This Week's Sky at a Glance, December 8 – 17

FRIDAY, DECEMBER 8

■ If you live in the world's mid-northern latitudes, M31, the Andromeda Galaxy, passes your zenith around 7 or 8 p.m. The exact time depends on how far east or west you live in your time zone. Binoculars will reveal the dim little glow of M31 just off the knee of the Andromeda constellation's stick figure. See the big evening constellation chart in the center of the December Sky & Telescope.

■ Before and during dawn Saturday morning the 9th, catch the lovely meetup in the eastern sky of Venus and the thin crescent Moon: Astarte and Diana, respectively, for the classically minded or those who remember their pre-dawn Edgar Allan Poe. The two goddesses are about 4° apart.

SATURDAY, DECEMBER 9

■ Orion comes into good view low in the east-southeast after dinnertime now. And that means Gemini is also coming up to its left (for the world's mid-northern latitudes). The head stars of the Gemini twins, Castor and Pollux, are at the left end of the Gemini constellation — one over the other, with fainter Castor on top.

SUNDAY, DECEMBER 10

■ At this time of year the Big Dipper lies down lowest soon after dark, due north. If you're as far south as Miami, it's entirely below the north horizon.

But by midnight the Dipper stands straight up on its handle in fine view in the northeast, while Cassiopeia has wheeled down to the northwest to stand nearly upright on the bright end of its W shape.

MONDAY, DECEMBER 11

■ Extraordinary event tonight! The brilliant orange-red supergiant Betelgeuse will be covered by a faint little asteroid, 319 Leona, for up to a second or two. But only if you're located near the middle of the narrow occultation track. It runs from central Asia across the Caspian Sea, northern Turkey, northern Greece, southern Italy and Spain, the Atlantic Ocean, southernmost Florida (around 8:17 p.m. EST), and finally east-central Mexico.

Amazingly, the 60-km asteroid and the supergiant star appear just about the same angular size from Earth's point of view. So the occultation will be partial for observers across most or all of the path width.

At least you won't need a finder chart for the star! Watch whether Orion's bright shoulder dims out and back. Much science is planned, including by amateurs, including attempts to detect detail on Betelgeuse's irregularly shining surface. Read all about it at Asteroid to Cover Betelgeuse on Night of December 11-12, with many links to interactive land maps, observing plans, detailed information about the star and the asteroid, and a link to a planned livestream.

TUESDAY, DECEMBER 12

■ With your telescope, take advantage of the moonless sky this week to try for the noteworthy dwarf galaxy IC 10, magnitude 10½, close to the bright end of Cassiopeia. IC 10 is a member of the Milky Way's Local Group of galaxies and may be a distant satellite of the Andromeda Galaxy, M31, which is fully 19° from it on the sky. Because it lies behind the winter Milky Way, IC 10 is dimmed fully 3 magnitudes by our galaxy's interstellar dust. In other words, if IC 10 were elsewhere it would appear 16 times as bright and would be much better known.

In the December Sky & Telescope article about it (page 57), the charts show where to pinpoint this small, weak little glow behind swarms of Milky Way stars, which are plotted there to magnitude 9. That's deep enough to enable you to star-hop to the exact spot. Once you know precisely where to examine, can you make it out in the December dark?

■ This evening Jupiter's brightest moon Ganymede will slip slowly into Jupiter's shadow, taking a few minutes to disappear centered on 7:00 p.m. EST. Ganymede will "star" just off Jupiter's celestial west limb.

■ New Moon (exactly new at 6:32 p.m. EST).

WEDNESDAY, DECEMBER 13

■ The Geminid meteor shower, usually the best of the year, should peak late tonight. And this year there's no interfering moonlight.

In early evening the meteors will be few, but those that do appear will be Earth-grazers skimming far across the top of the atmosphere. As the hours pass and the shower's radiant (near Castor in Gemini) rises higher in the east, the meteors will become shorter and more numerous — the most so between midnight and dawn.

Layer up even more warmly than you imagine you'll need; remember about radiational cooling! Bring a reclining lawn chair to a dark open spot where there are no bright local lights to get in your eyes. Lie back and gaze up into the stars. The best direction to watch is wherever your sky is darkest.

Be patient. As your eyes adapt to the dark, you may see a meteor every minute or two on average as night grows late.

