"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)

Some daily events in the changing sky for December 12 – 20.

The unusually big bright Moon shines among bright constellations in the east for the next couple of days, then departs to leave the winter stars to shine by themselves in early evening. (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.)

Alan MacRobert

Facing southwest in twilight

Watch Venus and Jupiter pulling farther apart in the southwest at dusk for the rest of December. The gap is between them is widening by about 1° per day.

Sky & Telescope diagram

Friday, December 12

  • The full Moon (exactly full at 11:37 a.m. EST) shines nearly between the horns of Taurus this evening, as shown above. The Moon is just a few hours from its closest perigee of 2008, making this the biggest and brightest full Moon since 1993 — by just a trace.

    Saturday, December 13

  • The annual Geminid meteor shower should be at its peak tonight, but the moonlight filling the sky will hide many of the meteors. Rates are best from about 10 p.m. until dawn. What with the moonlight, a steady watcher might see a meteor every few minutes on average. With the Moon right in Gemini, they'll appear to radiate almost from the direction of the Moon! Of course don't look there; watch whatever part of your sky is darkest.
  • Saturn is at western quadrature, 90° west of the Sun in the morning sky.

    Sunday, December 14

  • Once the Moon is up in the east this evening, look above it for Pollux and Castor, lower right of it for Procyon, and much farther to Procyon's lower right for bright Sirius on the rise.

    Monday, December 15

  • Late this evening the waning Moon forms a nearly equilateral triangle with Pollux high above it and Procyon to its upper right. Castor dangles above Pollux.

    Tuesday, December 16

  • Mira, the prototype red long-period variable star, is currently visible to the naked eye even through a fair amount of light pollution (look fairly early, before moonrise). By December 11th Mira had brightened to about magnitude 3.5. Here is its location in Cetus, and here is a closer-up chart with comparison-star magnitudes. Mira is due to reach peak brightness around December 22nd. It seems to be there already.

    Wednesday, December 17

  • Other red long-period variable stars due for maxima around now: R Aquilae, magnitude 6.1; R Trianguli, 6.2; R Canis Minoris, 8.0; R Geminorum, 7.1; R Aquarii, 6.5. All together now. . . .

    Thursday, December 18

  • Late tonight (on December 19th Universal Time), the asteroid 487 Venetia should occult a 9.5-magnitude star in Gemini for up to 6 seconds as seen from a track running from southern New England to British Columbia. Gemini will be in excellent view nearly overhead at the time. Maps and further information.
  • The Moon shines about 8° to the right of Saturn at dawn.

    Friday, December 19

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

    Facing southwest in early twilight

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

    Sky & Telescope diagram

    Saturday, December 20

  • Longest night of the year (in the Northern Hemisphere). The solstice occurs at 7:04 a.m. EST on the 21st, when the Sun is farthest south for the year and begins its six-month return northward.

    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 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 –0.8) is still hidden deep in the glow of sunset early this week. But by about the 18th, try scanning for it in early twilight just above your horizon far to the lower right of Jupiter. Bring binoculars.

    Venus, Jupiter, Moon

    Happy conjunction is happy. On the evening of December 1, 2008, the crescent Moon made an especially good smiley face under eye-like Venus and Jupiter (squint your eyes for a better impression of it) for surprised viewers glancing up in South Asia and Australia. This shot was taken by L. N. Naveen in Bangalore, India.

    Alan MacRobert

    Venus and Jupiter (magnitudes –4.2 and –2.0 respectively), continue pulling apart after their December 1st conjunction. The gap between them widens by 1° per day. Look southwest during twilight. Jupiter, to the lower right of dazzling Venus, is starting to get pretty low.

    In a telescope Venus is small (18 arcseconds wide) and gibbous (65% illuminated). Jupiter is 33″ wide, but it has a much lower surface brightness; being 7 times farther from the Sun, Jupiter is lit only about 1/49 as brightly.

    Mars remains hidden in the glare of the Sun.

    Saturn (magnitude +1.0, near the Leo-Virgo border) rises around 11 or midnight and shines high in the south by 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.

    Sad conjunction is sad. Hours later, by the time of twilight in North America on December 1st, the crescent Moon had moved over to the other side of Venus and Jupiter, creating a frowny face. (Tilt your head around to see it.) Sky & Telescope's Sean Walker took this shot from New Hampshire. In the time between, the Moon crossed right over Venus for western Europe. See our photo gallery.

    S&T: Sean Walker

    A telescope will show that Saturn's rings have closed to only 1° from edge on. They'll reach a minimum of 0.8° at the end of December, then start opening again before finally closing to exactly edge-on next September — when Saturn will, unfortunately, be in conjunction with the Sun.

    Uranus and Neptune (magnitudes 5.8 and 7.9, respectively, in Aquarius and Capricornus) are in the southwest and south right after dark. Use our article and finder charts or the chart in the September Sky & Telescope, page 63.

    Pluto is hidden in 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 (known as UT, UTC, or GMT) minus 5 hours.

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