NIGHT SKY HAPPENINGS

The Sky for December 2016
By Faylene Roth

Each season has its stars. The beacon of summer—the triangle formed by the bright stars in Lyra, Aquila, and Cygnus are slipping below the northwestern horizon. Cygnus the swan takes a nosedive towards the northwest horizon transforming into the asterism known as the Northern Cross. Pegasus, the signature constellation of autumn hangs high in the western sky. Clear cold nights provide good conditions for viewing our neighboring galaxy, Andromeda, which hangs off the northern most leg of the constellation Andromeda, attached to the northeast corner of the Great Square of Pegasus. The winter cluster of diamond-like stars within the constellations of Taurus, Auriga, Gemini, Orion, plus the dog stars, Sirius and Procyon, advance on the eastern horizon to claim dominance over the skies. Those who rise early to view Jupiter in the morning sky may glimpse the red-orange star above it that anticipates the distant promise of another spring.

Twilight extends the period of daylight in three stages at each end of the day. Morning twilight begins with astronomical twilight—about 1-1/2 hours (nearly 2 during summer months) before sunrise—as the eastern horizon brightens. Nautical twilight continues—as the overhead sky turns blue and color returns to the surrounding landscape—for another 30-40 minutes. The final stage—civil twilight—provides adequate light for most outdoor activities for the half hour before the sun crests the horizon. The opposite progression occurs after sunset.

VISIBLE PLANETS
Evening (Before Midnight)

Mars – Look for the small reddish orb hovering about 45 ̊ above the southwest horizon as astronomical twilight takes over the evening sky. The background constellations drift westward behind Mars—first Capricorn then Aquarius. Mars sets by 10:00pm, depending upon the horizon. (Magnitude +0.8)
Venus – Shining brilliantly, Venus claims the westward edge of Capricorn about 30̊ below Mars. It sets a few minutes later each day which closes the gap between the two planets to about 15̊ by month’s end. (Magnitude -4.2)

Morning (At Twilight)

Jupiter – Around middle of the night Jupiter crests the eastern horizon forming a large triangle with Regulus (Leo) to the west and red-orange Arcturus (Bootes) above. As twilight brightens the eastern sky, only Jupiter will remain visible. (Magnitude -1.7)

MOON HAPPENINGS
December 7 – Waxing first quarter lights the evening sky then sets soon after midnight.
December 13 – Full moon (5:05am) rises at 5:14pm.
December 20 – Dark evening skies return with the waning last quarter moon rising after midnight.
December 28 – New moon (11:53am) yields dark skies for several nights.

(The moon rises later each day—as little as 30 minutes to as much as one hour. Time of moonrise and moonset may also be delayed in mountainous terrain.)

Twilight is often the best time to look for Venus and Mercury because they frequently rise or set within one-half to one hour of sunrise or sunset. Twilight transitions between night and day in three stages at each end of the day. Morning twilight begins with astronomical twilight as the eastern horizon brightens —about 1-1/2 hours (nearly 2 during summer months) before sunrise when the sun is 18 ̊ below the horizon. Nautical twilight takes over for another 30-40 minutes—as the sun passes 12 ̊ below the horizon and the overhead sky turns blue and color returns to the surrounding landscape. The final stage—civil twilight—begins when the sun ascends to 6 ̊ below the horizon and provides adequate light for most outdoor activities for the half hour before the sun crests the horizon. The opposite progression occurs after sunset. Civil twilight covers the period after sunset during which daytime light quality persists for about one-half hour. Color then fades from the landscape during the 30-40 minute period of nautical twilight during which the overhead sky darkens while the western sky retains color. Astronomical twilight then transitions to night skies that are now darkened along the horizon.

Primary Sources: USGS; U.S. Naval Observatory; Your Sky at http://www.fourmilab.ch/yoursky/
To find out when the space shuttle and International Space Station are visible from your location, go to: http://spaceflight.nasa.gov/realdata/sightings/index.html and click on Sighting Opportunities.

The star chart approximates the sky from astronomical twilight to midnight. As the night and month progresses, the constellations shift toward the northwest. The celestial equator is measured in hours (h). The ecliptic is measured in degrees

Note: Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates 1.5̊. Middle, ring, and little finger touching represent about 5̊. The width of a fist is about 10̊. The fist with the thumb extended at a right angle equals 15̊. The hand stretched from thumb to little finger approximates 20̊-25̊. The diameter of both the full moon and the sun spans only 0.5̊. Adjust for the size of your hand.