This Week’s Sky at a Glance, December 4 – 12

Comet Catalina (C/2013 US10) is running about a magnitude fainter than predicted now that it's emerging into view low in the east at dawn. So it's on track to glow at 6th magnitude, not 5th, as it climbs high into the predawn eastern sky through December and January. See the December Sky & Telescope, page 45, or Get a Predawn Peek of Comet Catalina. With finder chart.

Friday, December 4

• The big Summer Triangle remains in the western sky after dark these cold evenings. Its brightest star is Vega, the brightest in the whole area. Look above Vega for Deneb. Farther to Vega's left or lower left is Altair.

• Before and during dawn on Saturday the 5th, bright Venus in the southeast anchors a diagonal line that stretches upper right past Spica to connect Mars, the waning Moon, and then Jupiter.


The thinning crescent Moon in the dawn will pair with Venus on the 7th, before occulting Venus in broad daylight that day for much of North America. (The Moon in these diagrams is drawn three times its actual apparent size.)

Big Dipper under the dim Little Dipper at dusk in December

The farther north you are, the higher above the northern horizon you'll find the Big Dipper at nightfall. This scene is drawn for north latitude 40° (New York, Denver, Madrid).

Saturday, December 5

• In early dawn on Sunday morning the 6th, the waning crescent Moon hangs roughly between Mars and Spica, as shown at right.

Sunday, December 6

• By about 10 p.m. now (depending in how far east or west you live in your time zone), the dim Little Dipper hangs straight down from Polaris.

• Before and during dawn tomorrow morning the 7th, Venus and the crescent Moon pair beautifully close together in the southeast, roughly as shown here. And read on....

Monday, December 7

The Moon occults Venus in broad daylight for essentially all of North and Central America. The Moon is a thin waning crescent about 43° west of the Sun. Venus will disappear behind its sunlit limb, which has a much dimmer surface brightness than Venus does. Venus will reappear from behind the Moon's dark limb up to an hour or more later.

The occultation happens in the middle of the day for the East, and in the morning for the West. In Alaska it happens during dawn. See the December Sky & Telescope, page 46, for timetables.

Tuesday, December 8

• Earliest sunset of the year (for 40° north latitude).

Two faint fuzzies naked-eye: The Andromeda Galaxy and the Perseus Double Cluster are two of the most famous deep-sky objects. They're both cataloged as 4th magnitude, and in a fairly good sky you can see each with the unaided eye. They're only 22° apart, high in the northeast or overhead these evenings.

But they look rather different, the more so the darker your sky. See for yourself. They're plotted on the all-sky constellation map in the center of the November Sky & Telescope.

Wednesday, December 9

• How well do you think you know the deep-sky scenery in Cassiopeia? Explore some telescopic sights you may never have heard of with Sue French's Deep-Sky Wonders article, photos and charts in the December Sky & Telescope, page 50.

For starters, Iota Cas (the star you hit when you extend the fainter end of Cassiopeia's W pattern) is a nice triple star at 100× in any telescope with a 3-inch aperture or larger.

Thursday, December 10

• Fomalhaut, the Mouth of the Southern Fish, shines due south as twilight grows late. Therefore, so does the right-hand side of the Great Square of Pegasus — much higher, or even overhead of you're as far south as Florida.

Friday, December 11

• Sirius is, famously, the brightest star in the night sky. Have you ever tried to catch Sirius actually rising? If you can find a good view down to the east-southeast horizon, watch for Sirius emerging into view. It's about two fists at arm's length below Orion's Belt. Sirius rises around 7:30 to 8:30 p.m. now, depending on where you live.

When a star is very low, it tends to twinkle quite slowly and often in vivid colors. Sirius is bright enough to show these effects especially well.

Saturday, December 12

• The Geminid Meteor shower is already active. It will reach its peak late on Sunday and Monday nights, December 13–14 and 14–15. The sky will be moonless. See the December Sky & Telescope, page 44.


Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or new jumbo edition), which shows stars to magnitude 7.6. Next up is the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5), and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet Roundup

Jupiter on Dec. 9, 2015

Jupiter's Great Red Spot has become strikingly brick red this season, making it easier to detect in a telescope than it has been for many years. Christopher Go in the Philippines shot this image on December 9th at 20:07 UT, when the GRS had just crossed the central meridian. The System II longitude on the central meridian was 240°. South here is up. Note the white turbulence downwind of the Red Spot (to the right; celestial east; following), the long-lived small white ovals in the South Temperate Belt south of it, and the continued dark-red narrowness of the North Equatorial Belt. Blue festoons extend from the NEB into the white Equatorial Zone.

Mercury is still very deep in glow of sunset, but it's on its way to a fine apparition later in December.

Venus, Mars, and Jupiter continue their display in the east before and during dawn. But they've drawn far apart; low Venus and high Jupiter are now separated by more than 40°. Fainter Mars is between them, closer to Venus. As for brightness, Venus shines at magnitude –4.2. Jupiter is –2.0, and Mars is +1.5.

Saturn is hidden deep in the sunrise.

Uranus (magnitude +5.8, in Pisces) and Neptune (magnitude +7.9, in Aquarius) are high in the southern sky early in the evening. Finder charts for Uranus and Neptune.


All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.


"Imagination will often carry us to worlds that never were. But without it, we go nowhere."

— Carl Sagan


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