Go Stargazing! December Edition

The ‘main event’ of December evenings occurs in the southwest at dusk, where you can watch Venus pull away from Jupiter.  Look southwest right as night falls for the two brightest things there except for the Moon.  The brighter one low in the southwest is Venus, which outshines everything else in the night sky.  Jupiter is the dimmer of the two, although it still outshines all the stars we ever see at night.  Venus and Jupiter begin the month about 2 degrees apart (your finger at arms length blocks about 1 degree.)  However, Venus will extend that gap quite noticeably each night, until it appears high above Jupiter on December 31. 

Mercury emerges from the Sun’s glare in time to form a nice pair with Jupiter on New Year’s Eve.  As you prepare to ring in 2009, take a moment to look at Mercury just to Jupiter’s left in late twilight.  That same night, the Moon will be near Venus. Saturn can be found high in the south at dawn.  Mars is lost in the Sun’s glare this month, and will remain out of sight into 2009.  It is directly behind the Sun (in conjunction with the Sun) on December 5.

Orion no céu
Creative Commons License photo credit: giumaiolini

The enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, sets in the west.  The Great Square of Pegasus is overhead at dusk.  The star in its upper left hand corner is also the head of Andromeda.  Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen.  Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from the Dipper.  In fall and early winter, while the Dipper is low and out of sight, Cassiopeia rides high.

Dazzling Orion rises in the east, reminding us that winter is on the way.  His belt points up to Aldebaran, the eye of Taurus the Bull.  By 9 pm tonight (7 pm by New Year’s Eve), the Dog Stars Sirius and Procyon will have risen below Orion in the east.  Sirius is the brightest star we ever see at night. 

Moon Phases in December 2008:

1st Quarter         December 5, 3:25 pm
Full                     December 12, 10:38 am
Last Quarter        December 19, 4:30 am
New                    December 27, 6:22 am

At 6:04 am on Sunday, December 21, the Sun is directly overhead at the Tropic of Capricorn, meaning that the North Pole is tilted as much as possible away from the Sun.  This is the winter solstice.  For people in the Northern Hemisphere, December 21 has less daylight and more night than any other day of the year. 

Sunset at Appalachian Trail
Creative Commons License photo credit: Pardesi*

However, the earliest sunsets occur on December 1 and 2.  We are already close enough to the solstice that the Sun’s apparent path across the sky on December 21 is only slightly lower than on any other day this month.  Meanwhile, Earth is about to make its nearest approach to the Sun, called perihelion, in January.  As a result, the Earth is speeding up.  The effect isn’t much (Earth’s orbit is nearly circular), but it’s enough to make both sunrise and sunset a little later each day this month and next.  With the Sun’s apparent height in the sky not changing that much in December and January, the small effect of Earth’s acceleration near perihelion dominates.  Since most of us sleep through sunrise and witness sunset, the days seem be slightly lengthening between the beginning of the month and the 21st, although they are actually getting slightly shorter.

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