Sky at a Glance | December 17th, 2010

The Moon takes three nights to pass from one end of the constellation Taurus to the other. (These scenes are drawn for the middle of North America. European observers: move each Moon symbol a quarter of the way toward the one for the previous date. For clarity, the Moon is shown three times its actual apparent size.)

Friday, December 17

  • This evening look lower left of the Moon, by a little more than a fist-width at arm's length, for the delicate Pleiades star cluster. (The Moon is still outside the scene at right.) Below the Pleiades by a roughly similar distance is orange Aldebaran. Far off to their left shines brighter Capella.

    Saturday, December 18

  • The Pleiades are only about 2° left or upper left of the gibbous Moon as seen from North America this evening, as shown at right. Use binoculars to penetrate the bright moonlight.

    Sunday, December 19

  • This evening the Moon, one night before its total eclipse when full, shines in the east with Aldebaran to its right or lower right and brighter Capella more than twice as far to its left.

    Monday, December 20

  • Total eclipse of the Moon late tonight, visible from all of North and Central America. The partial phase begins 1:33 a.m. Tuesday morning EST, total eclipse runs from 2:41 to 3:53 a.m. EST, partial eclipse ends at 5:01 a.m. EST. For our part of the world the Moon will be very high in the sky; in fact from the American Southwest and Southern California it will be near the zenith. The Moon will be centered between the bright constellations Orion, Auriga, Gemini, and Taurus. Read all about it: A Sky-High Lunar Eclipse. Includes a map of the eclipse's visibility worldwide.

    P.S.: Cloudy? Here's a webcast!

    Time-lapse of a lunar eclipse. Aligning his camera on the same star for nine exposures, Sky & Telescope contributing photographer Akira Fujii captured this record of the Moon’s progress dead center through the Earth’s shadow in July 2000. (This image is available as a print-quality JPEG (1.5 megabytes) for media use; see our press release.)
    Sky & Telescope / Akira Fujii

  • And, of course, the Moon is full (exactly so right around mid-eclipse).

    Tuesday, December 21

  • This is the longest night of the year in the Northern Hemisphere; winter begins at the solstice, 6:38 p.m. EST. Gather 'round the fire. In the Southern Hemisphere it's the shortest night of the year and the start of summer; dance in Midsummer's Night merriment.

    Wednesday, December 22

  • In early evening at this time of year, Cassiopeia floats as high in the north as it ever appears. It's loaded with telescopic sights to seek out. Some lesser-known ones are featured in Sue French's Deep-Sky Wonders column in the December Sky & Telescope, page 66.

    Thursday, December 23

  • Jupiter's largest and brightest moon, Ganymede, reappears out of eclipse from Jupiter's shadow around 8:08 p.m. EST. A small telescope will show it gradually swelling into view just east of the planet — right near Callisto and Io. Watch the pair become a triple!

    For a listing of all such Jupiter-satellite events this month, see the December Sky & Telescope, page 64.

    Friday, December 24

  • Christmas star: Brilliant Sirius, the brightest star in the night sky, rises around 7 or 8 p.m. (depending on where you live in your time zone). Orion's Belt points down nearly to Sirius's rising point, showing where to watch for its advent. When Sirius is low it often twinkles in vivid, flashing colors, an effect that binoculars reveal especially clearly.

    Saturday, December 25

  • Merry Sol Invictus! In the late Roman Empire, December 25th was the Birthday of the Unconquered Sun, marking the Sun's survival past its seasonal dark decline with the promise of a new spring and summer to come. Along with other solstice-period celebrations (including the birthday parties of numerous pagan deities), the date and the symbolism were taken over by Christianity and officially set to be Christmas in the 4th century. Carefully note the sunset point on your horizon from day to day. Can you see that it's already beginning to creep a little north?

  • Christmas is the time of year when iconic Orion finally clears the east-southeast horizon and sparkles in full view shortly after twilight ends (for skywatchers at mid-northern latitudes).

  • The eclipsing variable star Algol is at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple hours tonight centered on 1:08 a.m. Sunday morning Eastern Standard Time (10:08 p.m. Christmas evening Pacific Standard Time).



