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The Sky This Week, 2013 December 17 - 24

The brightest stars light our longest nights.
Orion and friends
Imaged 2011 DEC 31, Morattico, VA
Sirius is the bright star to the left of the utility pole.

The Moon brightens the overnight hours this week, beaming down from her perch high among the stars of the Great Winter Circle before easing southward into the rising stars of spring.  Last Quarter will occur on Christmas Day at 8:48 am Eastern Standard Time.  Look for bright Jupiter five degrees north of the just-past Full Moon on the night of the 18th.  On the late evenings of the 2ist and 22nd Luna will rise near the bright star Regulus in the constellation of Leo, the Lion.

The winter solstice falls on the 21st at 12:11 pm EST.  This is the moment when the Sun reaches an ecliptic longitude of 270 degrees, which also happens to be the time when it reaches its most southerly declination.  At this moment the Sun stands directly over the Tropic of Capricorn about 500 miles off the west coast of Chile.  The day of the solstice is the shortest for residents of the Northern Hemisphere, but careful skywatchers may have noticed that the sun is now setting several minutes later than it did around December 7th.  As mentioned a couple of weeks ago, this is due to the Equation of Time, which corrects apparent solar time to mean solar time.  Sunrise and sunset times are dictated by this equation and the changing declination of the Sun.  At the solstice the latter effect goes to zero, so the change in the equation of time is the dominant effect.  Thus our earliest sunset occurred on the 7th, but our latest sunrise won’t happen until January 4th.  The duration of daylight on the solstice will be just 9 hours 26 minutes here in Washington.

The bright Moon washes out much of the splendor of the winter sky early in the week, but fortunately our longest nights are accompanied by the year’s best display of bright stars.  We’ve mentioned Orion on many occasions, and on a clear night his distinctive figure shines through all the stray light the Moon can offer.  His three bright “belt” stars make an effective pointer to the brightest star in the entire sky.  Follow a line through these three stars to the left and you’ll run into the pulsing blue blaze of Sirius, commonly called the “Dog Star” due to its central location in the constellation of Canis Major, the Greater Dog.  Sirius shines at a magnitude of -1.4 and achieves its prominence due to its very close distance to our Sun.  Located just 8.6 light-years from the solar system, Sirius is about 25 times as bright as the Sun.  In 1844 the German astronomer Friedrich Bessel detected a small “wobble” in the proper motion of Sirius as it moved through space and proposed the idea that a massive but unseen companion was the cause.  On January 31, 1861, during the course of testing a 18.5-inch objective lens, American telescope maker Alvan Graham Clark finally spotted the elusive companion.  Now known as Sirius B, this object is the prototype of a class of stars known as “white dwarfs”.  These stars have exhausted all of their nuclear fuel and have collapsed to form a strange body with a radius similar to that of the Earth but with a mass approaching that of the Sun.

That bright object in the western twilight sky that's been attracting your gaze lately is poised to leave us.  Dazzling Venus has begun her descent toward the Sun and will soon vanish in the bright evening twilight.  As the week opens she is still prominent in the southwestern sky during the early evening, but by Christmas she will set nearly 25 minutes earlier.  By the end of the year she will be very difficult to spot.  Interestingly, we won't see her return to the evening sky until Christmas time next year.

Jupiter now rises at around 6:00 pm, which will put him well within range of backyard telescopes by 8:00.  He's now just a couple of weeks from opposition, when he will be at his closest to Earth and brightest in our skies.  Old Jove will be a great target for any telescopes left under the tree.  After the Moon, Jupiter is probably the most popular target for new telescope owners, and he rarely disappoints.  Look for his four bright Galilean moons which can be seen in any telescope.  If you have an instrument larger than four inches of aperture look for the dark parallel cloud belts surrounding the bright equatorial zone.  Subtle details are visible in his turbulent atmosphere in moments of steady air.

Mars now rises shortly before 1:00 am and is high in the southeast as morning twilight gathers.  Earth is gradually catching up to the red planet, and we'll pass him in early April of 2014.  Right now the telescopic view reveals a tiny ruddy gibbous disc, but his apparent size will steadily increase as opposition approaches.  By that time he will have grown to over twice the size he exhibits now.

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