Sky at a Glance | December 19th, 2008

Some daily events in the changing sky for December 19 – 27.

Facing southwest in early twilight
As Venus climbs higher and Jupiter slides lower, look for Mercury beginning to come into view. Bring binoculars.

Friday, December 19

  • Third-quarter Moon (exactly so at 5:29 a.m. EST).

    Saturday, December 20

  • Longest night of the year (in the Northern Hemisphere).

    Sunday, December 21

  • Shortest day of the year (in the Northern Hemisphere). The solstice occurs at 7:04 a.m. EST, when the Sun is farthest south for 2008 and begins its six-month return northward. This is the moment when winter begins in the Northern Hemisphere, summer in the Southern Hemisphere.

  • The bright eclipsing variable star Algol (Beta Persei) should be in one of its periodic dimmings, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 11:22 p.m. EST. Algol takes several additional hours to fade and to rebrighten. Use our comparison-star chart. (For all times of Algol's minima in December and January, good worldwide, see the January Sky & Telescope, page 74.)

    Monday, December 22

  • Take a look at Venus with binoculars after nightfall. It's closely equidistant from two stars, Iota and 30 Capricorni (magnitudes 4.3 and 5.4 respectively), about 0.8° from each. David Likuski finds that Venus is exactly equidistant from them around 6:34 p.m. Central Standard Time.

  • Mira, the prototype red long-period variable star, is visible to the naked eye even through serious light pollution. Today is the date predicted for Mira to reach its maximum brightness, which averages magnitude 3.4. It was already about that bright as early as December 10th. Here is Mira's location in Cetus, and here is a closer-up chart with comparison-star magnitudes.

    Tuesday, December 23

  • "Cassiopeia is open-cluster central," writes Gary Seronik in his Binocular Highlight column for the December Sky & Telescope (page 58). Oddly, however, only two clusters in Cas are Messier objects. Seronik charts three clusters for binoculars near Cassiopeia's eastern end (the bright end). Do you know them?

    Wednesday, December 24

  • Christmas star: This week 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 its rising point, showing where to watch for it. When Sirius is very low it often twinkles in vivid, flashing colors, an effect that binoculars reveal especially clearly.

  • Algol should be at minimum light for a couple hours centered on 8:12 p.m. EST.

    Thursday, December 25

  • Merry Sol Invictus! In the mid- to late Roman Empire, December 25th was the Birthday of the Unconquered Sun — celebrating the Sun's survival past another dark solstice with the promise of light and warmth in the spring and summer to come. The holiday, with its symbolism, was eventually taken over by Christianity.

    Friday, December 26

  • Orion season is well under way. The sparkly bright Hunter, steeply tilted, is already looming up low in the east-southeast as soon as darkness falls. He stands upright at his highest in the south by about 11 p.m.

    Facing southwest in early twilight. Bring binoculars.
    By the time the thin crescent Moon returns to the evening sky, Mercury will be in better view below Jupiter. (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.)

    Saturday, December 27

  • New Moon (exact at 7:22 a.m. EST).

  • Venus forms a nearly right triangle this evening with Delta and Gamma Capricorni, magnitudes 2.8 and 3.7 respectively. Both stars are a little over 1° from Venus. It's exactly a right triangle at 5:51 p.m. Pacific Standard Time, David Likuski writes to us.

  • Meanwhile Neptune, faintest of the major planets at magnitude 7.9, is 1½ north-northwest (right) of dazzling Venus, the brightest planet at magnitude –4.3. That's a difference of 80,000 times in brightness between them. If you've got a wide-field, low-power telescope, can you see them both in view at once? Or do you have to move glary Venus out of sight?

    Several faint stars are in the vicinity; identify which pinpoint is Neptune using the chart of the whole little scene in the December Sky & Telescope, page 55.

  • Algol should be at minimum light for a couple hours centered on 5:01 p.m. EST.

  • After dinnertime this week, it's Orion-Stack time. That is, Orion and some of his best-known companions form a big vertical stack in the southeast (if you live in the world's mid-northern latitudes). Start with Orion himself. In his middle, the three stars of Orion's Belt are stacked nearly vertically. The Belt points up toward orange Aldebaran, about two fist-widths at arm's length above. Poised higher over Aldebaran is the little Pleiades cluster. In the other direction, Orion's Belt points almost straight town to bright Sirius on the rise in Canis Major about two fists below.


    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 foldout map in 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 it's still less than one per square degree on the sky. Also plotted are many hundreds of good telescopic galaxies, star clusters, and nebulae.
    Sky & Telescope
    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; the standards are
    Sky Atlas 2000.0 or the smaller Pocket Sky Atlas) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion by Strong and Sinnott, the even more detailed Night Sky Observer's Guide by Kepple and Sanner, or the classic Burnham's Celestial Handbook). Read how to use them effectively.

    Can a computerized telescope take their place? 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 and a curious mind." Without these, they note, "the sky never becomes a friendly place."

    More beginners' tips: "How to Start Right in Astronomy".


    This Week's Planet Roundup

    Mercury (magnitude –0.7) is very low in the sunset but becoming easier to spot. Look for it in early twilight to the lower right of Jupiter. Bring binoculars. Mercury is closing in on Jupiter rapidly; they'll be in conjunction at December's end.

    Venus (magnitude –4.3) is the "Evening Star" blazing in the southwest during and after twilight. In a telescope it's still fairly small (19 arcseconds wide) and gibbous (62% illuminated).

    Mars remains hidden in the glare of the Sun.

    Jupiter (magnitude –1.9) has pulled away far to Venus's lower right in evening twilight. Look early and low; Jupiter now sets around twilight's end. It's 18° from Venus on December 19th and 24° from it on the 26th.

    Saturn's rings were tilted just 1.5° from edge-on when Christopher Go took this image on November 22, 2008. Note their very prominent shadow on Saturn's globe. South is up.
    Christopher Go

    Saturn (magnitude +1.0, near the Leo-Virgo border) rises around 11 p.m. and is highest in the south before dawn. Don't confuse Saturn with similarly-bright Regulus 22° (about two fist-widths at arm's length) to its upper right after they rise, and directly to its right at dawn.

    A telescope shows that Saturn's rings have closed to only 0.8° from edge on. This is their minimum tilt for a while; they'll start opening again a little in January and finally close to exactly edge-on next September (when Saturn will, unfortunately, be in conjunction with the Sun).

    Uranus and Neptune (magnitudes 5.9 and 7.9, respectively, in Aquarius and Capricornus) are in the southwest and south right after dark. Neptune is getting close to Venus. Use our article and finder charts or the chart in the September Sky & Telescope, page 63.

    Pluto is in conjunction with the Sun.

    All descriptions that relate to your horizon or zenith — 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 (known as UT, UTC, or GMT) minus 5 hours.


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