and Sunset Times for
The sun reaches its lowest point in the southern sky on December 21 at 5:04 am. It may not be obvious that this marks the shortest day of the year (9 hours 28 minutes). For a week before and a week after the winter solstice, the period of daylight varies by less than one minute. Twilight extends the effect of daylight by one and one-half hours in the morning and in the evening. Civil twilight provides sufficient light for most activities for one-half hour after sunset. Nautical twilight continues another half hour as color and shape fade from the landscape. During the next half hour, of astronomical twilight, the last rays of the sun disappear from the sky. Darkness overtakes the sky more quickly this time of year because the sun’s rays become very diffused as they pass through the atmosphere at such a low angle. Sunrise and sunset are calculated for a flat horizon. Actual times may vary depending upon the surrounding landscape.
December begins with a waxing crescent moon in the western sky. The moon appears fuller and higher each evening until December 12 when the full moon rises at 4:56pm. As the moon wanes, it rises more than an hour later each night until December 19 when a crescent moon rises after midnight. New moon occurs on December 27 with a waxing crescent returning to the evening sky by December 29.
STARGAZING AND STAR COUNTS
A group of stargazers from Moab gathered at Ken’s Lake on the evening of November 1st for stargazing, telescope viewing, and a star count. We took a tour of the night sky with the unaided eye and with telescopes provided by the Red Rock Astronomy Club. Then we turned our attention to the constellation Cygnus, the Swan, to determine the darkness of the night skies around Moab.
The darkness of the night sky can be measured by determining the limiting magnitude of visible stars. Stars visible with the unaided eye range from magnitude -1 to +7. (See note above on apparent magnitude.) By studying detailed star charts of Cygnus, we determined that our group could see stars in the range of magnitude +5 to +6. This was not quite as good as last year’s results which ranged from +6 to +7. Our observation site at Ken’s Lake still offers dark skies overhead, but artificial lights on the horizon were more apparent this year. The light dome over Moab was more prominent, and we observed commercial lights along the highway and from new construction nearby that were not present last year. High, thin clouds may have affected our viewing. Even though conditions seemed to clear as darkness progressed, there may have still been interference. As citizen scientists, we await the next star count for further evidence.
Last year more than 6,000 observations from around the world were submitted for the Great World Wide Star Count, sponsored by the University Corporation for Atmospheric Research (UCAR) in Boulder, Colorado. The star count was organized to raise public awareness of the importance of dark skies for astronomical research, human health, wildlife behavior, energy conservation, and security. Light fixtures with shields that direct light downward can minimize overhead light pollution. When light is focused where it is needed, lower wattage bulbs can be used, increasing energy efficiency, without sacrificing security. Check the International Dark-Sky Association website at www.darksky.org for more information on the benefits of dark skies.
For more information about future stargazing events, call 210-1151.
This year is a washout for the Geminid Meteor Showers, usually one of the best of the year. A high, bright, nearly full moon shines in the sky throughout most of this year’s event, December 6-19. Best viewing is after midnight during the first few days (the moon has set) and before midnight towards the end (the moon has not yet risen) . The radiant for the meteor shower is in the region of Castor and Pollux, the twin stars of Gemini. The Ursid Meteor Shower is a less spectacular event, but viewing will be more promising. This meteor shower peaks on the night of December 22. Look for these faint meteors December 17-26 just above the basket of the Little Dipper.
Note: Hold your hand at arm’s length to measure apparent distances in the sky. Adjust for the size of your hand. 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 hand stretched from thumb to little finger equals 20 degrees. The diameter of both the full moon and the sun spans only 0.5 degree.
Jupiter - Look for a conjunction of Jupiter and Venus on December 1 about one hour after sunset. The two planets will be about two degrees apart, forming a triangle with the waxing crescent moon. Shining at magnitude -2, Jupiter sets earlier each day which means it appears lower in the southwestern sky each evening.
Mercury - Mercury shines very low on the WSW horizon, with Jupiter, during the last four days of December. On December 31, about one-half hour after sunset, look for a conjunction between Mercury and Jupiter (within 1.2 degrees). A crescent moon joins them a few nights earlier on December 29. Mercury will be fainter (magnitude -0.7) and on the left.
Saturn - The only planet visible in the early morning is Saturn. Look for it high in the morning sky, shining at magnitude +1. By mid-month, it is rising a little before midnight.
Venus - Find Venus a little higher in the southwestern sky each night. Shining at magnitude -4, it moves eastward each evening through the constellation Capricornus. On December 31 it appears with a waxing crescent moon above the conjunction between Mercury and Jupiter.
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.
Primary Sources: USGS, U.S. Naval Observatory,
CONSTELLATIONS this MONTH
The eastern sky is brilliant with Magnitude 0, 1, and 2 stars. Aldebaran in Taurus, Rigel and Betelgeuse in Orion, Procyon in Canis Minor, Castor and Pollux in Gemini, and Sirius (brightest star in the sky) in Canis Major to the east of Orion’s belt.
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