Sky at a Glance | December 24th, 2010

Friday, December 24

  • Christmas star: Sirius, the brightest star in the night sky, rises in the southeast 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 it. When Sirius is low it often twinkles in vivid, flashing colors, an effect that binoculars reveal especially clearly. All stars do this, but only Sirius is bright enough to do it so visibly.

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

    Sunday, December 26

  • Two famous "enclosure" asterisms lie upper right of Jupiter these evenings: the small, dim Circlet of Pisces, and, farther on, the big, brighter Great Square of Pegasus. Also, far below Jupiter in early evening, look for Fomalhaut.

    Southeastward view around 3 a.m.
    Stick your head out around 3 a.m., and the waning Moon guides your way to Saturn.

    Monday, December 27

  • Last-quarter Moon tonight (exact at 11:18 p.m. EST). The last-quarter Moon always rises around the middle of the night, in a constellation that won't appear high in the evening sky until one season ahead. In tonight's case the Moon is in Virgo, a constellation best known in spring. Look for Saturn to the Moon's left, as shown here (on the morning of the 28th).

    Tuesday, December 28

  • Dawn and sunrise are now happening nearly as late by the clock as they're going to. Look southeast in early dawn Wednesday morning for the Moon, Saturn and Spica. For North America, Spica is just a few degrees from the Moon — upper left of the Moon at 3 a.m. as shown here, and above the Moon by dawn. Saturn at dawn is off to their upper right. Venus, much brighter, shines far to the Moon's lower left.

  • Jupiter's Red Spot crosses Jupiter's central meridian around 8:35 p.m. EST this evening.

  • The eclipsing variable star Algol is at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 9:58 p.m. EST this evening.

    Wednesday, December 29

  • A small telescope, or steadily held binoculars, will show the 5.5-magnitude star 20 Piscium just 4 arcminutes (six Jupiter diameters) to Jupiter's south or southeast, looking like a very out-of-place Galilean moon. With a slightly larger telescope, here's a chance to compare Jupiter's moons at high magnification with a star. In good seeing, their non-stellar nature is fairly plain.

    And don't miss Uranus, magnitude 5.8, currently 50 arcminutes to Jupiter's north-northeast. It looks even more un-starlike at high power.

  • Saturn's big new white spot should transit the planet's central meridian around 3:19 a.m. Thursday morning Eastern Standard Time.

  • During dawn Thursday morning, spot the waning crescent Moon with Venus to its left. Can you follow Venus with your unaided eyes right through sunrise?

    Thursday, December 30

  • Jupiter's Red Spot crosses Jupiter's central meridian around 10:14 p.m. EST; 7:14 p.m. PST.

    As the Moon wanes further, it points the way to Venus, Antares, and newly arrived Mercury.
  • At dawn Friday morning, Venus shines upper left of the Moon.

    Friday, December 31

  • The eclipsing variable star Algol is at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 6:47 p.m. EST.

  • After the noise and revelry at the turning of midnight, step outside into the silent, cold dark. If the sky is clear, Sirius will be shining at its highest in the south. To its upper right, Orion is just beginning to tilt westward. To Sirius's upper left shines Procyon. And (if you're near latitude 39° north) Capella and the Castor-Pollux pair straddle the zenith. This is a preview of how the constellations will look at dusk at winter's end.

  • At dawn on the morning of January 1st (are you really going to be up then?) look far below Venus in the southeast for the thin crescent Moon. Antares and Mercury are also in the scene, as shown at right.

    Saturday, January 1

  • Sirius, the Dog Star, sparkles low in the east-southeast after dinnertime. Procyon, the Little Dog Star, shines in the east about two fist-widths at arm's length to Sirius's left. If you live around latitude 30° (Tijuana, New Orleans, Jacksonville), the two canine stars will be 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 will be the higher one.



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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).

    Sky Atlas 2000.0 (the color Deluxe Edition is shown here) plots 81,312 stars to magnitude 8.5. That includes most of the stars that you can see in a good finderscope, and typically one or two stars that will fall within a 50× telescope's field of view wherever you point. About 2,700 deep-sky objects to hunt are plotted among the stars.
    Alan MacRobert

    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 emerges into dawn view and brightens late this week. On the morning of December 26th it's still very low and only magnitude +1.5, but by the 31st it's much higher and magnitude +0.3. Look for it in the east-southeast, about 60 to 45 minutes before your local sunrise time, very far to the lower left of bright Venus. Don't confuse Mercury with Antares, well to its right and more directly below Venus (for skywatchers at mid-northern latitudes).

    Venus (magnitude –4.7) blazes as the "Morning Star" in the southeast before and during dawn. In fact Venus rises some two hours before the first glimmer of dawn (for mid-northern latitudes) — a weird UFO of a thing in the east-southeast.

    Look for Saturn and Spica very far to Venus's upper right, and Arcturus even higher above Venus.

    Mars is lost in the glow of sunset.

    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.

    Jupiter (magnitude –2.4, at the Pisces-Aquarius border) shines in the south during twilight and southwest later in the evening. It's the brightest starlike point in the evening sky, then sets by midnight. 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 a month and a half ago. The original outbreak site transits Jupiter's central meridian about 3 hours and 40 minutes after the Great Red Spot.

    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 Saturn with Spica below it.

    A view of Saturn's dramatic new storm, as recorded by Sean Walker on December 25, 2010, with a 12½-inch reflector and an Imaging Source CCD camera. South is up.
    S&T: Sean Walker

    In a telescope, Saturn's new white spot has grown big, long, and prominent! See our article, Saturn's New Bright Storm, which includes its predicted transit times.

    Saturn's rings have widened to 10° from edge-on.

    Uranus (magnitude 5.8) is only about 1° from Jupiter and closing.

    Neptune (magnitude 7.9, in Capricornus) is still in the southwest right after dark. It shared the same telescopic view with 5th-magnitude Mu Capricorni. See our finder charts for Uranus and Neptune online or (with article) in the September Sky & Telescope, page 56.


    "Science is built up of facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house."
    — Henri Poincaré (1854–1912)


    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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