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The Sky This Week, 2010 December 7 - 14

Mercury's last evening stand, bright stars in Moonshine
GRC_Jup_2010_outbreaksmall.jpg

 Jupiter, showing South Equatorial Belt Plume
Cylindrical projection map based on observations made
2010 NOV 29, 01:11 and 01:29 UT


The Moon begins the week as a thin waxing crescent in the southwestern twilight sky. She quickly climbs northward and grows fatter during the week. First Quarter occurs on the 13th at 8:59 am Eastern Standard Time. Look for Luna six degrees above and to the left of fleet Mercury shortly after sunset on the 7th. On the night of the 13th she stays in the company of bright Jupiter, six degrees above the giant planet.

The 7th marks the earliest sunsets of the year for temperate northern hemisphere latitudes. Here in Washington sunset occurs at 4:46 pm EST. Sunset remains at this early hour for most of the week, finally setting a minute later on the 13th. From then on Old Sol sets a bit later each evening, slipping below the horizon at 4:50 pm on the day of the winter solstice on the 21st and a full 10 minutes later at 4:56 pm on New Year’s Eve. However, our latest sunrises won’t occur until early January, so the duration of the year’s shortest day will fall squarely on the equinox. This seeming discrepancy is a result of two primary factors that are at work at this time of year: the rapid change in the value of the "Equation of Time" and the very slow change in the Sun’s declination. A more in-depth explanation of these solstice phenomena may be found elsewhere on the USNO’s web site.

As the Moon’s crescent waxes steadily through the week, her brightening glow begins to wash out the fainter stars of the autumnal constellations. By the time she reaches Jupiter at week’s end the only bright stars of fall are on the meridian in the form of the Great Square of Pegasus, the diverging chains of Andromeda, the wish-bone shaped Perseus, and the prominent "W" of Cassiopeia. In all of these constellations, only one star is brighter than magnitude 2.00, but just to the east you’ll find winter’s bright beacons rising to take over the night watch. By 10:00 pm the Great Winter Circle has cleared the horizon, and within its confines are over a dozen stars that beat the magnitude 2.00 criterion.

The fleet planet Mercury rapidly drops from the evening sky this week, ending his last evening apparition for the year. Your best shot at finding him is during twilight on the 7th, when the Moon is just over six degrees from the planet. Over the next few evenings Mercury starts to fade dramatically, and by week’s end he’ll be lost in the twilight glare.

Jupiter is still the planet of choice for evening skywatchers. He pops into view almost immediately after sunset and crosses the meridian at the end of evening astronomical twilight. This puts him in perfect viewing position at around 6:30 pm. With the cold weather of late you should put the telescope out about an hour earlier so it can cool to the ambient air temperature. This will ensure that the turbulence that may distort your views of Old Jove are in the atmosphere and not your optics. The planet’s missing South Equatorial seems to be in a full-fledged revival, with a dark plume of material spreading out in the belt’s once-prominent location. The plume now extends nearly halfway around the planet.

The pre-dawn sky hosts stately Saturn and dazzling Venus among the rising stars of spring. As the week opens both planets flank the bright blue star Spica by about 10 degrees, but Venus is adding almost a degree per day to her gap with the star. Venus is at her greatest brilliancy for this current morning apparition right now. If you are in a very dark location see if you can spot your shadow cast by the light of her brilliant cloud tops.

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