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This Week's Sky at a Glance

by Alan M. MacRobert

Some daily events in the changing sky for December 7 – 15.

Sizes of Comet Holmes and Moon compared, Dec. 1
"It's one thing to be told that Comet Holmes is bigger than the Moon," writes S&T's Dennis di Cicco, "but it's another to see it. I made this composite view of Comet Holmes on Dec. 1st with the full Moon inserted at the proper scale." And now the comet is even larger! Use binoculars, and look early in the week; by about the 13th moonlight begins returning. The star cluster in the comet's tail near bottom is NGC 1245. Click image for larger view.
S&T: Dennis di Cicco
Comet Holmes remains high in a moonless sky. It continues fading, but it's big — much bigger than the Moon now, as shown at right. Look for it in Perseus west of Alpha Persei. Binoculars give a beautiful view. See our update with finder chart, full story, and reader photos.

And don't look now, but another comet is on the way in! Comet 8P/Tuttle is still very dim at about magnitude 8.5 (just about as predicted), but it should reach 6th magnitude from late December through mid-January. And while Comet Holmes stays stuck in Perseus for months, Comet Tuttle (being much nearer Earth) will dive from near the north celestial pole across the evening sky to the southern horizon during the same time. See the article and charts in the January Sky & Telescope, page 73.

Friday, December 7

  • The red long-period variable star T Cephei, one of the brightest in the sky, should be at its peak of about magnitude 6.0 this week. See the article and finder chart in the December Sky & Telescope, page 78.

    Saturday, December 8

  • Bright Mars and Orion rise in the east around the same time at nightfall, and they shine at about the same height through the evening. Show someone how to spot Orion, to the right of brilliant Mars by two or three fist-widths at arm's length.

    Sunday, December 9

  • New Moon (exact at 12:40 p.m. Eastern Standard Time).

    Monday, December 10

  • The Great Square of Pegasus is high overhead after dinnertime in December. Think there's nothing in the Great Square to see with a telescope? Then you haven't read Sue French's "Deep-Sky Wonders" in the December Sky & Telescope, page 80.

    Tuesday, December 11

  • Also high overhead are Andromeda and Triangulum. Every skygazer learns to find the Great Andromeda Galaxy, M31. But what about the other big member of our Local Group of galaxies, M33 in Triangulum? It's not hard to locate once you know where: On the opposite side of Beta Andromedae from M31. Or alternatively: not quite halfway from Beta Andromedae to Alpha Arietis. M33 is much dimmer than M31; you'll need a good sky. See Gary Seronik's "Binocular Highlight" in the December Sky & Telescope, page 58. And compare the sky chart there with with the photo of the scene on page 54.

    Wednesday, December 12

  • By 10 p.m. brilliant Sirius is up in good view, sparkling whitely in the southeast below Orion. Sirius, orange Betelgeuse in Orion's shoulder, and Procyon to their left form the bright Winter Triangle. But this season it really ought to be the Winter Diamond -- with the addition of bright Mars to the Triangle's upper left. Mars and Sirius shine just about equally bright, on the Diamond's opposite ends. In the coming days and weeks, watch the diamond's top (Mars) skew to one side.

    Bright Geminid near Orion
    During the 2004 Geminid meteor shower, Alan Dyer caught a bright fireball with a tripod-mounted digital camera. He used a wide-field, 16-mm lens for a 1-minute exposure at f/2.8 with an ISO setting of 800. Expect to shoot a lot of frames before you get this lucky. Click image for larger view.
    Alan Dyer
    Thursday, December 13

  • The Geminid meteor shower peaks tonight and tomorrow night. From late evening until dawn, you might see a meteor a minute on average if you have excellent sky conditions. See our short article online and the full version in the December Sky & Telescope, page 71.

    Friday, December 14

  • It's not even winter yet, but already the Big Dipper is beginning its long annual rise in the evening sky, coming up bowl-first. By 9 p.m. you can spot it creeping up through the bare tree branches the north-northeast. The Dipper will float highest overhead on the warm evenings of May and June.

    Saturday, December 15

  • Whenever the Big Dipper is starting its rise, the Little Dipper hangs straight down from Polaris as if from a nail on the north wall of the icy winter sky (per Leslie Peltier). This week the Little Dipper assumes that position around 9 p.m., depending on where you live in your time zone. By mid-January, the very coldest time of year, it's there at 7 p.m. right after dinnertime.


    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).

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; the standard is Sky Atlas 2000.0) 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 enchanting though increasingly dated Burnham's Celestial Handbook). Read how to use them effectively.

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



    This Week's Planet Roundup

    Mercury is lost in the glare of the Sun.

    Venus (magnitude –4.2, passing from Virgo into Libra) is the bright "Morning Star" in the southeast before and during dawn. Look for Spica, much dimmer, off to its upper right. Arcturus shines much farther to Venus's upper left.

    Mars on Dec. 11, 2007
    Mars as it appeared on the night of Dec. 11. Note the North Polar Hood of clouds (top) breaking up, and the strong morning clouds now showing up (lower left). The bright region at center is Chryse; below it are Margaritifer Sinus and Aurorae Sinus. Dark, two-pronged Sinus Meridiani is nearing the evening limb at right. The time was 3:47 UT December 12th; the central-meridian longitude was 40°.
    S&T: Sean Walker
    Mars blazes bright yellow-orange (magnitude –1.4) in Gemini. It rises as early as 6 p.m. and is high up in fine view in the east by 9, awaiting your telescope. Mars shines very high in the early-morning hours — passing near the zenith, in fact, for observers at mid-northern latitudes (around 1 or 2 a.m.).

    In a telescope Mars appears about 15.6 arcseconds in diameter this week, essentially the same as the 15.9" it will display when nearest Earth on December 18th. For all about observing Mars with a telescope this season, see the guide and surface-feature map in the November Sky & Telescope, page 66.

    Jupiter is lost in the sunset.

    Saturn (magnitude +0.7) rises around 11 p.m. and is highest in the south before dawn. Fainter Regulus (magnitude +1.4) is 9° to Saturn's upper right after they rise, and directly right of it as they fade out in dawn's glow.

    Uranus (magnitude 5.8, in Aquarius) is still well placed in the south right after dark, high above Fomalhaut.

    Neptune (magnitude 7.9, in Capricornus) is getting lower in the southwest. Finder charts for Uranus and Neptune are in the July Sky & Telescope, page 60, and online.

    All descriptions that relate to your horizon or zenith — including the words up, down, right, and left — are written for the world's midnorthern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America. Eastern Standard Time (EST) equals Universal Time (UT, UTC, or GMT) minus 5 hours.

    To be sure you always get the current Sky at a Glance, bookmark this URL:
    http://SkyandTelescope.com/observing/ataglance?1=1



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