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The Sky This Week, 2018 December 18 - 25

The Long Night Moon, and a Christmas comet.
Periodic Comet 46P/Wirtanen, imaged 2018 December 18 from Alexandria, Virginia.
Periodic Comet 46P/Wirtanen, imaged 2018 December 18 from Alexandria, Virginia
imaged with a Canon EOS Rebel SL2 DSLR and an Anteres Sentinel 80mm f/6 achromatic refractor.

The Moon brightens the mid-winter night sky this week, beaming down from the stars of the Great Winter Circle as she waxes to Full Moon, which occurs on the 22nd at 12:49 pm Eastern Standard Time.  December’s Full Moon is popularly known as the Long Night Moon or Moon before Yule.  Look for Luna nestled among the stars of the Hyades star cluster just to the west of the bright star Aldebaran in Taurus, the Bull on the evening of the 20th.  During the course of that evening you can watch the Moon occult a number of the brighter stars in the Hyades.  Use binoculars to watch these stars “wink out” as the Moon’s dark limb moves eastward through the cluster.

The winter solstice occurs on the 21st at 5:23 pm EST.  This is the moment when the center of the Sun’s apparent disc reaches an ecliptic longitude of 270 degrees.  This is also the time when Old Sol reaches the southernmost declination in his apparent path around the sky.  The solstice marks the shortest day of the year for residents in the Northern Hemisphere.  Here in Washington we’ll have just 9 hours and 27 minutes of daylight that day.  This is also the mid-point in the solstice “season” that plays out over the course of the month.  We have already experienced the year’s earliest sunset, which occurred at 4:46 pm EST on December 7th in Washington.  By the year’s end we’ll add 10 minutes to that time, but our latest sunrise won’t occur until after the start of the new year, when the Sun rises at 7:27 am on January 4th.  This seeming “lag” between latest sunrise and earliest sunset is an artifact of our method of timekeeping, where the seconds that define our time-scales are defined by the inflexible oscillations of atoms.  If we used sundials as our time reference these phenomena would all coincide with the solstice.  We have an in-depth discussion of solstice phenomena on our website. 

The Moon’s bright glare casts a silvery glow over the winter evening landscapes and in the process washes out most of the stars associated with the autumnal sky.  One exception is the star Fomalhaut, which can be found above the southwest horizon in the early evening.  This solitary star is the 17th brightest in the sky, and it is notable for being the first star to have a planet directly imaged, by the Hubble Space Telescope, in optical wavelengths.  Located just over 25 light-years away, it shines with a luminosity of about 16 Suns.  

By the late evening Fomalhaut is gone, but in the east you have a veritable riot of bright, colorful stars to enjoy.  9 of the 25 brightest stars in the sky are located within the Great Winter Circle, which includes the distinctive outline of Orion, the Hunter.  Unlike Fomalhaut, Orion’s stars are very far away, ranging from some 640 light-years for ruddy Betelgeuse to around 2000 light-years for Alnilam, the middle star in Orion’s belt.  Alnilam is one of the most luminous stars in the sky, shining with more power than half a million Suns!  It is a very young star, perhaps only five million years old.  Its life will be brief by cosmic standards, and in another five million years it will “die” in a spectacular fashion as a “supernova” explosion.

The Moon also washes out the soft greenish glow of Comet 46P/Wirtanen, which passed close to the Earth on the 16th.  If you live in a dark location you should be able to spot it in binoculars over the course of the next couple of weeks, and it may be visible to the naked eye once the Moon moves into the morning sky.  This week the comet moves into the western part of the constellation of Auriga, the Charioteer, and it will be in the same binocular field of view as the bright star Capella between the 21st and 25th.

Mars is still keeping pace with the advancing Sun and can be seen on the meridian as evening twilight grows deeper.  You’ll find him below a nice binocular asterism, the Circlet of Pisces, throughout the week. 

Venus continues to dazzle in the morning sky.  You should have no trouble spotting her before sunrise in the southeast.  She is at her peak brightness for this current morning apparition, and if you can keep the direct rays of the Sun behind an obstruction you should be able to spot her well into the morning hours.

If you have a flat southeastern horizon try to spot Mercury about 40 minutes before sunrise.  The fleet planet is speeding to the far side of the Sun, so this will be the last week to get a morning view of him this year.  

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