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29 Nov 2017

December Astronomy: The Month Brings a 'Supermoon' and Potentially Great Conditions for the Geminids Meteor Shower

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Evenings during twilight in early December 2017 feature a half-dozen stars of first magnitude or brighter, including the Summer Triangle of Vega, Altair and Deneb well up in the west, getting lower as month progresses; Fomalhaut, mouth of the Southern Fish, in the south; and Capella, the Mother Goat star, ascending in the northeast, with red-orange Aldebaran, eye of Taurus, to its lower right.

Binoculars and an unobstructed view are needed to spot Saturn, with Mercury close to its lower left, very low in the southwestern twilight glow 2.8 to 2.3 degrees apart Dec. 1-3, some 40 minutes after sunset. But both sink lower each evening, with Mercury fading to the equal of Saturn by Dec. 3, and fading rapidly thereafter. Mercury passes inferior conjunction, nearly between Earth and the sun, on Dec. 12, and Saturn hides in conjunction on the far side of the sun on Dec. 21.

In evening twilight late in month—or later in the evening in early December—watch the eastern horizon for the rising of reddish Betelgeuse and blue-white Rigel. About midway between them, Orion’s vertical three-star belt confirms their identity as Hunter’s shoulder and foot.

Our morning chart (below) depicts the sky about 45 minutes before sunrise, but we advise you to begin viewing at least a half-hour earlier than that, to allow time to locate all the bright objects before they are drowned out in the brightening twilight. Steady Jupiter in the southeast is the most prominent object during the half-hour beginning 75 minutes before sunup. (Venus doesn’t rise until 45 minutes before sunrise on Dec. 1; 30 minutes before on Dec. 12; and only 15 minutes before sunrise on Dec. 23.) Binoculars are increasingly useful for spotting Venus before sunup on the last possible date before superior conjunction, beyond the sun on Jan. 9.

Next in prominence after Jupiter are twinkling blue-white Sirius in the southwest to west-southwest, until it sinks from view late in the month; golden Arcturus high in the east; blue-white Vega rising higher in the northeast; and Capella sinking in the northwest. Mercury, very low in the east-southeast to southeast, attains magnitude +1 by Dec. 20, and magnitude 0 by Dec. 24, and for the rest of month ranks second in brilliance, after only Jupiter, since Sirius has departed.

Other bright morning objects include the Winter Hexagon, which has Sirius and Capella marking its southern and northern vertices, and red Betelgeuse inside. But Rigel, Aldebaran, Sirius and Betelgeuse exit the morning sky in December, leaving only the upper arch of Procyon, Pollux (with Castor 4.5 degrees to its right, not shown) and Capella.

Regulus, heart of Leo, is high in the southwest to west-southwest following the Hex across the sky. I like to imagine Leo chasing his menu, which includes some beef (Aldebaran in Taurus), two dogs (Sirius and Procyon in Canis Major and Canis Minor), a mother goat (Capella in Auriga) and even some human fare (Rigel and Betelgeuse of Orion, the Hunter, becoming the Hunted; and Pollux and Castor, of Gemini, the Twins).

Following Regulus across the sky are Arcturus and Spica. The latter star marks the ear of grain in the hand of Virgo, the next zodiac constellation east of Leo. The zodiac constellations serve as hosts to the bright objects of the solar system, namely the sun, moon and planets.

On Dec. 1, dim red Mars appears just 3 degrees from Spica and 16 degrees to the upper right of Jupiter. Jupiter itself is 4 degrees west-northwest of the third-magnitude star Alpha in Libra on Dec. 1; within 1 degree of that star Dec. 18-26; and as close as 0.7 degrees from Alpha Lib on Dec. 21-23. On Dec. 31, Mars is 3 degrees to the upper right of Jupiter, and 20 degrees to the lower left of Spica. On the last morning of 2017, Alpha Lib lies nearly midway between the planets, 1.5 degrees from Mars and 1.7 degrees from Jupiter—a beautiful sight for binoculars! The star’s Arabic name, Zubenelgenubi, meaning “Southern Claw,” refers to its assignment to a larger early version of the Scorpion, the next zodiacal constellation east of Libra. The pairing of Jupiter with Alpha Lib on Dec. 22 is the first of a triple conjunction between them. Jupiter, retrograding, will pass closely north of the star on June 3, 2018, and then, after resuming direct (eastward) motion, will pass closely north of it a third time, on Aug. 15.

Mercury, pulling away from its inferior conjunction of Dec. 12, brightens rapidly in the morning sky and holds a steady 28 degrees to the lower left of Jupiter Dec. 21-27. Look 8 degrees to the upper left of Antares, heart of the Scorpion, Dec. 20-27. During Dec. 28-30, Mercury is 9 degrees to the left or lower left of Antares. On Dec. 31, Mercury is 31 degrees to the lower left of Jupiter and 10 degrees to the lower left of Antares. On New Year’s morning, Mercury reaches greatest elongation, 23 degrees from the sun. On the mornings of Jan. 6 and 7, Mars will appear only 0.3 degrees from Jupiter. Within a week later, on Jan. 13, Mercury will pass 0.8 degrees south of Saturn. Wouldn’t a good pair of binoculars to view these events make a great gift for a skywatcher?

