This Week's Sky at a Glance, December 10 – 18

FRIDAY, DECEMBER 10

■ Comet Leonard (C/2021 A1) is right now at its best! It's about magnitude 5.5, low due east tomorrow morning the 11th. Go out just before the very first light of dawn, meaning about 90 or 100 minutes before your sunrise time. The comet will be about 10° above horizontal.

You'll almost certainly need binoculars or a wide-field telescope to detect it. Use the finder chart at the bottom of See Comet Leonard at its Best to help you locate the exact place among the stars you need to examine.

On Sunday morning the 12th Comet Leonard may be just a trace brighter, but it'll be much lower and more involved in morning twilight.

Later this week the comet will reappear very low in the west during evening twilight; see December 16th below. But by then it will be fading. For the world's mid-northern latitudes, tomorrow morning is your last chance to catch it at peak brightness in a dark sky.

■ First-quarter Moon tonight (exactly so at 8:36 p.m. EST). At dusk the Moon shines far off the upper-left end of the Venus-Saturn-Jupiter line, turning it into an immense, gently curving series of four objects.

■ Algol, high overhead in Perseus after dark, should be at minimum light (magnitude 3.4 instead of its usual 2.1) for a couple hours centered on its mid-eclipse time of 8:27 p.m. EST.

SATURDAY, DECEMBER 11

■ Venus and Jupiter continue to blaze during and after twilight in the southwest and south-southwest, respectively. As twilight fades, dimmer Saturn emerges between them.

Jupiter and Saturn are far past opposition, so don't be disappointed by the less-than-optimal telescopic views. Venus, on the other hand, is enlarging while becoming a thinner, more dramatic crescent. It will continue to do so into the beginning of January.

■ Not that you'd have any possible way to see it, but this evening Pluto is less than 1° from Venus, 100 times farther in the background. And it's a lot farther from the illuminating Sun as well as from us. All this means that Pluto, at magnitude 14.4, is 50 million times fainter that Venus!

■ Have you seen any early Geminid meteors yet? The shower is due to peak in two days, late on the night of December 13-14. See Bob King's Precious Hours with the Geminids.

SUNDAY, DECEMBER 12

■ Orion strides up clear of the east-northeast horizon by 7 p.m. now. By 9 or 10 he displays himself high in the southeast in full wintry glory.

■ The Big Dipper lies shyly down at its lowest just after dark, due north. It's entirely below the north horizon if you're as far south as Miami.

But by 11 or midnight the Dipper stands straight up on its handle in fine view in the northeast. By that time Cassiopeia has wheeled over to the high northwest and stands nearly upright on the bright end of its W shape.

MONDAY, DECEMBER 13

■ The Geminid meteor shower, often the best of the year, should be at its peak late tonight. The light of the waxing gibbous Moon will wash out faintest meteors until the Moon sets around 3 a.m. By then the shower will be in full force, with its radiant high overhead. You have about three excellent dark-sky hours between then and the first light of dawn. Under a really dark sky during that time, you might see two or three meteors per minute on average. Under suburban light pollution, maybe more like one a minute.

In early evening the meteors will be fewer because the shower's radiant point (in Gemini) will be low. But those that do appear will be Earth-grazers skimming far across the top of the atmosphere.

Layer up even more warmly than you imagine you'll need; remember about radiational cooling! Find a dark open spot with no local lights to get in your eyes, lie back in a reclining lawn chair, and gaze up into the stars. Relax and be patient.

Want to do more? This year the International Meteor Organization is asking for help in determining the effect of bright moonlight on meteor activity, by means of careful counts for at least one hour and careful determinations of your limiting magnitude. See Bob King's Precious Hours with the Geminids.

TUESDAY, DECEMBER 14

■ The Summer Triangle is sinking lower in the west as the season turns, and Altair is the first of its stars to go (as seen from mid-northern latitudes). Start by spotting bright Vega, magnitude zero, in the northwest right after dark. The brightest star above it is Deneb. The Triangle's third star, Altair, 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?

WEDNESDAY, DECEMBER 15

■ This evening, spot orange Aldebaran about two fists at arm's length lower left of the Moon. Now look in the opposite direction from the Moon a little less far, and just a bit up from there, and you're at the brightest two or three stars of Aries. Of all the zodiac constellations, Aries has by far the smallest distinctive pattern of its main stars.

