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The Sky This Week, 2014 December 2 - 9

The Moon lights the sky for The Hunter.
Orion Rising Thumbnail
Orion Rising, Blue Ridge Regional Park, Bluemont, VA

The Moon passes through the bright stars of the Great Winter Circle this week, a fitting backdrop for the most northerly Full Moon of the year, which occurs on 6th at 7:27 am Eastern Standard Time.  December’s Full Moon is popularly known as the Moon Before Yule, Cold Moon, or Long Night Moon.  The latter is particularly appropriate as we approach the year’s longest nights around the time of the Winter Solstice.  Watch Luna glide just a degree north of the bright star Aldebaran on the night of December 5th.  On the following night her bright crisp disc hovers above the figure of Orion, the Hunter.  

For most folks living in the mainland United States this week begins the series of phenomena associated with the winter solstice.  For the next 10 days we will experience the earliest sunsets of the year.  Here in Washington they occur at 4:46 pm EST.  By the 12th the sunset time slowly begins to creep a bit later; however, the time of latest sunrise is still advancing.  That event falls on the several days before and after January 4th.  Thus, when we measure the total length of daylight/night, we find that the year’s shortest day indeed does fall on the solstice on December 21st.  This “lag” in the times of sunrise and sunset is a result of our method of keeping time by using a standard second and by the slightly elliptical orbit of the Earth around the Sun.  If we still used sundials for keeping time the effect wouldn’t exist.

This is probably the one time of the year when the Full Moon gets some competition from the surrounding sky.  I’ve always liked the coincidence of having nine of the 25 brightest stars in the sky surrounding the most recognized and brightest constellation, Orion.  In addition to their brightness, the stars of the Great Winter Circle also offer a wide range of colors which are easily perceived with the naked eye.  From the reddish glimmers of Betelgeuse in Orion and Aldebaran in Taurus we find a nice blue-tinted contrast in the stars Rigel and Sirius.  The most northerly of the circle’s stars is Capella, which glows with a subtle golden yellow hue.  To my eyes the three “belt stars” of Orion have the most striking icy colors, probably because they serve to remind me that I forgot some piece of cold-weather observing gear when I trek away from the city lights.  Although they appear “cold” in hue, these are among the hottest and brightest stars in the galaxy.  They are the epitome of what we astronomers call “hot, young stars”, and like many of their Hollywood counterparts, they live their lives quickly and “burn out” at a relatively young age.  Compared to our Sun they are true powerhouses, shining with luminosities over 100,000 times that of our little star and surface temperatures five to six times hotter.  At this intensity they will exhaust their hydrogen fuel very quickly and have life spans measured in tens of millions of years.  Our much more modest Sun, leading a much more sedate life, should be around for a few billion years.  Fortunately for us, these stars are very far away, on the order of 700 to 800 light years.  If we could somehow move one of them to the same distance of Sirius, just over 8 light-years distant, it would be as bright as the Full Moon in our sky and we would all perish from the intense ultraviolet radiation pouring off its surface!

On a cheerier note, bright Venus is gradually working her way back into the evening sky.  You can catch her just after sunset, low in the southwest in bright evening twilight.  She sets about 40 minutes after the Sun on the 2nd, but gains 10 minutes by the week’s end.

Mars spends the week drifting eastward into the stars of the dim constellation of Capricornus, the Sea-Goat, located in the southwestern sky.  By the week’s end he drifts about five degrees south of Dabih, the constellation’s second-brightest star. From now through the year’s end the red planet sets at the same time each night, 8:13 pm here in the Washington area.

Jupiter now rises at around 10:00 pm EST, and late-night skywatchers will see his unmistakable glow dominating the eastern sky.  The giant planet reaches the first stationary point in this year’s apparition on the 9th.  For the next few months he’ll trace a westward track against the stars near the bright star Regulus in Leo.