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The Sky This Week, 2018 December 11 - 18

Fleeting phenomena for frosty nights.
Waiting for darkness, looking over the dome of the USNO's 26-inch
Waiting for darkness,
Looking over the dome of the USNO's 26-inch "Great Equatorial" telescope, 2018 December 11
imaged with a Canon EOS Rebel SL2 DSLR.

The Moon waxes in the evening sky this week, climbing through the sparse star fields of the autumnal constellations as she wends her way northward along the ecliptic.  By the end of the week she lies just west of the rising stars of the Great Winter Circle.  First Quarter occurs on the 15th at 6:49 am Eastern Standard Time.  Look for the Moon just below ruddy Mars on the evening of the 14th.

The annual Geminid meteor shower peaks during the overnight hours of the 13th-14th.  This shower is one of the year’s most consistent displays, but it doesn’t get the same notoriety as August’s Perseids.  This is probably due to the observing conditions for most of us.  In August you can lay on a blanket in shorts and a T-shirt to enjoy the show.  In mid-December it’s hard to remain still while staring at the sky without getting very cold very quickly.  However, one of the great advantages of the Geminids is that the shower’s radiant point in the sky, near the bright star Castor in Gemini, is quite high in the east by 10:00 pm for northern hemisphere observers.  From a dark location it’s possible to see up to 100 meteors per hour during the shower’s peak times.  On the 13th the Moon sets shortly after 10:30 pm EST, so scattered moonlight shouldn’t be a factor.  Geminid meteors are fairly slow-moving compared to the zippy Perseids, and the brighter members can be quite colorful. The parent body of the Geminids is the very unusual asteroid 3200 Phaethon, the first asteroid to be discovered by a spacecraft.  Its orbit resembles that of a comet, but it doesn’t show a cometary composition.  Instead, each time it passes close to the Sun dust from its surface “sputters” off and gets distributed along its orbital path.  It is these dust grains that create the meteors that grace our skies each year.                                           

In addition to the Geminids, this week presents us with another ephemeral event.  On the evening of the 16th a member of Jupiter’s “comet family” will pass within 11.5 million kilometers (7.2 million miles) of the Earth and will be very well-placed for observation in the evening sky.  Comet 46P/Wirtanen orbits the Sun with a period of 5.44 years and is one of many comets whose orbit is shaped by the gravity of the solar system’s largest planet.  It was discovered in January, 1948 by American astronomer Carl Wirtanen at Lick Observatory in California.  This year’s unusual close approach won’t happen again for at least 20 years.  Currently the comet is moving northeastward through the stars of the constellation of Taurus, The Bull.  It is glowing at fifth magnitude and should brighten a bit more over the next several nights.  On the evenings of the 15th and 16th it will be located about five degrees southeast and east (respectively) from the Pleiades star cluster and should be a fairly easy binocular target, appearing as a fuzzy glowing ball with a faint greenish tint.  If the current brightness trend continues it may reach a third magnitude and would thus be an easy naked-eye target from dark-sky sites.  It will cross the meridian at around 10:00 pm, at which time the first-quarter Moon should be low in the southwest.  Over the course of the next couple of weeks it will move into the constellation of Auriga, the Charioteer, passing the bright yellow-hued star Capella just before Christmas.

Ruddy Mars is now the sole bright planet to grace the evening sky.  You’ll find him on the meridian as evening twilight deepens.  Glowing at zero magnitude, he is now some ten times fainter than he was at opposition back in late July, his distinctive tint and position in a sparse field of bright stars means that you shouldn’t have any trouble finding him.  Viewed through the telescope, his now-shrunken disc may display a few tenuous surface features, but it will also have a distinctive gibbous phase. 

Bright Venus beams down in the southeast pre-dawn sky, and you should be able to view her until sunrise.  Right now she’s been easy to spot as I take the dog out for his morning “constitutional”, even without my glasses on!  She is currently shining at a blazing negative 4.8 magnitude, and if you know where to look for her she should be visible even after the Sun comes up.

The elusive planet Mercury reaches greatest elongation west of the Sun on the 15th.  At this time he will be just over 20 degrees from the Sun and should be visible in the southeast, 10 degrees above the horizon 45 minutes before sunrise.  

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