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The Sky This Week, 2017 December 12 - 19

See the year's best meteor shower.
Gemini and Orion Rising
imaged from near Bluemont, Virginia in 2009.
Castor is the upper of the two bright stars at the left.

The Moon wanes in the morning sky this week, passing a couple of rising planets in the pre-dawn eastern sky as she drifts through the late spring constellations.  Look for Luna near Mars on the morning of the 13th.  She is close to bright Jupiter on the following morning.  New Moon occurs on the 18th at 1:30 am Eastern Standard Time.  

The celestial highlight for the week occurs on the evening of the 13th and morning of the 14th with the peak of the annual Geminid meteor shower.  This annual display usually takes a back seat to the August Perseids, primarily because it’s more comfortable to sit outside for a few hours in the summer than in December’s chill.  However, the Geminids have gradually evolved to become the year’s most prolific and consistent shower, with “zenith hourly rates” that have steadily increased over the past century.  One great advantage for the Geminids is that the radiant, the point in the sky from which the meteors seem to originate, is very close to the bright star Castor, the second-brightest of Gemini’s “Twin Stars”.  Castor now rises at around local sunset, and by 10:00 pm it is high in the eastern sky.  This means that you stand a very good chance of seeing a number of Geminids from this time forward through the night.  Geminid meteors are slower than the very swift Perseids, and most of their members are third magnitude or brighter.  By the time the radiant is overhead at around 2:00 am, an observer at a very dark site may spot up to 100 meteors per hour.  Observers in city exurbs may see one per minute, and even urban dwellers might spot 30 to 50 brighter shower members per hour.  The Geminids are quite unusual in that they are associated with the orbit of an asteroid rather than a periodic comet.  The parent body is a small chunk of rock known as (3200) Phaethon, which was the first asteroid to be discovered by a satellite.  In mythology Phaethon asked his father, Helios the Sun god, to drive his fiery chariot across the sky for one day.  He lost control of the god’s fierce horses and the chariot brushed the Earth with flames, creating the deserts of North Africa and the Middle East.  (3200) Phaethon is an unusual type of asteroid of the Apollo family, passing just 21 million kilometers (13 million miles) from Old Sol’s surface.  Each year at this time in December Earth passes close to the orbital path of Phaethon, running into debris sputtered off its surface during its close solar encounters.  In 2017 Phaethon itself is closer to us than at any time until 2093, which could mean a stronger Geminid display than usual.  If the weather looks poor for the 13th/14th, you can still see a decent display on the nights before and after the peak.  The waning crescent Moon won’t interfere.

Take advantage of the moonless nights this week to explore the Milky Way between the “feet” of Gemini and the bright yellow star Capella, located high in the northeastern sky in the late evening.  This area of the galaxy lies opposite the dense star clouds of the summer Milky Way, but it is chock full of bright galactic star clusters.  It’s a wonderful area to sweep with a modest-aperture low-power telescope.

The Moon scoots by ruddy Mars and bright Jupiter in the pre-dawn sky between the mornings of the 13th and the 15th.  Mars isn’t particularly bright, but his distinctive ruddy hue should make him apparent.  Jupiter will be hard to miss, shining at a healthy magnitude of -2.  By the end of the week the giant planet approaches the third-magnitude star Zubenelgenubi, a favorite of crossword enthusiasts that is the second-brightest star in the rather obscure Zodiacal constellation of Libra, the Scales.

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