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The Sky This Week, 2013 December 3 - 10

Seven Sisters and the earliest sunset.
Pleiades_131130_01small.jpg

Messier 45, the Pleiades or Seven Sisters 

Imaged 2013 November 30 from Great Meadow, near Old Tavern, VA,
80mm (3.1-inch) f/6 Antares Sentinel refractor & Canon EOS Rebel T2i DSLR


The Moon returns to the evening sky this week, waxing through her crescent phases as she climbs through the dim stars of the autumnal constellations. First Quarter occurs on the 9th at 10:12 am Eastern Standard Time. Look for the Moon just over six degrees north of dazzling Venus on the evening of the 5th. For the rest of the week Luna eschews the company of bright objects.

December 7th marks the beginning of the series of phenomena associated with the winter solstice. This is the evening of the year’s earliest sunset, which in the Washington, DC area occurs at 4:46 pm EST. From this evening onward Old Sol will set a little bit later on successive nights. The change is very incremental at first, but by the time the solstice occurs on the 21st sunset will be four minutes later. By the end of the year sunset will occur at 4:58 pm. The trade-off comes with the time of latest sunrise. That won’t occur until January 4th, 2012, when the Sun peeks over the horizon at 7:27 am. The shortest day of the year still falls halfway between these dates on the solstice itself, marking the astronomical beginning to the winter season. The reason for this seeming discrepancy has to do with the "equation of time", which is the formula used to correct "sundial" or "apparent" solar time to "mean" solar time. This is graphically displayed on Earth globes as the "figure-8" diagram, the "annalema", that’s usually printed over the expanse of the Pacific Ocean. The rate of change of the equation of time reaches its maximum near the time of the winter solstice, causing the times of sunrise and sunset to be "skewed" in the weeks surrounding the solstice itself.

With a brightening Moon gradually adding more light to the sky in the evening hours, it becomes more difficult to explore some of the fainter treasures of the "deep sky". Fortunately we have a wonderful group of stars that are now becoming very prominent in the eastern sky once evening twilight has ended. By 9:00 pm the Pleiades are high in the heavens as they approach the meridian. Almost everyone in the northern hemisphere knows this small group which resembles a tiny replica of the stars of the Big Dipper. This is one of the most recognized asterisms in all of popular sky lore, and stories relating to the group may be found in the records of virtually every culture that has left written or oral history in their wake. In Greek mythology the stars represent the seven daughters of Atlas who were saved from the lustful pursuit of Orion by the god Zeus, who plucked them from Earth and placed them in the sky, forever out of reach from the Hunter. Temples in bor the Old World and the New are aligned to the point on the horizon where they rise, and even J.R.R. Tolkien included them in the lore of Middle Earth, where they were known by the elvish name "Remmirath" or "The Netted Stars". The Pleiades form a genuine star cluster and are located relatively nearby on the cosmic scale at a distance of between 400 to 450 light-years. While most of us can see six or seven stars with the unaided eye, I know of a few individuals who can accurately place 14 stars under very good conditions. In reality there are some 1000 stars associated with the cluster, and each increase in optical aid will reveal more of these luminaries. The dominant stars are very energetic blue giants that formed around 100 million years ago, and under dark-sky conditions you can still see wisps of the nebula that they condensed from in a telescope, faintly glowing around the brightest members. The brightest star is Alcyone; the other sisters are Celaeno, Electra, Maia, Merope, Sterope, and Taygeta.

Venus reaches her greatest brilliancy this week, glowing at a dazzling -4.9 magnitude in the southwestern sky during the early evening hours. This is the best time of the current apparition to see Venus against a dark sky. She gets a visit from the waxing crescent Moon on the evening of the 5th, but by the time the Moon comes around to call on her again she will be rapidly falling into the Sun’s twilight glow.

Jupiter now rises at around 7:00 pm, and he should be high enough above the horizon to turn the telescope toward him by 9:00. The giant planet is at a very high northern declination, so he will dominate the sky throughout the rest of the night. Old Jove’s disc is large enough to resolve in any telescope, and even a simple spyglass will show his four large Galilean moons. If you have a four-inch or larger aperture scope, take a look at the planet’s dusky cloud belts and bright zones. If you’re viewing him at around 10:00 pm on the evening of the 6th, look for the pink oval in the South Equatorial Belt that betrays the famous Great Red Spot, and Earth-sized jovian storm that has persisted for nearly three centuries! By the end of the week Jupiter has a very close encounter with the third-magnitude star Wasat in the constellation of Gemini.

Look for ruddy Mars high in the southeast in the hours before dawn. His distinctive reddish tint is a dead giveaway. He’s now drifting into the western reaches of the constellation Virgo, which is where he’ll spend the rest of this upcoming apparition.

 

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