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28 Nov 2019

December Astronomy: Early in the Month, Venus Dances With Saturn; Later in the Month, It Rendezvouses With the Moon

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Venus becomes ever more prominent in the evening, while Jupiter, and then Saturn, sink into the southwest twilight glow in December. Mars, slowly brightening, appears in the southeast morning sky, with bright Mercury below it in the first half of the month.

Do not miss Venus and Saturn within 5 degrees Dec. 7-14 (see right), with their closest pairing 1.8 degrees apart on Dec. 10; and the spectacular Venus-moon conjunction on Dec. 28.

Three planets span 18 degrees in the southwest at dusk in early December. On Dec. 1, find Venus, magnitude -3.9, with Jupiter, magnitude -1.8, just 8 degrees to the lower right, and Saturn, magnitude +0.6, 11 degrees to Venus’ upper left. Venus is now on the far side of its orbit, with its light taking 12 minutes to reach us, compared to the sun’s eight minutes. Light reflected from Jupiter and Saturn takes 51 and 90 minutes, respectively, to travel to Earth on Dec. 1.

As stars appear at dusk, find the Summer Triangle of Vega, Altair and Deneb dropping downward from high in the west; Fomalhaut, mouth of the Southern Fish, crossing through south; Capella, the mother goat star, ascending in the northeast; and Aldebaran, eye of Taurus, climbing in the east-northeast to east. In twilight before month’s end, Orion’s bright stars Rigel and Betelgeuse, and his three-star belt midway between them, appear in the east, followed by Gemini’s twin stars, Castor and Pollux, rising farther north along the horizon.

Note the first-magnitude star Aldebaran, eye of Taurus, low in the east at dusk on Dec. 1. Visualize: As Earth passes between Aldebaran and sun, the star is at opposition and up all night. The revolution of Earth and other planets around the sun is counterclockwise, from the viewpoint of an observer above the north side of our solar system. So Earth on Dec. 1 is moving directly away from a point 90 degrees east of the sun and 90 degrees west of Aldebaran; that’s 2 degrees west of the third-magnitude star Lambda in Aquarius, in the southern sky at nightfall. Inner planets move faster, so Venus, now on the far side of its orbit and gaining on us, continues to move farther out from last August’s place on far side of the sun, until reaching greatest elongation, 46 degrees east of the sun, on March 24, 2020. Venus will overtake us 10 weeks later, when it passes inferior conjunction, nearly between the Earth and the sun, on June 3.

We’re leaving Jupiter and Saturn behind. Earth’s faster revolution around the sun will cause Jupiter to pass behind the sun on Dec. 27, and Saturn to do so on Jan. 13. During this month, before these solar conjunctions, when will you last spot Jupiter low in the southwest at dusk? When will you last see Saturn? Pick a spot where nearby mountains won’t block your view. Using binoculars can help you delay these planets’ departures by a few days.

In morning sky, on Dec. 2, Earth is heading toward Leo, 10 degrees east of Regulus. Speedy Mercury, a few days past greatest elongation on Nov. 28, is leaving us behind and heading toward superior conjunction on the far side of the sun, on Jan. 10. When will you last see Mercury before then? Mars begins December at faint magnitude +1.7. We’re gaining on the red planet, so watch it brighten until its closest approach and opposition in October 2020, when it will gleam at magnitude -2.6.

Follow the moon and planets: The earliest sunsets of the year in December provide a convenient opportunity for families to enjoy the night sky, and this year, the planets provide much to see. In the southwest, the brightest planet, Venus, is easy to spot by half an hour after sunset. In early December, find bright Jupiter to Venus’ lower right, and Saturn to Venus’ upper left. In December’s first week, Jupiter and Saturn are 18 degrees apart, with brilliant Venus between them, while the waxing moon moves farther from the planets nightly. Each evening in early December, Venus moves farther from Jupiter and closer to Saturn. On Dec. 2, foreground Venus appears nearly midway between the distant giants. On Dec. 3, the moon reaches first-quarter phase, when it appears half-full. On the evenings of Dec. 10 and 11, Venus will pass less than 2 degrees to the lower left of Saturn. Can you still spot Jupiter those evenings, before it sinks into even brighter twilight? Find Jupiter 17 degrees to the lower right of the Venus-Saturn pair on Dec. 10.

