Sky at a Glance | December 20th, 2013

Pre-dawn view
If you're out before first light in this dark time of the year, watch the waning Moon passing under the springtime Sickle of Leo, now showing in a pre-dawn preview.

Friday, December 20

  • At this cold time of year the Big Dipper, highest in the warm late spring, may be the last thing on your mind. But look low in the north at nightfall and there it is, resting upright on or near the horizon — if you're not too far south!

  • With a telescope this evening, watch Jupiter's moon Io disappear into eclipse in Jupiter's shadow at 9:15 p.m. Eastern Standard Time. Io will fade away just off Jupiter's western limb. Forty-five minutes later, Ganymede emerges across the western limb onto dark sky. Then around 11:23 p.m. EST, Jupiter's Great Red Spot (strong orange this season) transits the planet's central meridian.

    To find all such events for every time zone worldwide, see "Action at Jupiter" in the December Sky & Telescope, pages 51 and 52.

    Saturday, December 21

  • Today is the shortest day of the year in the Northern Hemisphere; the longest day in the Southern Hemisphere. Winter in the north, and summer in the south, begin at the solstice: 12:11 p.m. Eastern Standard Time. This is when the Sun reaches its farthest point south for the year and begins its six-month northward return. Happy Yule.

  • If there's one constellation that everyone should know at this time of year, it's wintry Orion climbing up in the east-southeast. As always when Orion is on the rise, the three-star Belt of Orion in its middle is nearly vertical. Show someone Orion this week and they'll know it for a lifetime.

    Sunday, December 22

  • One of the nearest galaxies is dusty NGC 253 in Sculptor. Fewer deep-sky delvers know this narrow spindle, magnitude 7.8, than ought to — no doubt because it's officially in a dim, obscure southern constellation. But it's actually an easy star-hop just 7° south of 2nd-magnitude Beta Ceti, as shown in Gary Seronik's Binocular Highlight in the December Sky & Telescope, page 45.

    Monday, December 23

  • With fall gone and winter here, the Great Square of Pegasus is again tipped up onto one corner after dinnertime, now descending on the western side of the sky. The main line of Andromeda's stars extends up from its top corner.

    Tuesday, December 24

  • The last-quarter Moon shines near Mars after they rise late tonight and into the Christmas-morning dawn.

    Wednesday, December 25

  • A Christmas deco star. At this time every year Sirius rises in the east-southeast down below Orion around 7 or 8 p.m. (depending on your location). When Sirius is still low, binoculars often show it twinkling in vivid colors. All stars do this when low, but Sirius is the brightest, making the effect more pronounced.

  • Merry Sol Invictus! In late Roman times when the solstice fell on December 25th, this date was celebrated as Dies Natalis Solis Invicti, the Birthday of the Unconquered Sun — when the Sun began to reverse its long decline with the hopeful promise, in the cold and the dark, of a new spring and summer to come.

    Thursday, December 26

  • Sirius and Procyon in the balance. Sirius sparkles low in the east-southeast after dinnertime. Procyon shines in the east about two fist-widths at arm's length to Sirius's left. If you live around north latitude 30° (Tijuana, New Orleans, Jacksonville), these Big and Little Dog Stars are at the same height above your horizon soon after they rise. If you're north of that latitude, Procyon will be higher. If you're south of there, Sirius leads.

    Friday, December 27

  • Yes it's cold out. That's no reason to be cold or to shun the beauties of winter astronomy. Learn to be a toasty winter observer with Tony Flanders' "Dress for Stargazing Success" in the December Sky & Telescope, page 66.

    Dawn view
    The waning crescent Moon now passes Alpha Librae, Saturn, and Antares in the eastern dawn.
  • In Saturday's cold dawn, Alpha Librae (Zubenelgenubi) shines 1° to 3° lower left of the waning crescent Moon (for North America), with Saturn 5° farther onward, as shown here. The Moon occults (covers) Alpha Librae for Hawaii (timetable).

    Saturday, December 28

  • Very late tonight and tomorrow night, little Mars is passing 3/4° south of Gamma Virginis (Porrima), magnitude 2.7, a famous close double star for telescopes at high power. They rise together after midnight but are best observed before dawn.


    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. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; 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, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.

    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 (able to point with better than 0.2° repeatability, which means heavy and expensive). As Terence Dickinson and Alan Dyer say in their invaluable 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. 20, 2013.
    Jupiter on December 20th, imaged by Christopher Go with a Celestron-14 scope in the Philippines. South here is up. The Great Red Spot is rotating into view at the following (right-hand) limb. Notice the tumultuous interface between the Equatorial Zone and the North Equatorial Belt. Also notice the thin, very dark red barge in the North North Temperate Belt near the central meridian. The reddish oval just north of its following end is the North North Temperate Zone Little Red Spot (NNTZ LRS). The System II longitude on the central meridian at the time of the picture was 148°.

    Mercury is hidden in the glare of the Sun.

    Venus (magnitude –4.8) shines brightly in the southwest during dusk. It's moving lower rapidly now, as it wanes to a thinner crescent: from 14% sunlit on December 20th to just 7% lit on the 27th. During this time the crescent enlarges from 51 to 57 arcseconds from horn to horn. The best time to observe it in a telescope is late afternoon, before sunset. Here's our article See Venus's Thin Crescent.

    Mars (magnitude 1.0, in the head of Virgo) rises around midnight or 1 a.m. By dawn it's at its highest in the south, but in a telescope Mars is still very small, only 6.4 arcseconds in diameter. Can you see its gibbous shape?

    Jupiter (magnitude –2.7, in Gemini) rises in the east-northeast around the end of twilight, with Pollux and Castor to its left. It blazes highest around 1 a.m. In a telescope Jupiter has grown to a big 46 or 47 arcseconds wide as it nears its January 5th opposition. Read all about observing Jupiter in the January 2014 Sky & Telescope.

    Saturn (magnitude +0.6, in Libra) is well up in the southeast as dawn begins to brighten. Look for it far to the lower left of Mars and Spica, and far below brighter Arcturus.

    Uranus (magnitude 5.8, in Pisces) is high in the south right after dark.

    Neptune (magnitude 7.9, in Aquarius) is moving lower in the southwest now after dark. 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.


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