December opens with a few brightly moonlit nights. On Saturday evening, Dec. 2, the moon rises 20 minutes before sunset and is nearly full. An hour after sunset, find the Pleiades 10 degrees to the moon’s upper left, and Aldebaran, whose name means “the Follower” (of the Pleiades), within 14 degrees below the Pleiades and 7 degrees to the moon’s lower left. The moon creeps closer to Aldebaran throughout that night. During morning twilight on Sunday, Dec. 3, use binoculars to spot Aldebaran very close to the moon’s upper left. A “supermoon,” the closest full moon of 2017, occurs at 7:47 a.m. that morning, just more than an hour after moonset in Palm Springs. The moon reaches its least distance from Earth just 17 hours later, at 12:46 a.m. on Dec. 4.

Moonrise early on Sunday evening, Dec. 3 (as seen from Palm Springs), occurs at 5:11 p.m. Although more than nine hours past full, the moon, rising 33 minutes after sunset, will be impressive. Note Aldebaran 8 degrees to the moon’s upper right.

After it’s full, moonrise occurs later each night: On Monday, Dec. 4 at 6:10 p.m.; on Dec. 5 at 7:13 p.m. (northernmost moonrise of the month); on Dec. 6 at 8:19 p.m.; on Dec. 7 at 9:25 p.m.; and on Dec. 8 at 10:30 p.m.

On Dec. 5, two weeks have elapsed since Nov. 21, when Earth passed between the sun and the Pleiades (Seven Sisters) star cluster. Starting on Dec. 5, the moon is well below the horizon at the end of evening twilight, nearly 1 1/2 hours after sunset, and the sky is then dark and moonless. Starting on the evening of Dec. 5, face east-northeast to east at dusk and watch the Pleiades emerge and ascend in the deepening twilight. The scene is well described in lines by Alfred Lord Tennyson.

After the full moon, you can follow its motion in the morning sky, averaging 13 degrees per day eastward through the constellations of the zodiac. Watch the waning gibbous moon pass 9 degrees south of Pollux on Dec. 6, and leapfrog over Regulus on Dec. 8 and 9. On Dec. 12, find the waning crescent moon 9 degrees above Spica. On Dec. 13, the moon is 4 degrees to the upper left of Mars and 9 degrees to the lower left of Spica.

The moon rises very late on the night of Dec. 13 (as a 13 percent crescent at 3:21 a.m. on Dec. 14), leaving the peak of the Geminid meteor shower, overnight on Dec. 13-14, excellent for viewing meteors. Best hours are from Wednesday at 9 p.m. until first light of dawn at 5:15 a.m. on Thursday, although a few long trails from meteors grazing Earth’s atmosphere might be seen anywhere in the sky as Castor, near the shower’s radiant, rises soon after 6 p.m.

On the morning of Dec. 14, perhaps after a few good hours of meteor-watching, look for the moon passing 4 degrees north of Jupiter. On Dec. 15, find the moon within 12 degrees to the lower left of Jupiter. On the 16th, the last easy old crescent moon will appear 23 degrees to the lower left of Jupiter. If you have a very good low horizon between east-southeast and southeast, use binoculars to find Antares rising 9 degrees to the lower right of the crescent moon in brightening morning twilight.

In case the sky is very clear just after 6 a.m. on the morning of Sunday, Dec. 17, we have a site selected in northwestern Palm Springs to attempt a sighting of a very old moon, just a little more than 16 hours before new. We’ll announce the details in the “Impromptu Star Parties” link mentioned below.

We’re still checking out sites in Desert Hot Springs and the eastern Coachella Valley to attempt a sighting of a very young moon shortly after 5 p.m. on Monday, Dec. 18. The moon’s age will be just more than 18.5 hours. As the date draws near, visit the same link for details. Twelve days later, on Saturday, Dec. 30, the daytime waxing gibbous moon rising at 2:53 p.m. is covering Aldebaran. Telescopes may show the star emerging along the moon’s bright edge at 3:51 p.m.

The website of the Astronomical Society of the Desert, at www.astrorx.org, has a listing of our evening star parties at two locations. The primary, more-accessible location is at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument (on Highway 74, within four miles south of Highway 111 in Palm Desert). A session is scheduled there on Saturday, Dec. 23, from 5-8 p.m. Sawmill Trailhead, our high-altitude site (elevation 4,000 feet—wear warm clothes), will have a star party starting at dusk on Saturday, Dec. 9. The society’s website includes maps and directions for both locations. Star parties can be cancelled in poor observing weather. See the link to “Impromptu Star Parties” on that website.

On Friday, Jan. 5, I will present a summary of the coming year’s sky events. The summer and early autumn of 2018 will be outstanding for viewing planets in the evening sky, and there will be total lunar eclipses in January 2018 and January 2019. The talk will be held at the Portola Community Center, 45480 Portola Ave., in Palm Desert. Doors open at 6:30 p.m. for refreshments, and the talk begins at 7 p.m.

Also, check out the Sky’s the Limit Observatory and Nature Center in Twentynine Palms, at skysthelimit29.org.

Wishing you clear skies!

Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky watching opportunities for a variety of groups in the Coachella Valley. Robert D. Miller did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

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