SKY HAPPENINGS

The Sky for December 2011
By Faylene Roth

DAYLENGTH
The shortest period of daylight this month is 9 hours, 28 minutes during a six-day period December 19-24 when the length of the days vary by only a few seconds. Civil twilight contributes a half hour of light before sunrise and after sunset. Nautical twilight adds an additional half hour at each end of the day, but it contributes much less light than summer twilights because the sun is now below the bulge of the earth’s equator which blocks our access to its light (and warmth). Pitch darkness follows in another half hour at the end of astronomical twilight when all color has vacated the horizon.

MOON HAPPENINGS
Dec. 2 – First Quarter Moon sets soon after midnight.
Dec. 10 – Full Moon occurs at 7:36am and rises at 5:15pm.
Dec. 17 – Last Quarter Moon rises shortly after midnight.
Dec. 24 – New Moon occurs at 11:06pm.
(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

METEOR EVENTS
The Geminid Meteor Shower, generally one of the year’s best, is well-known for multicolored meteors and fireballs that produce very few trails. The source for this prolific meteor shower is the asteroid Icarus. Unfortunately, a full moon on December 10 obscures all but the brightest of this year’s show. A more promising event occurs on December 20. The Delta Arietids provide early evening potential on a mostly moonless night. Aries is a small constellation comprised of two fairly bright stars above and to the left of Jupiter. This shower produces up to 10 meteors per hours. A few days later, on December 22, the Ursid Meteor Shower continues throughout the night. The Ursids emanate from the region of the Little Dipper, so good viewing begins around 11:00pm. A nearly new moon promises a dark night for viewing up to 20 meteors per hour.

LUNAR ELCIPSE
Early morning risers will observe this month’s full moon being overtaken by the Earth’s shadow on December 10. A lunar eclipse occurs during a full moon when the sun, earth, and moon are on the same plane. As the earth moves between the sun and moon, it first casts a partial shadow, the penumbra. When the Earth has moved far enough to block all light on the moon, it casts a full shadow, called the umbra, which marks the period of total eclipse. Totality lasts about 51 minutes. Observers in Moab can watch the beginning of the eclipse and may see the moon enter totality before it disappears behind the rocks. The following timetable is based on information provided by the Clark Planetarium in Salt Lake City.

Penumbral phase begins about 5:46am MST
Total phase begins about 7:06am MST
Greatest eclipse occurs about 7:32am MST
Moonset (in Moab) is about 7:27am MST
Totality ends about 7:57am MST

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

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.

VISIBLE PLANETS

Jupiter -Find Jupiter high overhead in the evening sky, then dropping below the western horizon around midnight. The brilliant yellow planet is the brightest object overhead and appears on the edge of the constellation Pisces . The two bright stars above and to the left of Jupiter form the constellation Aries. (Magnitude -2.8)

Mars - Wait until after midnight to look for Mars. It rises in the eastern sky below and to the left of Regulus (bright star at Leo’s heart). Look for it overhead in the early morning twilight. It appears farther east each day, moving about 15 degrees farther left of Regulus by month’s end. It’s red-orange orb brightens significantly this month as its orbit brings it closer to Earth. (Magnitude +0.6 to +0.2)

Saturn - Regulus (Leo), Mars, Saturn, and Spica (Virgo) form a line across the eastern sky in the early stages of morning twilight. Saturn (in Virgo) appears about 45 degrees below Mars and about 10 degrees above and to the right of Spica. Mars shines a little brighter, but Saturn’s disk appears larger. (Magnitude +0.7)

Venus - On December 1 Venus adorns the tip of the teapot in Sagittarius, outshining Jupiter in the overhead sky. It sets within a few hours of the sun this month and will be difficult to see without a clear view of the western horizon. By the last week of December, Venus appears farther east and has moved into Capricorn. (Magnitude -4.0)


Note: Apparent magnitude values range from -4 to +6 for most planets and visible stars. The lower the value the brighter the object. A decrease of 1.0 magnitude is 2.5 times brighter.

Cygnus the Swan takes a nosedive on the western horizon as it glides through the Milky Way towards the center of our galaxy. In this position, it is often called the Northern Cross.

Hold the star chart high above your head and match the compass directions to the direction you are facing. Adjust the star chart by orienting Ursa Major (Big Dipper) to match its position in the sky.

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.