Sky at a Glance | December 18th, 2009

Some daily events in the changing sky for December 18 – 26.

Moon and planets as twilight fades
The waxing crescent Moon points the way to Mercury low in the cold twilight. The 10° scale as about the size of your fist held at arm's length. (These scenes are drawn for the middle of North America. European skywatchers: move each Moon symbol a quarter of the way toward the one for the precious date.)
Alan MacRobert

Friday, December 18

  • As the glow of sunset fades, look for Mercury glimmering low in the southwest about 6° or 7° lower right of the thin crescent Moon, as shown at right (seen from most of North America). This evening is also when Mercury is at greatest elongation, 20° east of the Sun.

    Saturday, December 19

  • Jupiter and Neptune are in conjunction, with 8th-magnitude Neptune 0.6° to dazzling Jupiter's north. To identify Neptune among similarly faint stars, use our finder chart.

    Sunday, December 20

  • Jupiter and Delta Capricorni shine left of the Moon this evening, as shown at lower right.

  • Mars reaches its stationary point; it ceases its normal eastward motion against the stars and begins retrograding (moving westward), what with opposition hardly six weeks away.

    Moon and Jupiter shortly after dark
    After dark on Sunday and Monday, the Moon poses by bright Jupiter and faint Neptune, which is invisible to the naked eye.

    Monday, December 21

  • Jupiter and Delta Capricorni shine below the Moon, as shown at right.

  • The December solstice occurs at 12:47 p.m. Eastern Standard Time. This is when the Sun is farthest south for the year and begins its six-month return northward. Winter begins in the Northern Hemisphere — where this is the shortest day of the year. In the Southern Hemisphere it's the longest day, and summer begins.

    Tuesday, December 22

  • How well do you know the sights of Auriga, high overhead after dark at this time of year? See Fred Schaaf's description for binoculars, with a finder photo, in the January Sky & Telescope, page 44. Go deeper with a telescope using Sue French's "Deep-Sky Wonders" article on page 65.

    Wednesday, December 23

  • The Rabbit's Ruby: The super-red carbon star R Leporis, a long-period variable, is brightening toward a maximum predicted for mid-January, when it should be about magnitude 6.8. It's located in front of the forehead and ears of Lepus, the Hare, under Orion's feet. See the article, photo, and chart in the January Sky & Telescope, page 66.

  • The bright eclipsing variable star Algol should be in one of its periodic dimmings, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 12:35 a.m. EST tonight (9:35 p.m. PST). Algol takes several additional hours to fade and to rebrighten. (Algol's minima times listed in the December S&T are incorrect; use the December times listed in the January issue, page 62).

    Orion's Belt points the way down to rising Sirius. Betelgeuse, Sirius, and Procyon form the nearly equilateral Winter Triangle.

    Thursday, December 24

  • First-quarter Moon (exact at 12:36 p.m. EST).

  • 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 low it often twinkles in vivid, flashing colors, an effect that binoculars reveal especially clearly.

    Friday, 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 dark decline with the promise of a new spring and summer to come. Along with other solstice celebrations (including the birthday parties of numerous pagan gods), the December 25th date and symbolism were taken over by Christianity 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?

    Saturday, December 26

  • 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 down to bright Sirius on the rise in Canis Major, about two fists below.

  • Algol should be at minimum light for a couple hours centered on 9:24 p.m. EST. (Algol's minima times listed in the December S&T are incorrect; use the December times listed in the January issue, page 62).


    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'll need a detailed, large-scale sky atlas (set of charts; 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 more detailed and descriptive 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? 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 and a curious mind." Without these, "the sky never becomes a friendly place."

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


    This Week's Planet Roundup

    Jupiter on Dec. 14, 2009
    Jupiter continues to shrink as it moves westward toward conjunction; turn a telescope on it in late twilight while it's still at its highest. The North Equatorial Belt remains massively broad and dark, while the South Equatorial Belt has greatly faded and is divided into two narrow parts. Christopher Go in the Philippines took this image on December 14th at 10:17 UT, when System II longitude 281° was on the planet's central meridian. South is up.


    Stacked-video images like this can show much more detail on a planet than the same telescope will reveal to the human eye.

    Christopher Go

    Mercury (magnitude –0.5 fading to +0.5) is visible low in the southwest in twilight. Look about an hour after sunset. Mercury remains near elongation all week but loses brightness quite noticeably during this time.

    Venus is lost in the sunrise. Not until late winter (Northern Hemisphere winter) will it emerge into view again, after sunset.

    Mars (magnitude –0.5, in Leo) rises around 8 p.m. local time, far below Castor and Pollux a bit north of east. A little later, twinkly Regulus rises about a fist-width beneath it. Mars and Regulus are very high in the south in the hours before dawn, now lined up horizontally.

    In a telescope, Mars is 11.5 to 12 arcseconds wide and growing. The north polar cap is in good view, bordered by a wide dark collar. Do you see other surface features? Identify them with the Mars map and observing guide in the December Sky & Telescope, page 57. Mars is on its way to opposition in late January, when it will reach 14.1 arcseconds wide.

    Jupiter and Neptune on July 9, 2009
    Jupiter and Neptune had a previous conjunction last July. On the morning of July 9th, Bob Kimmel of Rockledge, Florida, took this image. Why are they so different in brightness? Jupiter is a bigger planet, but the main reasons are that Jupiter is closer to the Sun (so it gets illuminated more brightly) and also closer to Earth.


    Kimmel used an 8-inch f/5 Newtonian reflector with a Canon D20a DSLR camera body on his back patio. This is an unguided 20-second exposure at ISO 3200.

    Bob Kimmel

    Jupiter (magnitude –2.2, in Capricornus) shines brightly in the south-southwest in twilight, and lower in the southwest after dark. It sets by 8 or 9 p.m. Jupiter is moving lower west toward the sunset week by week, sets earlier, and continues to shrink as Earth pulls farther away from it in our faster orbit. In a telescope Jupiter is currently 36 arcseconds wide, compared to 49″ around its time of opposition last August.

    Saturn (magnitude +1.0, in the head of Virgo) rises in the east around midnight and shines highest in the south before and during dawn. Its rings are still narrow, tilted 4.8° from edge-on to us.

    Uranus (magnitude 5.8, just below the Circlet of Pisces) is highest in the south right after dark.

    Neptune (magnitude 7.9, in Capricornus) lurks closely in the background of Jupiter, which shines 10,000 times brighter. Nevertheless Neptune is detectable in good binoculars. Jupiter and Neptune reach conjunction, with Neptune 0.6° north of Jupiter, on the evening of December 19th. By the 25th they widen to 1.0° apart. To identify Neptune (and Uranus) among other faint points, use our finder charts for these two planets.

    Pluto is in conjunction, behind the glare of 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 (also known as UT, UTC, or GMT) minus 5 hours.

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