Venus dazzles at dusk

As twilight fades to darkness, three bright planets dominate the western sky. Venus pops into view first. Earth’s nearest neighbor shines brilliantly against the backdrop of Sagittarius the Archer. The planet appears brightest during December’s first week, when it gleams at magnitude –4.9 — outshining the night sky’s brightest star, Sirius, by 25 times. (Technically, this is the magnitude Venus would have if Earth had no atmosphere, but our blanket of air doesn’t dim this beacon much.)

Although it’s easy to admire Venus’ brilliance, the planet’s telescopic appearance provides an even greater thrill. As Venus draws closer to the Sun in our sky, its apparent size increases dramatically while its phase wanes to a thin crescent. On December 1, the world measures 39" in diameter and sports a 29-percent-lit phase. By the 15th, the planet has grown to 49" across but the Sun illuminates only 16 percent of its Earth-facing hemisphere. And if you can track Venus as it sinks into bright twilight at month’s end, you’ll see a disk that spans 60" and appears just 3 percent lit. Under good viewing conditions, you can even detect the planet’s phase through binoculars.

Saturn lies to the upper right of Venus in neighboring Capricornus the Sea Goat. The ringed planet shines at magnitude 0.6 and easily overshadows this constellation’s faint stars.

A telescope offers great views of the planet and its rings. In mid-December, Saturn’s disk appears 16" in diameter while the rings span 36" and tilt 18° to our line of sight. Any scope also reveals the planet’s brightest moon, 8th-magnitude Titan.

The third and loftiest member of our planetary trio is Jupiter. The giant world resides in far eastern Capricornus as December begins but crosses into Aquarius the Water-bearer at midmonth. Jupiter shines at magnitude –2.2 and stands out against this dim backdrop.

Relish spending time viewing Jupiter through a telescope this month. Any scope shows the giant planet’s 37"-diameter disk and banded atmosphere. Also look for Jupiter’s four bright satellites: Io, Europa, Ganymede, and Callisto. They stand out against the blackness of space near the planet unless one or more of them are passing in front of or behind the gas giant’s disk.

You might catch a brief glimpse of Mercury after sunset during December’s final days. The innermost planet passes 4° south (upper left) of Venus on the 29th and stands 6° above Venus on New Year’s Eve. Binoculars will provide the best views of magnitude –0.8 Mercury.

If you’re up before dawn, plan to enjoy Mars for the first time in several months. By late December, the Red Planet rises two hours before the Sun and hangs low in the east-southeast at twilight commences. Look for the magnitude 1.5 planet 5° to the lower left of Antares in Scorpius. The star shines a bit brighter than Mars and possesses a similarly ruddy color.

A total solar eclipse takes place December 4, though totality touches solid ground only in Antarctica. Observers in far southeastern Australia, the southern tip of New Zealand, and the southernmost parts of South America and Africa can witness a minor partial eclipse if they use safe Sun-viewing techniques. From Hobart, Tasmania, maximum eclipse occurs at 9h06m UT, when the Moon covers 21 percent of the Sun’s diameter and the two appear 4° above the horizon.

The starry sky

The inconspicuous constellation Fornax the Furnace passes nearly overhead from midsouthern latitudes these December evenings. Because it lies at declinations between –24° and –39°, it escapes the notice of many Northern Hemisphere observers.

Indeed, Fornax was one of the “new” constellations French astronomer Nicolas Louis de Lacaille introduced in 1752. On his 1756 planisphere, he called it le Fourneau (French for “the furnace”), and a few years later renamed it Fornax Chimiae the Chemical Furnace. Scientists eventually shortened it to Fornax, and we continue to use this name today.

Although Fornax holds many wonderful deep-sky objects, it is best known for its multitude of galaxies. The greatest concentration lies in the constellation’s southeastern corner, where you’ll find the Fornax galaxy cluster. This impressive group contains more than 50 galaxies and lies some 62 million light-years from Earth.

The brightest and easiest to spot is 9th-magnitude

NGC 1316, which shows up quite easily through a 7.5-centimeter telescope on a dark night. Unlike most galaxy clusters, in which the biggest member lies near the center of the group, NGC 1316 resides in the cluster’s outskirts.

The barred spiral NGC 1365 is another easy and attractive target. The bar in this 10thmagnitude island universe shows up nicely through 20-cm and larger scopes. NGC 1316 and NGC 1365 are just two of the many sights awaiting you in the Fornax Cluster.

Fornax also offers a couple of nice double stars. Point your scope at the constellation’s brightest star, Alpha (α) Fornacis, and you’ll see 4thand 7th-magnitude stars separated by about 3". The gap between these two has increased over the past few decades.

Scan 8° west of Alpha and you’ll land on Omega (ω) For. It’s easier to split these 5th- and 8th-magnitude suns because a more comfortable 11" separate the two.