Into Tonight’s Sky: December Darkness

 

JAMESTOWN, N.D. (NewsDakota.com) – Below is the monthly editorial provided by Dr. Tim Bratton, Professor Emeritus of History at the University of Jamestown. He is also an Amateur Astronomer.

This December the naked-eye planets set too soon after sunset to be readily visible, or else rise before dawn, making it a quiet month. However, there will be a major meteor shower and a lunar occultation of Aldebaran as compensation.

Dec. 3 (Sun.): The final Full Moon of 2017 occurs at 9:47 a.m. CST, when it will be 16.2 degrees beneath the NW horizon, 33.2 minutes of arc in apparent diameter, and 222,452 miles away. This month’s Full Moon is called the “Moon Before Yule” or the “Long Night Moon,” since evenings are longest around the winter solstice. Since the Sun is lowest in the sky in December, and the Full Moon must be opposite the Sun, it follows that this Full Moon climbs highest in the night sky.

Dec. 4 (Mon.): The Moon will reach perigee (its closest approach to the Earth this month) at 2:46 a.m., when it will be 55.8 degrees above the SW horizon, 99.8% illuminated, 33.22 arc-minutes in apparent span, and 222,135 miles away. This combination of a nearly full Moon and perigee makes it a “Super Moon,” although it appears actually only 7% larger than its average size; most casual observers would not notice the difference.

Dec. 10 (Sun.): Last Quarter Moon occurs at 1:52 a.m., when it will be 17 7/12 degrees above the E-ESE skyline, 237,049 miles distant, and 31 arc-minutes across. Today’s sunset at 4:46 p.m. will be the earliest of 2017, although only by a few seconds; the Sun sets about this time from Dec. 6 through Dec. 14. Thereafter sunsets will happen later, but the days will continue to shorten until winter solstice on Dec. 21 because sunrises will be taking place later also.

Dec. 13 (Wed.): This is St. Lucia’s Day in Sweden, when the eldest daughter of the household, wearing a white gown and a wreath with four lit candles in her hair, serves sweet buns to other members of her family. In English-speaking countries, this is St. Lucy’s Day. Before the more accurate Gregorian Calendar was adopted in 1582, Dec. 13 was the day of the winter solstice according to the older Julian Calendar. Today is also Hanukkah, the beginning of the Jewish eight-day feast celebrating the victory of the Maccabees over the Syrian monarch Antiochus IV Epiphanes in 165 B.C. Hanukkah is celebrated on the 25th day of the month Kislev, but since the Jewish calendar is lunar, not solar, the date of that feast moves from year to year.

Dec. 14 (Thurs.): The Geminid meteor shower takes place between Dec. 4 and 17, but its climax is expected to occur between midnight and 4 a.m. this morning. The waning crescent Moon, which rises at 4:25 a.m., won’t interfere much with the shower this year. The radiant (apparent point of origin) of the shower lies just to the upper right of the white star Castor (Alpha Geminorum, magnitude 1.58), the slightly fainter of the two brightest suns of the “Twins” of Gemini. The radiant will be highest in the southern sky at 2 a.m., when it will have climbed 76.1 degrees above the horizon. Although the Geminids appear to come from the radiant, they will fan out and develop trails as they move away from their point of origin, so be sure to glance around this area. If you live in a really dark rural area with little man-made lighting, you could see as many as 120 Geminids per hour; from light-polluted Jamestown, you might see 15-20 of the brightest specimens per hour.

The Geminids are composed of debris in the orbital path of the Apollo (Earth-crossing) asteroid 3200 Phaethon, which might be the rocky and inactive nucleus, less than 1¼ miles wide, of an extinct comet. An alternate theory maintains that Phaethon always was an asteroid, but that a collision with some other object knocked enough grain-sized debris (with an estimated density of 2 grams per cubic centimeter, much heavier than the usual cometary rubbish) from the former’s surface to produce the shower. About 65% of these meteors are white, 26% are yellow, and the remaining 9% are blue, orange, red or green; their bright colors are caused by their high metallic content. About 4% of Geminids leave trains in their wake. About 13% reach zero magnitude or brighter. They are medium-speed meteors, burning up within a second after entering Earth’s atmosphere at an average velocity of 21¾ miles per second. Around 1960 the Geminids replaced the August Perseids as the most active meteor shower, but fewer people see the former because of the frigid weather. As both the orbit of Phaethon and its particles pass near Jupiter, that giant planet might modify their path over time.

