations hung in the air, telling me that the humidity was very nearly 100 percent. Clouds returned later that night. It got me to thinking about the actual chances of seeing Comet 46P/Wirtanen because December is the cloudiest time of the year for the East Coast, averaging about three clear-to-mostly-clear evenings per month. Late November and January are not that much better. What I have gleaned from a lifetime of interest in astronomy and making observations is that when an important event occurs, you prepare for it regardless of the meteorological expectations because if it is clear, you want to be able to take the picture or make the observation with a minimum of fuss and frustration. So make sure you’ve got the binoculars or camera and tripod ready and that you know how to operate them in the dark. I am going to concentrate on the two best evenings to make an observation, the 15th and 16th of December when Wirtanen will be passing the Pleiades or Seven Sisters. On both evenings the comet will be in the same binocular field of view as the Pleiades, which if it is clear, should make it relatively easy to spot if 46P/Wirtanen continues to brighten as expected. Currently, it is on track to do precisely that. If you are outside around 11 p.m., Orion will be in the southeastern sky, its three bright belt stars pointing upward towards a V-shaped grouping of stars, the head of Taurus the Bull. If you fire your imaginary rocket upwards, following the path of Orion’s belt, you will pass under Aldebaran, the brightest star of Taurus. In suburbia the “V” should be observable without optical aid on a clear night, but if not, use those trusty binoculars to spot it. Continue upward just a little farther, following the line of sight of Orion’s belt stars, and you will come across a gossamer clump of haze and dim luminaries. That will be the Pleiades, bright enough even to be visible from mid-sized towns. Use averted vision to make the Pleiades appear brighter. Check them out with binoculars, and you’ll see a tight grouping of dozens of blue stars, an indication that the Pleiades are relatively young, about 115 million years of age. Wirtanen will be on the Aldebaran side of the Pleiades on the nights of December 15 and 16. Here’s my prediction. The weather will be cooperative on both of these evenings, and everyone who dares to be ready and brave the cold will see a faint comet. More about Wirtanen in next week’s StarWatch.
 

1164    DECEMBER 9, 2018:   Big Week for Comet Wirtanen
Years past in the 1970’s right before Thanksgiving at the end of an astronomy class at William Allen High School, a student by the name of Tony asked if I had ever heard of a person named Whipple? At that time there was a popular TV commercial about a grocery sales clerk named Mr. Whipple who was always being reprimanded for squeezing the Charmin bathroom tissue in the supermarket in which he worked. “Don’t squeeze the Charmin” was the catchphrase. I responded to Tony by saying, “Isn’t he the guy that squeezes the Charmin?” I could see immediately by Tony’s posture that I had picked the wrong Whipple. So I countered by saying, “Dr. Fred Whipple, Mr. Comet, Harvard astronomer, the person who first postulated the dirty snowball theory about comets and explained the morphology of these hairy stars.” Tony lit up like a searchlight and then exclaimed enthusiastically, “He’s my uncle.” We talked for a while, and I gave Tony some suggestions regarding questions he might ask Fred Whipple. When I saw him in class after the holiday break, he said to me that he had made the biggest mistake of his life. Tony continued, “I asked my uncle what he did for a living, and three hours later, he was still talking about his work.” I love that story as much as I enjoy watching comets when they become bright enough to be seen at least with binoculars. Comet 46P/Wirtanen is such an object, and it’s due to make its best showing this week. There are some pros and cons about this comet, but I’ve found it from suburbia with binoculars and with minimal hassle. Wirtanen will be passing some prominent sky objects like the Pleiades which should make it easy for the general public to find with binoculars. A new map is included with this issue of StarWatch at http://astronomy.org/StarWatch/December/index-12-18.html#12-9-18. Also read last week’s StarWatch. Wirtanen is a small comet; its nucleus is only about six blocks in diameter, and it has been around the sun many, many times, hence much of its “oomph” is gone. It’s icy core, however, has continued to outgas with hyperactivity, allowing ultraviolet energy from the sun to cause this gas to fluoresce or glow. The comet is passing very close to Earth, about 7.2 million miles, the tenth closest approach of a comet since 1950. The closeness of the comet will counteract its brightness because its nearness makes the object appear huge in the sky and that causes its brightness to be spread over a large area of the heavens, making Wirtanen’s actual surface brightness appear dim. I’ve seen 46P/Wirtanen on two recent nights, December 5 and 7, but I have found it well over a dozen times. I have also compared it with other objects like star clusters in Auriga and Gemini to see if there could be any confusion. There is none. Wirtanen is its own unique object, a fuzzy, low surface brightness “blob,” now about twice the diameter of the full moon. Wirtanen should remain a binocular object from suburbia, but it still has some brightening ahead of itself, about three times, which should improve visibility, but it will also get a little bigger as it makes its closest approach to Earth on December 17. Wirtanen’s increase in size will counteract some of its increase in brightness. Another nemesis, the moon, enters the scene on the 15th when it will be at first quarter, only about 1/12th the brightness of a full moon, but nonetheless adding luminescence to the Holiday heavens, diminishing the contrast between the sky and the comet, making Wirtanen more difficult to see until after moonset. Although this is not a flashy comet, 46P/Wirtanen is definitely worth seeing, and this is the week to make your best attempts at viewing it.

[Comet Wirtanen Locator Map]
The path of Comet 46P/Wirtanen during the time when it will be at its brightest is shown in this star chart. Although the size and shape of Wirtanen is shown correctly, the brightness of the comet is not. It will appear more like a greyish cotton ball and without the cyan color. Color only becomes apparent with longer exposures taken though a camera. Use binoculars to see Wirtanen because of its large size and faintness. Gary A. Becker map using Software Bisque’s The Sky.
 

1165    DECEMBER 16, 2018:   
 

1162    DECEMBER 23, 2018:   
 

1162    DECEMBER 30, 2018:   
 

[December Star Map]

[December Moon Phase Calendar]
 

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