What’s in the southern sky this month?

You don’t want to miss Jupiter this month. The giant planet reaches opposition and peak visibility December 7 against the stunning backdrop of Taurus the Bull. Shining at magnitude –2.8, Jupiter dominates the northeastern sky once darkness falls. Although the gas giant lies well north of the celestial equator — less than optimal for us in the Southern Hemisphere — it remains an attractive sight all night.

The best views through a telescope come when it climbs highest in the north around midnight local time. Be patient for moments of good seeing when indistinct features snap into focus. Even the smallest scopes show detail in the bright zones and darker belts that run parallel to the planet’s 48″-diameter equator. And keep an eye out for Jupiter’s four Galilean moons, which shift position from hour to hour and night to night.

As Jupiter climbs higher in the northeast, Venus dips lower in the west. The brightest planet gleams at magnitude –4.3 in mid-December, making it four times brighter than Jupiter. It slowly pulls away from the Sun from night to night. Venus begins December in Sagittarius, enters Capricornus a week later, and crosses into Aquarius at month’s end.

A telescope shows no detail in Venus’ thick cloud cover, but a careful eye reveals subtle changes to the planet’s size and shape. During December, Venus’ diameter grows from 17″ to 22″ while its phase wanes from 68 to 56 percent lit.

As Venus draws away from the Sun, Saturn steadily approaches our star. The 1st-magnitude ringed planet remains a fine sight to Venus’ upper right on December evenings. You can find it among the faint background stars of Aquarius the Water-bearer.

Great views of Saturn through a telescope come when it lies high in the west soon after darkness falls. Expect to see a disk that measures 17″ across surrounded by a ring system that spans 39″ and tilts 5° to our line of sight. Now’s the time to grab a final view of the rings before they turn edge-on to us in March. And be sure to watch for 8th-magnitude Titan, Saturn’s largest moon, as well as the 10th-magnitude trio of Tethys, Dione, and Rhea.

Around the time Venus sets, Mars pokes above the northeastern horizon. The Red Planet’s appearance improves dramatically this month. It nearly doubles in brightness, climbing from magnitude –0.5 to magnitude –1.2. You can find it against the dim backdrop of Cancer the Crab, where it begins its retrograde loop (moving west) December 7. It then lies just 2.2° north-northwest of the bright Beehive star cluster (M44).

Mars’ apparent diameter also swells during December, growing from 11.6″ to 14.2″. During periods of steady seeing, you should be able to see some of its dark surface markings.

Our final planet makes a brief appearance before dawn in late December. Mercury reaches greatest elongation on the 25th, when it lies 22° west of the Sun and stands 7° high in the east-southeast 45 minutes before sunrise. That morning, a telescope shows a disk that spans 6.6″ and appears 63 percent illuminated.

The starry sky

On any December evening, look about halfway up the southeastern sky and you can’t miss Canopus. This –0.7 magnitude luminary of Carina the Keel ranks as the second-brightest star in the night sky.

But I want you to focus your gaze this month on the faint constellation Pictor, which lies just above Canopus. It is one of 14 created and named by French astronomer Nicolas Louis de Lacaille (1713–1762) during his visit to the Cape of Good Hope in the mid-18th century. Today we call it Pictor, the Latin word for painter, but Lacaille used the name le Chevalet et la Palette, meaning “the easel and the palette.”

Lacaille later changed the name to Equuleus Pictorius, which eventually led to the shortened version we use today. So, over the years, the constellation representing the painter’s equipment came to represent the actual artist.

Observationally, Pictor does not get much respect. Its brightest star, Alpha (α) Pictoris, glows dimly at magnitude 3.3. Yet Pictor has been the site of several significant discoveries.

In the late 1890s, Jacobus Cornelius Kapteyn (1851–1922) found that one of its stars has a proper motion of 8.7″ per year, pushing Groombridge 1830 in Ursa Major into second place. The discovery of Barnard’s star and its proper motion of 10.4″ per year relegated Kapteyn’s star to its current position in second place.

In 1925, a bright nova designated Nova Pictoris 1925 or RR Pictoris rose to prominence, reaching magnitude 1.2 in early June. After a drop to around 4th magnitude, it rose again to near its earlier peak before irregularly fading below naked-eye visibility in 1926.

Then in 1983, the Infrared Astronomical Satellite discovered an excess of infrared radiation around Beta (β) Pic and a few other stars. The Hubble Space Telescope later imaged Beta’s dusty disk. It was the first time scientists had seen this signature of a planetary system in formation. It turns out Beta is only about 20 million years old.

Alas, there’s not many easy deep-sky objects in Pictor. The most impressive is the double star Iota (ι) Pic, composed of a magnitude 5.6 primary and magnitude 6.4 secondary separated by an easy 12.3″.�

Star Dome

The map below portrays the sky as seen near 30° south latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.

The all-sky map shows how the sky looks at:
11 P.M. December 1
10 P.M. December 15
9 P.M. December 31

Planets are shown at midmonth