Sky at a Glance | December 23rd, 2011

What to See with Your New Telescope. As the gift-giving season comes to an end, maybe you've got a shiny new telescope to call your own. Read our article on how to get started using it! What to See with Your New Telescope.


Evening twilight
Watch the Moon pass Venus on the evenings after Christmas. (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. In the Far East, move it halfway. For clarity, the Moon is shown three times actual size.)

Friday, Dec. 23

  • Vega, the "Summer Star," still shines brightly in the northwest early these evenings. To Vega's upper left, the Northern Cross in Cygnus is swinging around on its way to planting itself upright on the northwest horizon around 9 p.m. (as seen from mid-northern latitudes).

  • Jupiter's Great Red Spot crosses Jupiter's central meridian around 7:12 p.m. EST.

    Saturday, Dec. 24

  • Christmas star: On this date every year, you can go out around 8 p.m., spot Orion well up in the southeast, and look down below it for bright Sirius on the rise. When Sirius is low it often twinkles with vivid flashing colors, which show up especially well in binoculars. All stars do this — but Sirius is so bright that the effect is especially pronounced.

  • New Moon (exact at 1:06 p.m. EST).

    Sunday, Dec. 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 new life and light 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 the setting Sun is 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).

  • And this year, the very thin waxing crescent Moon is visible low in the west-southwest 30 to 60 minutes after sunset, as shown at the top of this page.

  • Jupiter's Great Red Spot crosses Jupiter's central meridian around 8:51 p.m. EST.

    Monday, Dec. 26

  • Venus shines left of the crescent Moon as twilight fades, a lovely sight. They're about 7° apart as seen around the times of twilight for North America. And use binoculars to resolve the wide double star Alpha Capricorni near the Moon's right or upper right, as seen at the top of this page.

    Tuesday, Dec. 27

  • The Moon shines high above Venus at dusk, as shown at the top of this page.

  • Jupiter's Great Red Spot transits Jupiter's central meridian around 10:29 p.m. EST.

  • The eclipsing binary star Algol is at minimum brightness, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 10:41 p.m. Pacific Standard Time (1:41 a.m. on the 28th Eastern Standard Time).

    Dawn view
    Mercury is now descending in the dawn day by day, while Antares is moving upward to start its long journey toward the summer evening sky. Here they're 8° apart.
  • If you live near 40° north latitude (New York, Denver, Madrid) Mercury is now level with Antares low in the dawn, as shown here. Seen from north of 40°, Mercury is higher. Seen from south of there, Antares is higher.

    Wednesday, Dec. 28

  • Sirius, the Dog Star, sparkles low in the east-southeast after dinnertime this week. 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° north (Tijuana, New Orleans, Jacksonville), the two canine 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 will be the higher one.

  • Jupiter's inner moon Io crosses Jupiter's face from 7:57 to 10:06 p.m. EST, followed by its tiny black shadow (more easily visible) from 9:11 to 11:20 p.m. EST.

    Thursday, Dec. 29

  • Two of Jupiter's moons emerge out of eclipse from the planet's shadow early this evening: Europa at 7:51 p.m. EST, and Io at 8:36 p.m. EST. A small telescope will shown them gradually swelling into view just east of the planet.

    Friday, Dec. 30

  • Look upper right of the Moon after dinnertime for the Great Square of Pegasus balancing on one corner. It's a little larger than your fist at arm's length.

  • Algol is at minimum brightness for about two hours centered on 10:30 p.m. EST.

  • Jupiter's Great Red Spot crosses Jupiter's central meridian around 7:59 p.m. EST.

    Saturday, Dec. 31

  • After the noise and revelry at the turning of midnight, step outside into the silent, cold dark. Sirius is at its highest due south — a preview of where it will be at dusk when winter nears its end.

    To its upper right, Orion is just beginning to tilt westward. High to Sirius's upper left shines Procyon. And look high overhead; bright Capella and the Castor-Pollux pair straddle the zenith (if you're at mid-northern latitudes). Lower in the east Mars is glowing yellow-orange, presaging its opposition to come in early March.


    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'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 the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts effectively.

    You'll also want a good deep-sky guidebook, such as Sue French's new 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 classic if dated Burnham's Celestial Handbook.

    Can a computerized telescope replace charts? 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

    Mars on Dec. 12, 2011, at 9:05 UT
    On the morning of December 12th, Mars was 7.7 arcseconds wide and quite gibbous. "The red planet continues its approach each day," writes S&T's Sean Walker. "Note the broken dark ring within the North Polar Cap." Above center is the Aurorae Sinus / Tithonius Lacus region; at lower left is Mare Acidalium.


    Although this stacked video image shows far more than you're likely to see in any telescope visually, the North Polar Cap and its bordering dark areas are becoming more visually apparent. South is up. S&T's Sean Walker took this image with a 12.5-inch Newtonian reflector and a DMK21AU618 video camera.

    S&T: Sean Walker

    Mercury (about magnitude –0.3) is now having an excellent apparition in the dawn. About an hour before your local sunrise, look for it low above the southeast horizon, very far lower left of the Saturn-Spica pair.

    As dawn brightens, you can also look for fainter Antares beginning to emerge about 7° below or lower right of Mercury.

    Venus (magnitude –3.9) shines as the “Evening Star” in the southwest during twilight. It’s getting a little higher every week, on its way to a grand apparition high in the evening sky this winter and much of the spring (for the Northern Hemisphere).

    Mars (magnitude +0.5, at the hind leg of Leo) rises around 11 p.m. below Regulus and the Sickle of Leo. It's highest in the south before the first light of dawn, with Regulus now to its right. In a telescope Mars is a small blob 8 arcseconds wide.

    Jupiter (magnitude –2.7, at the Aries-Pisces border) blazes high in the southeast after dusk and a little higher in the south around 8 p.m. It sets around 2 or 3 a.m. In a telescope Jupiter appears 45 arcseconds wide; see our observing guide.

    Jupiter on Dec. 29, 2011
    Jupiter's Great Red Spot (GRS) was just about to depart around Jupiter's preceding limb when Christopher Go imaged the planet at 10:56 UT December 29th. South is up. "The wake following the GRS is very active and complex," he writes. "The North Equatorial Belt is very narrow, and the dark red barges are very impressive! The northern hemisphere is very quiet."

    Saturn (magnitude +0.7) rises around 2 or 3 a.m. and glows in the southeast well before dawn. Spica, similar in brightness at magnitude +1.0, is 5° or 6° to its right. Brighter Arcturus shines far to their upper left.

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

    Neptune (magnitude 7.9, at the Aquarius-Capricornus border) is getting low in the southwest right after dark. Use our printable finder charts for Uranus and Neptune, or see the September Sky & Telescope, page 53.


    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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    "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)


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