The Sky This Week
U.S. Naval Observatory's Weekly Blog

The brightest lights on the longest nights

by Geoff Chester, USNO Public Affairs
14 December 2021

Full Moon and Orion rising, Ocean City, Maryland
imaged 2020 December 29 with a Canon EOS Rebel SL2 DSLR.

The Moon brightens the sky as she waxes among the bright stars of the winter sky.  Full Moon occurs on the 18th at 11:35 pm Eastern Standard Time.  December’s Full Moon is popularly known as the Cold Moon, Long Night Moon, and Moon Before Yule.  Since the Full Moon occurs in the opposite part of the sky from the Sun, which reaches his most southerly declination on the solstice, Full Moon occurs at Luna’s highest declination for the year.  Her silvery glow adds a sublime illumination to the barren landscapes of boreal winter.

The winter solstice occurs on the 21st at 10:59 am EST.  This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of 270 degrees, marking the onset of astronomical winter.  It is also the date of the longest night of the year.  Here in Washington Old Sol is above the horizon for just 9 hours 23 minutes on that date.  However, if you consult an almanac, you will notice that the time of sunset on the evening of the solstice is four minutes later than it was on December 7th.  That almanac will also show that the latest sunrise won’t occur until January 4th, when sunrise will be four minutes later than it is on the 21st.  By the end of the year sunset will be later still, and most people will probably begin to notice the gradually lengthening days.

Reports from the early morning of the 14th indicate that the Geminid meteor shower was very strong this year.  If you missed the peak you still may be able to catch several shower members before morning twilight on the 15th and 16th.  The waxing Moon encroaches on the evening and early morning hours’ viewing, but you will have an hour or so after moonset to look for these stragglers.  I managed to glimpse several despite moonlight and high clouds on the evening of the 13th from my suburban front yard.  

December’s Full Moon occurs at the apex of one of the most visually attractive asterisms in the sky, Popularly known as the Great Winter Circle or Winter Hexagon, the asterism is delineated by seven colorful first magnitude stars.  It surrounds the magnificent constellation of Orion, the Hunter.  Orion is visible from every inhabited part of the Earth, and is one of the few constellations that can be easily identified from urban locations.  The Hunter’s brightest stars are the rose-tinted Betelgeuse and icy-blue Rigel.  Between these two are three blue-hued stars that mark Orion’s belt.  All of these stars are quite far away, ranging in distances from 500 to 2000 light years.  To appear as bright as they do in our sky, these luminaries must be intrinsically extremely bright.  Alnilam, the middle star in the Belt, is thought to shine with the equivalent luminosity of nearly half a million suns!  Many of Orion’s stars are blue supergiant stars like Alnilam, and they will exhaust their nuclear fuel in a matter of a few million years.  They will then evolve into red supergiants like Betelgeuse, a highly unstable star that will ultimately collapse on itself in a cataclysmic supernova explosion.  Betelgeuse’s reddish tint is due to its relatively cool surface, and its brightness is due to the vastness of that surface.  If Betelgeuse occupied the Sun’s position in our solar system, our Earth would orbit inside its outer layers!

Venus reaches a stationary point in her journey against the background stars on the 18th.  After that date she begins westward motion along the ecliptic as she begins to pass between the Earth and the Sun.  All of this leads to a precipitous drop from the evening sky between now and the end of the year.  She still dominates the western evening twilight sky, but soon she will only be visible in bright twilight.

Saturn appears in the later stages of twilight above and to the left of Venus’ dazzle.  The ringed planet is pale in comparison to his brighter neighbor, but there are no other comparably bright objects nearby.  His yellow tint stands out in contrast to the intense white of Venus and the creamy hue of Jupiter to the east.

Jupiter should be easy to find very shortly after sunset.  He lies just west of the meridian during evening twilight, which is the best time to train the telescope on him.  Earth’s rotation carries Old Jove quickly toward the horizon, so by the time it is fully dark he is becoming too low for a detailed view.
 
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