Dec. 18 (Mon.): New Moon takes place at 12:31 a.m., when it will be 62.6 degrees below our northern horizon, 252,524 miles from Earth, and 29 arc-minutes across. As it will be passing then 3½ degrees north of the Sun, there cannot be a solar eclipse this month. The Moon also reaches apogee, its farthest distance from the Earth this month, at 7:26 p.m., when the two bodies will be 252,651 miles apart.

Dec. 21 (Thurs.): Winter Solstice occurs at 10:28 a.m. when the Sun reaches its lowest point 23 13/30 degrees below the celestial equator. Today will be the shortest day of the year for Jamestown, with only 8 hours and 32 minutes of daylight. At local noon (actually 12:33 p.m.) the Sun will climb only 20 degrees above the southern horizon. Winter in Jamestown for 2017-18 will last 88 days, 23 hours, 45 minutes, and 36 seconds until the Sun crosses the celestial equator at the spring or vernal equinox on March 20th next year.

Dec. 24 (Sun.): Christmas Eve. What was the “Star of Bethlehem,” which was mentioned only in the Gospel of Matthew? Given the latitude and longitude of that city, it would have been impossible for any star, including a supernova (exploding star), to remain directly overhead all night long. Meteors would have burned up too quickly, and comets would have moved noticeably from night to night. The best candidate for the “Star” would have been a close conjunction of two or more of the brightest planets; the Magi were Zoroastrian (Persian) astrologers, for whom spectacular conjunctions in specific constellations heralded the birth of great kings. Familiar with Hebrew scripture, the Magi would have known that the Messiah was expected to be born in Bethlehem, then a small village; thereafter it would have been a simple matter to discover the only male infant born during such an impressive conjunction. Unfortunately, there were several major planetary conjunctions in the first decade B.C., and the identity of the bodies that constituted the “Star” is still under dispute.

Dec. 25 (Mon.): Christmas Day.

Dec. 26 (Tues.): First Quarter Moon transpires at 3:19 a.m., when it will be 27.8 degrees below our WNW-NW skyline, 239,617 miles away, and 30.72 arc-minutes across.

Dec. 30 (Sat.): This year’s final lunar occultation of the bright orange star Aldebaran (Alpha Tauri, magnitude 0.85) occurs today. At 5:15:56 p.m. the star will vanish behind the central dark eastern rim of the Moon, which will be then 19¼ degrees above the eastern horizon, 93.2% illumined, 32.72 arc-minutes in apparent span, and 222,964 miles away. The star will return to view on the lower bright western lunar limb at 6:12:54 p.m. Both events occur while twilight is still in progress, so use binoculars to observe Aldebaran’s disappearance; the Moon’s glare will make the star’s return harder to see. Such occultations were used in the past to prove that the Moon had a negligible or no atmosphere, since the star would have winked slowly out of view had any gases refracted its light. Aldebaran is the fiery “eye” of the V-shaped constellation of the Bull.

Dec. 31 (Sun., New Year’s Eve): At 7:20 a.m., an hour before sunrise and while nautical twilight is in progress, bright cream-colored Jupiter (mag. –1.8) will be 23.85 degrees above the SSE horizon, 32.99 seconds of arc in apparent diameter, 99½% lit, 504,963,830 miles from the Sun, and 554,388,435 miles from Earth. Only 3 degrees to its upper right will be Mars (mag. 1.49); the “Red Planet,” 20.65 times fainter than its giant neighbor, will be then 4.78 arc-minutes in span, 93.3% lighted, 151,648,105 miles from the Sun, and 182,165,495 miles from Earth. However, the Earth is catching up with Mars, and the two worlds will have a close encounter next year. By the end of the first week of January, Jupiter and Mars will be merely 0.2 degree apart, a conjunction that might remind one of the “Star of Bethlehem.” Using binoculars, look 6 1/6 degrees above the ESE-SE skyline for pinkish-yellow Mercury (mag. –0.3); at that time it will be 6.9 arc-seconds across, 59% illumined, 91,991,855 miles from Earth, and 35,959,025 miles from the Sun. Mercury is making a good appearance as a “morning star.

Have a blessed holiday season and a Happy New Year!


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