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Sky Happenings December 2007

The Sky for December 2007

By Faylene Roth

DAYLIGHT
Day length changes little around the solstice (the shortest period of daylight) because the sun appears to move towards the east as the Earth passes through the elliptical end of its orbit. However, the latest sunrise and earliest sunset do not occur on the same date. The earliest sunset occurs almost two weeks before the winter solstice, while the latest sunrise occurs in January. In December the sun sinks low in the sky which

Moab UT (at City Hall)
38O34’ N Latitude
109O33’ W Longitude
4048 ft - 1234 m
MAJOR CONSTELLATIONS OF NOVEMBER

Overhead
Andromeda
Aries
Pegasus
Perseus
Pisces

Northward
Cassiopeia
Cepheus
Ursa Major
Ursa Minor

Eastward
Auriga
Canis Major
Canis Minor
Gemini
Orion
Taurus

Southward
Aquarius
Cetus


Westward
Cygnus
Lyra

causes later sunrises and earlier sunsets. However, from November to February, the tilt of the Earth’s axis and its increased speed as it approaches perihelion (point nearest to sun) make the solar day a little longer than a 24 hour clock day. The difference can be detected at solar noon, when the sun reaches its zenith after 12 noon clock time. By early December solar noon is delayed enough to postpone the time of sunset by a few seconds each day. After the solstice, sunrise should begin to occur earlier as the sun begins to move higher in the sky. However, the longer solar day continues to delay sunrise, solar noon, and sunset. By early January the sun’s position in the sky has risen high enough to override the effect of the longer solar day. At that point the sun begins to rise earlier each day.

MOON HAPPENINGS

December 1 begins with a third quarter moon high in the morning sky just below Saturn. On the morning of December 5 the waning moon will appear alongside Venus and the bright star Spica in the constellation Virgo. New Moon occurs on December 9 at 10:41am. The 13th Full Moon (we had two in May) of the year occurs at 6:16pm on December 23. It rises with a still brilliant Mars in the eastern sky just before sunset. On the evening of December 27, a waning gibbous moon rises less than one degree from the bright star Regulus in the constellation Leo. Look for them around 10:00pm. Saturn rises about one-half hour later.

METEOR SHOWERS

The Geminid Meteor Shower is one of the most rewarding displays of the year, producing 60 or more meteors per hour. Increased meteor activity in the vicinity of Gemini is evident from December 6-19. A waxing crescent moon sets by 8:00pm, leaving dark skies for viewing this abundant, colorful meteor shower. Look eastward towards Gemini (highlighted by the bright red planet, Mars) soon after sunset. Peak viewing occurs after midnight, on December 13, when Gemini is overhead. A more moderate meteor shower occurs to the north, in late December, between Ursa Major and Ursa Minor. The Ursid Meteor Shower produces around 10 meteors per hour and peaks on December 22. The source of this meteor activity is from previous passes of Comet Tuttle (see COMET VIEWING).

COMET VIEWING

Be sure to check the progress of Comet 17P/Holmes as it passes through Perseus during December. Its outburst of dust and ice on October 24, 2007, increased its brightness by a million times in just a few days. By mid-November the diameter of the dust and ice cloud forming its coma was larger than the diameter of the sun. Comet Holmes went through a similar outburst and increase in brightness in 1892 when it was first discovered. Normally it is no brighter than the planet Pluto. The comet is fading in brilliance as it expands, but it could repeat its performance of 1892 when the first outburst was followed by another a few months later. Comet Holmes has an orbital period of seven years. Another Comet, 8P/Tuttle, is due to rendezvous with the sun around the beginning of the new year. Look for it as it passes through Cassiopeia and Andromeda during December. Its brilliance is similar to that of a dim star when viewed with the unaided eye. Comet Tuttle, with a period of 13.6 years, is the source of the Ursid Meteor Shower.

VISIBLE PLANETS
Jupiter - disappears from the evening sky by December 7 as its orbit takes it behind the sun; reaches conjunction with the sun (opposite side of sun from Earth) on December 23, but we will not see it; reappears in the morning sky in January.
Mars - at its nearest and brightest for the next nine years on December 18; outshines all the planets but Venus; due south at midnight on December 24 when it reaches opposition (opposite side of Earth from sun).
Saturn - rises before midnight throughout the month and visible high in the morning sky above Venus; on the morning of December 1 look for Saturn just 2 degrees above the waning third quarter moon (that is a little more than the width of your little finger held at arm’s length).
Venus - shines brightly as the morning star throughout the month; on the morning of December 5 a waning crescent moon hovers near Venus and Spica (brightest star in Virgo).

WINTER SOLSTICE
Winter begins in Moab on December 21 at 11:08pm. The solstice occurs when the sun’s path across the sky reaches its lowest point in the southern sky. Seasonal changes occur because the Earth’s axis tilts away from the plane of its orbit at a 23.5 degree angle. In winter that means the northern hemisphere tilts away from the sun so that the sun’s rays strike the Earth at a shallow angle. Even though the sun is closer to the earth in winter than in summer, much of its heat is lost as it passes through more atmosphere and is dispersed over a greater area.

Mars stands out in the eastern sky. Its reddish light sits
alongside the slightly irregular parallelogram
of Gemini. On December 24 it will be
due South and almost directly
overhead at midnight shining as
bright as Sirius, the Dog Star,
to the South in Canis Major.

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 night sky from astronomical twilight to midnight. As the night and the month progress, the constellations will shift toward the northwest.



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