THURSDAY, DECEMBER 16

■ Left of Orion after dinnertime is the constellation Gemini. Castor and Pollux, the head stars of the Gemini stick-figure twins, are at its left end; the twins are lying on their sides. Castor is the higher one.

■ Comet Leonard has been emerging just above the southwest horizon in evening twilight, a little bit higher each day. But you'll be looking through a not-fully-dark sky.

This evening offers an easy pointer to the exact spot to examine with binoculars. In the southwest, draw a line from modest Saturn diagonally down through brilliant Venus, extend the line on by half again as far or just a little more, and there you are!

When to look? Fifty minutes after sunset might be a time to start hunting with binoculars or a wide-field telescope. But the best time window (if any) between a too-bright sky and a too-low comet will depend on your latitude and the clarity of the air.

The farther south you are the better. If you're as far north as 40°N (Denver, New York), Comet Leonard will be 8° above the horizon 50 minutes after sunset. Seen from 30°N (New Orleans, the Gulf Coast) it will be 10° up at that time. Good luck.

FRIDAY, DECEMBER 17

■ Have you ever watched a Sirius-rise? Find an open view right down to the east-southeast horizon. Watch there for Sirius to come up 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, a name that means “The Announcer.” What Mirzam announces is Sirius. You’re not likely to mistake the two; the second-magnitude Announcer is only a twentieth as bright as the King of Stars about to make his entry.

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

SATURDAY, DECEMBER 18

■ As twilight fades out this evening, use binoculars or a telescope to try for Comet Leonard 5° below Venus in the southwest. It should still be about magnitude 6. Good luck.

■ Full Moon tonight (exactly full at 12:35 a.m. tonight EST; 9:35 p.m. PST). The Moon shines between the horntip stars of Taurus, Beta and Zeta Tauri, as shown above. It's at apogee, making this a "mini-moon" very slightly smaller than average: the opposite of a "supermoon" at perigee.

By midnight the Moon is very high in the south, not far from the zenith. The full Moon of the Christmas season rides higher across the sky at midnight than at any other time of year, thus "giving lustre of midday to objects below."

Why? December is the month of the solstice, when the Sun is farthest south in the sky. So, this is when the full Moon (opposite the Sun) is farthest north. In crossing the sky, it behaves like a pale, cold June Sun in the night.

 

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This Week's Planet Roundup

Mercury is hidden deep in the sunset.

Venus, a dazzling magnitude –4.8, shines in the southwest during and after twilight. It's drawing nearer to Earth and nearer to our line of sight to the Sun. Thus we see its crescent enlarge from a big 45 arcseconds to 50 arcseconds tall this week, while it thins from 20% to only 13% sunlit. Any telescope is enough to keep watch!

Mars, far and faint at magnitude +1.6, is low in the dawn in Libra. Look for it a little above the southeast horizon about 50 minutes before sunup, way down below Arcturus and Spica.

Jupiter, crossing from Capricornus into Aquarius, shines in the south-southwest at dusk about three fists at arm's length upper left of Venus. At magnitude –2.2 it's second in brightness only to Venus.

Look two fists lower left of Jupiter for Fomalhaut, magnitude +1.2.

Saturn, in Capricornus, is midway between Jupiter and Venus. At magnitude +0.7, Saturn is only 1/15 Jupiter's brightness.

Saturn sets around 8 p.m. Jupiter follows it down a little more than an hour later.

Uranus (magnitude 5.7, in Aries north of the head of Cetus) is very high in the southeast in early evening. See Bob King's story and finder chart.

Neptune (magnitude 7.9, in Aquarius below the Great Square of Pegasus and the Circlet of Pisces) is almost high in the south-southwest in early evening.

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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 (also called UT, UTC, GMT, or Z time) minus 5 hours.

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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 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 magazine of the American Astronomical Society.

Once you get a telescope, to put it to good use you'll need a 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 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 magnitude 9.75). And be sure to read How to Use a Star Chart with a Telescope.

You'll also want a good deep-sky guidebook, such as the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. 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."

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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 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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