By then, the moon, shifting its place eastward by an average of 13 degrees per day against background stars, has moved into the constellation Taurus, the Bull. On Tuesday, Dec. 10, note the first-magnitude star Aldebaran, eye of the Bull and follower of the Pleiades, to the lower left of the moon. On the next evening, find Aldebaran to the upper right of the full moon. Continue to follow the moon for several more evenings after Dec. 11—if you’re OK with staying up later each night.

Alternatively, early risers can shift their moon-watching time to mornings, one hour before sunrise. At that time on Dec. 11, the moon and Aldebaran are within 3 degrees and about to set in the west-northwest, while Orion is setting in west, and Sirius, the brightest star, is low in the west-southwest. Mercury, just risen in the east-southeast, is in the head of Scorpius on the morning of Dec. 11, while the moon is just above the head of Taurus—opposite to Scorpius in the zodiac, the belt of constellations where the sun, moon and planets are always found. Look in the southeast for faint Mars, 18 degrees to the upper right of Mercury on Dec. 11, and for a first magnitude star, Spica, nearly 21 degrees to the upper right of Mars. Spica is in Virgo, another zodiac constellation. Note that Mercury, Mars and Spica are in a nearly straight line.

Mornings the next two weeks, watch the waning moon pass several bright stars and planets in the zodiacal band: Pollux and Castor of Gemini on Dec. 14; Regulus, heart of Leo, on Dec. 17; Spica in Virgo on Dec. 20 and 21; Mars in Libra on Dec. 22 and 23; and Antares in Scorpius on Dec. 24. By then, Mercury has moved closer to the sun and may be too difficult to observe in bright twilight.

The new moon is invisible as it passes the sun late on Christmas Day. At dusk on Dec. 27, the young crescent can be seen low in the southwest to the lower right of Venus. Using binoculars, can you spot Saturn within 7 degrees to the lower right of the moon?

Whatever you plan for Saturday, Dec. 28, be sure to include time for views of the southwest sky at dusk! At sunset, Venus will be within 2 degrees above and slightly to the right of the moon. Can you spot Venus before sunset? By 40 minutes after sunset, the view of Venus just to the upper right of the crescent moon will be spectacular! Keep watch as the moon and Venus sink toward the horizon. They’ll be little more than one degree apart as they set.

For a preview of sky events through August 2020, with monthly all-sky charts for dusk and dawn, visit the Sky Calendar extra content page at www.abramsplanetarium.org/msta.

Check the website of the Astronomical Society of the Desert at www.astrorx.org, and come to our free evening star parties, offered monthly at two locations.Our primary, more accessible venue is 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. Our next session there will be on Saturday, Dec. 21, from 5 to 8 p.m.Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will next host a session on Saturday, Dec. 28, starting at dusk.

At various locations, some members of the Astronomical Society of the Desert will host observing sessions for special events, including gatherings of moon and planets. These might be announced on short notice, so periodically check the link to Impromptu Star Party Dates, on the society’s website.

You can also pre-register for one of the Stargazing Parties offered several times each month at the Rancho Mirage Library Observatory. Visit www.ranchomiragelibrary.org/observatory.html; click on Stargazing Parties, then sign up for their eNewsletter to receive registration information. If you go farther afield, check out the Sky’s the Limit Observatory and Nature Center in Twentynine Palms, at www.skysthelimit29.org.

Wishing you clear skies!

Robert C. Victor was staff astronomer at Abrams Planetarium at Michigan State University. Now retired, he often collaborates in producing new issues of the Sky Calendar he originated in 1968, and enjoys providing sky watching opportunities for a variety of groups. 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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