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

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

    Pocket Sky Atlas
    The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but that's less than one star in an entire telescopic field of view, on average. By comparison, Sky Atlas 2000.0 plots 81,312 stars to magnitude 8.5, typically one or two stars per telescopic field. Both atlases include many hundreds of deep-sky targets — galaxies, star clusters, and nebulae — to hunt among the stars.
    Sky & Telescope

    Once you get a telescope, to put it to good use you must have a detailed, large-scale sky atlas (set of charts). The standards are the
    Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger and deeper Uranometria 2000.0 (stars to magnitude 9.75). And read how to use your charts effectively.

    You'll also want a good deep-sky guidebook, such as Sky Atlas 2000.0 Companion by Strong and Sinnott, or the more detailed and descriptive Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

    Can a computerized telescope take their place? I don't think so — not for beginners, anyway, and especially not on mounts that are less than top-quality mechanically. 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 Dec. 13, 2010
    By December 13th, the dark markings issuing from the South Equatorial Belt Outbreak formed a very obvious, turbulent diagonal line most of the way around the planet. It's now detectable visually in almost any telescope capable of showing belts on Jupiter. Note also the activity in the North Equatorial Belt and the great blue festoons in the bright Equatorial Zone. Christopher Go took this image at 11:15 UT, when the central-meridian longitude (System II) was 267°. South is up.

    Mercury is hidden in the glare of the Sun.

    Venus (magnitude –4.8) blazes in the southeast before and during dawn, at its greatest height as the "Morning Star." In fact Venus rises some two hours before the first glimmer of dawn (for skywatchers at mid-northern latitudes) — a weird UFO of a thing in the east-southeast.

    Look for fainter Spica well to Venus's upper right, and for Saturn above or upper right of Spica. Far to Venus's upper left shines Arcturus.

    Mars is lost deep in the glow of sunset.

    Jupiter (magnitude –2.4, at the Pisces-Aquarius border) shines in the south at dusk and southwest later in the evening. It's the brightest starlike point in the evening sky. In a telescope Jupiter has shrunk to 40 arcseconds wide. Jupiter's South Equatorial Belt continues re-forming, as dark markings spread east and west all around the globe from the storm spot that broke out in the SEB's latitude more than a month ago. The original outbreak site transits Jupiter's central meridian about 3 hours and 40 minutes after the Great Red Spot.

    Jupiter on Dec. 15, 2010
    Jupiter's other side, on December 15th. The central-meridian longitude (System II) was 163°. South is up. Note the thin streamer of dark material all the way across the disk just above the bright Equatorial Zone. It's from the South Equatorial Belt Outbreak on the far side of the planet, now more than a month old.

    As for the Great Red Spot, it's near System II longitude 157°. Assuming it stays there, here's a list to print out of all the Great Red Spot's predicted transit times for the rest of this observing season.

    Saturn (magnitude +0.8, in Virgo) rises around 1 a.m. and is high in the south-southeast before and during dawn, far upper right of brilliant Venus. Don't confuse it with Spica below it. Saturn's rings have widened to 10° from edge-on.

    In a telescope, see if you can detect the new white storm known as the North Electrostatic Disturbance, seen with dramatic clarity in the stacked-video image below. Visually the spot is much less obvious. Saturn's rings have widened to 10° from edge-on.

    Uranus (magnitude 5.8) is only about 1½ east of Jupiter and closing.

    Saturn on Dec. 13. 2010
    Saturn's new white spot leaps out from this stacked-video image made by Christopher Go with his 11-inch scope. "I was even able to see it distinctly visually!" he writes. He took the image at 20:39 UT December 13th. The spot was at System III longitude 261° and is moving toward higher longitudes. South is up.

    Neptune (magnitude 7.9, in Capricornus) is still in the southwest right after dark. See our finder charts for Uranus and Neptune online or (with article) in the September Sky & Telescope, page 56.


    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) equals Universal Time (also known as UT, UTC, or GMT) minus 5 hours.


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