CVIndependent

Mon12102018

Last updateTue, 18 Sep 2018 1pm

30 Nov 2018

December Astronomy: Venus Amazes in the Morning Twilight, While the Bright Winter Stars Debut at Night

Written by 

In December 2018, Venus is an outstanding predawn sight for the unaided eye, as well as binoculars and telescopes.

Who would fail to wonder at the sight of brilliant Venus near a predawn crescent moon? Separated in time by about a month, Venus-moon pairings on Dec. 3 and Jan. 1 provide easy chances to locate Venus in the daytime with the unaided eye. Telescopic views reveal Venus’ changing phase—28 and 48 percent on those respective dates—coupled with a shrinking apparent size as the planet recedes from Earth. When Venus appears half-full just a few days into the new year, it will be “rounding the bend,” moving from the near side into the far side of its nearly circular orbit around the sun, as Venus’ orbit is viewed nearly edge-on from planet Earth.

Venus, about 25 percent full in early December, gleams at magnitude -4.9, as bright as it ever gets. Rising in a dark sky more than three hours before sunup, it’s truly very impressive!

At dawn: On Saturday, Dec. 2, an hour before sunrise, Venus appears 17 degrees to the lower left of a waning crescent moon, and 7 degrees to the lower left of blue-white first-magnitude star Spica. On Monday, Dec. 3, Venus will appear only 4 degrees to the lower right of the moon. To catch Venus at its best, I am leading sky-watching sessions in Palm Springs from 5 to 5:45 a.m. on both those mornings, if sky is clear, on the pedestrians-only bridge over Tahquitz Creek, at Camino Real between North and South Riverside drives. Parking is available on Camino Real, both north and south of the bridge.

On Dec. 4, the moon has moved 10 degrees to the lower left of Venus, and Mercury is visible in binoculars 17 degrees to the lower left of Venus. On Dec. 5, the last old crescent moon easy to see with the unaided eye is 22 degrees to the lower left of Venus, with brightening Mercury just 4 degrees below the thin moon. Mercury then begins a nearly month-long appearance low in the east-southeast to southeast, to the lower left of Venus.

On Dec. 6, if skies are very clear just 30 minutes before sunrise, if you use binoculars and have an unobstructed view toward the east-southeast, you’ll have a chance to spot a very thin, old crescent moon only 17 hours before new, rising 35 degrees to the lower left of Venus and 10 degrees to the lower left of Mercury. We might even spot Jupiter just 3 degrees to the lower right of the hyper-thin moon. I have selected a site in a residential area on the high slopes of northwestern Palm Springs with an excellent view of these rare events on the morning of Dec. 6; watch the Independent’s social media for an announcement of the location if the weather allows for it.

From Dec. 7-21, the moon is absent from the morning twilight sky. Within a few days, Jupiter becomes easy for unaided eye, staying 9 degrees to the lower left of Mercury through Dec. 11 as both rise higher daily. For several mornings, the arrangement of three planets from upper right to lower left is Venus-Mercury-Jupiter. But Mercury, entering the far side of its orbit beyond the sun, passes just 0.9 degrees to the upper left of Jupiter on Dec. 21—look for the striking pair 26 degrees to the lower left of Venus. Thereafter, until Mercury sinks into the sun’s glare in early January, the order of the three planets is Venus-Jupiter-Mercury.

The moon returns to the morning sky, fully illuminated, low in the west-northwest in dawn mid-twilight on Dec. 22. The waning gibbous moon will pass 8-12 degrees to the left of the Twins, Pollux and Castor, on Dec. 24, and 3 degrees to the right of Regulus, heart of Leo, on Dec. 26. A fat crescent moon, just past last quarter (half-full) phase, will appear within 7 degrees to the upper left of Spica in Virgo on Dec. 30. The waning crescent moon will appear within 4 degrees to the upper right of Venus on New Year’s morning, and slide downward past Antares, Jupiter and Mercury over the next three mornings.

At dusk: Mars is high in the sky, halfway from horizon to overhead, in the southern sky in December. Although fading slowly from magnitude 0 to +0.4, Mars shouldn’t be confused with any star this month, while the red planet passes through the background of Aquarius and Pisces, which include no stars brighter than magnitude 3. Saturn can be glimpsed very low in the southwest to west-southwest in early December, before its conjunction with the sun on Jan. 1.

In early December, the Summer Triangle of Vega, Altair and Deneb is still well up in the western sky. Fomalhaut, mouth of the Southern Fish, is in the south. The earliest harbinger stars of winter—Capella in the northeast and Aldebaran lower in the east-northeast—have arrived. Aldebaran, eye of Taurus and “Follower” of the Pleiades star cluster, is at opposition to the sun on Dec. 1 and visible from dusk to dawn.

The waxing moon appears at dusk as a thin crescent very low in the southwest to west-southwest, 3 degrees to the lower right of Saturn on Dec. 8, and 9 degrees to the upper left of Saturn on the next evening. The moon appears almost half-full, 4 degrees to the lower left of Mars, high in south on Dec. 14, and almost full, 3 degrees to the upper right of Aldebaran in the east, on Dec. 20. On Dec. 22, the full moon appears very low in the east-northeast at dusk, opposite to the sun’s direction below the west-southwest horizon. By then, Orion’s Betelgeuse and Rigel have appeared above the eastern horizon. The rest of early winter’s bright stars soon follow. Watch for their risings a little later in the evening in December, or in twilight by mid-January, in this order: Castor, Pollux, Procyon and finally Sirius, the brightest nighttime star. Sirius completes the nearly equilateral Winter Triangle with Procyon and Betelgeuse. As a check, Orion’s belt points downward toward Sirius rising in the east-southeast. When Sirius first appears, Altair is still visible, low, just north of west (if mountains don’t block your view)—and both the Summer and Winter Triangles can be seen simultaneously.

After nightfall: Here’s an easy chance to see our solar system’s two outermost planets. At the end of evening twilight, as soon as the sky becomes fully dark, 5.7-magnitude Uranus is visible in binoculars, closely north-northeast to north of 4.3-magnitide Omicron in Pisces. Point your telescope at Mars, and you’ll find eighth-magnitude Neptune in the same low-power field for four evenings: within 1 degree east-northeast of Mars on Dec. 5; one-third of a degree east-northeast of Mars on Dec. 6; one-third of a degree west-southwest of Mars on Dec. 7; and 1 degree west-southwest of Mars on Dec. 8. To distinguish Uranus and Neptune from faint stars, use these finder charts provided by Sky and Telescope magazine: www.skyandtelescope.com/observing/ice-giants-neptune-and-uranus.

Star parties: The website of the Astronomical Society of the Desert at www.astrorx.org has a listing of our evening star parties at two locations. Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will have a star party starting at dusk on Saturday, Dec. 8. Our primary, more-accessible star-party site is the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument, on Highway 74, within 4 miles south of Highway 111 in Palm Desert. Our next monthly star party there is scheduled for Saturday, Dec. 15, from 5 to 8 p.m. Star parties can be cancelled in poor observing weather. Also, check the Impromptu Star Parties link. I’ll be offering sky watches in Palm Springs at the pedestrian bridge over Tahquitz Creek at North Riverside Drive and Camino Real.

Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. Subscriptions are $12 per year at www.abramsplanetarium.org/skycalendar for three printed issues mailed quarterly. The January 2019 issue of the calendar and this column in Coachella Valley Independent will preview the total lunar eclipse happening after nightfall on Jan. 20, and the four bright planets in the morning sky.

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.

Leave a comment

Make sure you enter all the required information, indicated by an asterisk (*). HTML code is not allowed.