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The Sky This Week, 2018 December 4 - 11

The earliest sunset, and bright lights for winter nights.
Orion, imaged 2016 January 3 from Mollusk, Virginia.
Orion, imaged 2016 January 3 from Mollusk, Virginia.
with a Canon EOS Rebel T2i DSLR.

The Moon begins the week as a waning crescent in the pre-dawn sky and ends the week with her return to the evening twilight hours.  New Moon occurs on the 7th at 2:20 am Eastern Standard Time.  Try to catch a glimpse of Luna’s thin crescent five degrees above elusive Mercury half an hour before sunrise on the 5th. 

This is the week when we observe the earliest sunsets for the year.  This kicks off about four weeks of phenomena associated with the winter solstice.  Although we are now experiencing the earliest sunsets, the latest sunrises won’t occur until the first week of January.  In between is the solstice itself, which falls on December 21st, when we experience the shortest day of the year.  So why is there a seeming discrepancy between the shortest day and the times of earlies sunset and latest sunrise?  It’s a matter of the way we keep time versus the Sun’s apparent place in the sky.  Since the orbit of the earth is not perfectly circular, the time when the Sun appears to cross your local meridian (“noon” as displayed on a sundial) isn’t always an exact 24-hour interval.  Sometimes this “apparent Sun” is a little fast, and sometimes it’s a little slow compared to a uniform time-scale kept by a “mean time” scale such as the one provided by USNO’s atomic clocks.  The difference between time defined by the “apparent Sun” and the “mean Sun” gives us the discrepancy between sunrise/sunset times and the length of the longest and shortest days on the solstices.  We have a very thorough explanation of these effects on one of our Web pages for those who wish to delve into the deeper details.  That said, by the end of the year sunset will occur 10 minutes later than it does this week. 

December is the time when I like to welcome the bright stars of winter back to the evening sky.  As we plunge into the year’s longest nights we are rewarded for the Sun’s absence by the appearance of 9 of the sky’s 25 brightest stars.  The central figure in this glittering array is Orion, the Hunter, which is probably the most recognized star pattern in all the heavens.  Orion is characterized by three bright stars arranged in a straight line which portray the Hunter’s “belt”.  They are framed by four stars of equal and greater brightness, and it doesn’t take a huge leap of imagination to see a striding figure stepping up into the sky.  Orion’s brightest stars are the ruddy-hued Betelgeuse marking one of his shoulders and steely-blue Rigel marking his opposite knee.  Betelgeuse is a prime example of a highly-evolved “red supergiant” star.  These stars have exhausted the nuclear fuel in their cores and are fusing hydrogen in an expanding shell that causes the outer layers of the star to expand to a huge girth.  If Betelgeuse occupied the Sun’s place in our solar system the Earth would be orbiting inside its outer layers!  Rigel is a classic “blue supergiant” star, one whose mass is over 20 times that of the Sun.  This makes Rigel very hot, and it shines with the luminosity of 120,000 Suns!  In fact, almost all the bright stars in Orion are similar to Rigel, which means that they are quite far away, between 1,000 to 2,000 light-years distant.  Surrounding Orion is a large circle or hexagon of bright stars that form the asterism I call the Great Winter Circle.  Starting at Rigel, move clockwise to see the stars Sirius, Procyon, Pollux, Castor, Capella, and Aldebaran.  How thoughtful of the universe to provide these bright beacons to cheer us up on these long Boreal winter nights!

You might be able to catch a fleeting glimpse of Saturn on the evening of the 8th.  If you have a clear southwest horizon and can spot the thin sliver of the crescent Moon, the ringed planet will be just over three degrees to the left.  You will most likely need binoculars to see this appulse, which will be the last evening meeting between the Moon and Saturn this year.

Mars continues his eastward trek across the stars of the dim constellation of Aquarius.  The red planet begins the week just southeast of the 3.7-magnitude star Lambda Aquarii, which should be easy to spot in binoculars.  On the evening of the 7th Mars is just east of the distant planet Neptune.  The latter is a very dim 8th-magnitude object just a quarter-degree southwest of the brighter planet.  You should be able to pick Neptune out in binoculars, but a telescope will reveal its tiny disc that betrays its planetary nature.  Neptune is a very remote place, some 4.5 billion kilometers (2.8 billion miles) from Earth. 

You’ll find Venus cheerfully greeting the rising sun in the southeast each morning.  The dazzling planet now shines at a very bright -4.8-magnitude.  If you’re in a dark-sky location and are up and out at 5:30 am on mornings after the 6th, try to spot your shadow cast by Venus’ bright glow.

Fleet Mercury moves up from the horizon during the course of the week.  He’ll be best placed for viewing next week, but you’ll have the waning crescent Moon to help you find him on the morning of the 5th.

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