For more see Get Ready to Greet the Geminid Meteor Shower. We've also put up a press kit of text and images for free use by anyone: A Great Year for the Geminids.

THURSDAY, DECEMBER 14

■ The Pleiades and Aldebaran below them are well up in the east after dark, and Orion is rising below them. Orion's whole main figure, formed by his seven brightest stars, takes about 1 hour 20 minutes to clear the horizon. By 8 p.m. or so Orion is up in fine view.

■ Algol, high overhead in Perseus, should be at its minimum brightness, magnitude 3.4 instead of its usual 2.3, for a couple hours centered on 9:27 p.m. EST; 6:27 p.m. PST. Algol takes several additional hours to fade and to rebrighten. Estimate its brightness using this comparison-star chart:

FRIDAY, DECEMBER 15

■ The Summer Triangle is sinking lower in the west now, and Altair is the first of its stars to go (for mid-northern skywatchers). Start by spotting bright Vega, magnitude zero, in the northwest right after dark. The brightest star above Vega is Deneb. Altair, the Triangle's third star, is farther to Vega's left or lower left. How late into the night, and into the advancing season, can you keep Altair in view?

SATURDAY, DECEMBER 16

■ Have you ever watched a Sirius-rise? Find an open view right down to the east-southeast horizon, and watch for Sirius to rise about two fists at arm's length below Orion's vertical belt. Sirius rises sometime around 8 p.m. now, depending on your location.

About 15 minutes before Sirius-rise, a lesser star comes up barely to the right of there: Beta Canis Majoris or Mirzam. Its name means “The Announcer,” and what Mirzam announces is Sirius. You’re not likely to mistake them; the second-magnitude Announcer is only a twentieth as bright as the king of stars about to make his royal entry.

When a star is very low it tends to twinkle slowly, and often in vivid colors. Sirius is bright enough to show these effects well, especially with binoculars.

SUNDAY, DECEMBER 17

■ This evening, spot Saturn about 3° upper right of the Moon as shown above. The Moon is 1.3 light-seconds from us. Saturn is currently 84 light minutes away in the background, almost 4,000 times as far from us.

■ Algol again dips to minimum brightness, this time for a couple hours centered on 6:17 p.m. EST.

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This Week's Planet Roundup

Mercury this week fades and settles down out of sight in the afterglow of sunset.

Venus, magnitude –4.1, shines as the bright "Morning Star" in the southeast before and during dawn. It's not quite as high as it was a month or two ago.

Venus rises above the east horizon about 1½ hours before dawn's first light. Watch for it to come up about three fists at arm's length lower right of Arcturus, the brightest star twinkling in the east.

Mars is out of sight behind the glare of the Sun.

Jupiter (magnitude –2.7, in Aries) is that bright white dot dominating the high southeast early these evenings. It stands highest in the south around 8 or 9 p.m.

Saturn (magnitude +0.9, in Aquarius) glows yellowish high in the south-southwest just after dark. Fomalhaut, similarly bright, twinkles almost two fists at arm's length to Saturn's lower left. Saturn declines toward the southwest as evening progresses and sets by about 10 p.m. So get your telescope on it early!

Uranus, magnitude 5.6 in Aries, awaits your binoculars in the darkness 14° east of Jupiter. In a telescope at high power Uranus is a tiny but distinctly nonstellar ball, 3.8 arcseconds in diameter. Locate and identify it using the finder charts in the November Sky & Telescope, pages 48-49.

Neptune, fainter at magnitude 7.9, is at the Aquarius-Pisces border 24° east of Saturn, high in the early-evening dark. Neptune is only 2.3 arcseconds wide: harder to resolve as a ball than Uranus, but definitely nonstellar at high power.

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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 and graphics that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time minus 5 hours. UT is also known as UTC, GMT, or Z time.

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For a more detailed 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.

Once you get a telescope, to put it to good use you'll need a much more detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows all stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to mag 9.75). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.

Can computerized telescopes replace charts? Not for beginners I don't think, especially not on mounts and tripods that are less than top-quality mechanically. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning the sky. And 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."

But finding a faint telescopic object the old-fashioned way with charts isn't simple either. Learn the tricks at How to Use a Star Chart with a Telescope. 

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Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the naked-eye heavens above. It's free.

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"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things."
            — John Adams, 1770

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