This Week’s Sky at a Glance, December 23 – 31

Venus and Mars, Dec. 23, 2016

The temporary "Southwest Triangle" of Venus, Mars and Fomalhaut continues to shrink, as Venus approaches Mars and the stars slide westward behind the planets.

Moon, Saturn, Antares at dawn, post-Christmas 2016

The waning Moon points the way to Saturn and Antares very low in the dawn.

Friday, December 23

Sirius and Procyon in the balance. This week Sirius, the Dog Star, sparkles low in the east-southeast after dinnertime. Procyon, the Little Dog Star, shines in the east about two fist-widths at arm's length to Sirius's left.

If you live around latitude 30° (Tijuana, New Orleans, Jacksonville), the two canine stars will be at the same height above your horizon soon after they rise. If you're north of that latitude, Procyon will be higher. If you're south of there, Sirius will be the higher one.

Saturday, December 24

• This is the time of year when M31, the Andromeda Galaxy, passes your zenith soon after dark (if you live in the mid-northern latitudes. It goes precisely across your zenith if you live at latitude 41° north.) Binoculars show M31 just off the upraised knee of the Andromeda constellation's stick figure; see the big evening constellation chart in the center of Sky & Telescope.

Sunday, December 25

• Right after dark you'll find the Pleiades high in the east, with Aldebaran and the Hyades below them. Far below these, Orion is beginning to clear the horizon. By about 9 p.m. Orion is much higher and Sirius is sparkling below it, completing this famous tall stack of December stars.

Got a new scope for Christmas? Not quite sure what to do with it? Read What to See with Your New Telescope.

Monday, December 26

• Saturn already? Antares already? Before 2017 even begins, you can catch the very beginning of their apparition that will culminate in the warm evening sky next summer. In early dawn tomorrow the 27th, if the air is very clear, go out and look very low in the southeast. Saturn is below the thin waning crescent Moon, and twinkly Antares is off to their right, as shown above.

Binoculars will help. The best viewing will probably be about 45 to 35 minutes before your local sunrise time. (Though in a telescope, Saturn so low will look awfully blurred through the thick air.)

Tuesday, December 27

• Venus now stays up a full two hours after complete darkness (at least if you can see down to the west-southwest horizon). So you have plenty of time this evening to notice that just to Venus's left is little Delta Capricorni, magnitude 2.8. They're 1.1° apart (for North America). And below Venus by about 1.4° is fainter Gamma Cap, magnitude 3.6.

Wednesday, December 28

• After dinnertime at this time of year, the Great Square of Pegasus balances on one corner high in the west. The vast Andromeda-Pegasus constellation complex runs all the way from near the zenith (Andromeda's foot) down through the Great Square (Pegasus's body) to fairly low in the west (Pegasus's nose).

Thursday, December 29

• Orion is now up in the east-southeast soon after nightfall, with his three-star Belt nearly vertical. The Belt points up toward Aldebaran and, even higher, the Pleiades. In the other direction, it points down to where bright Sirius will rise around 7 or 8 p.m. (depending on your location) and twinkle furiously.

• New Moon (exact at 1:53 a.m. on the 29th Eastern Standard Time). A new lunar month begins. Unlike a calendar month, which averages 30.437 days long, a lunar month (from one new Moon to the next) averages 29.531 days. This means that, on average, you'll see the Moon in the same phase about one day earlier every calendar month.

Friday, December 30

• Can you spot the fingernail-thin crescent Moon in twilight? It's less than two days old as seen after sunset from North America. Look low in the southwest 30 to 50 minutes after the Sun goes down. The Moon is located about 30° (three fists at arm's length) to the lower right of shiny Venus.

A similar distance to the Moon's upper right sparkles Altair.

Saturday, December 31

• As twilight fades, catch the crescent Moon, Venus, and Mars forming a diagonal, gently curving line in the southwest. Left of the line twinkles Fomalhaut, 25 light-years away — the same distance as Vega, the brightest star in the northwest.

• Mars is passing faint Neptune, which is only 1/1000 as bright at 8th magnitude. They're just 0.2° apart early this evening, with Neptune above Mars for mid-northern observers. Use high power to try to discern Neptune's slightly nonstellar disk, just 2.2 arcseconds in diameter.

• After the noise and whoopla at the turning of midnight tonight, step outside into the silent, cold dark. Shining in the south will be Sirius at its highest. The other bright stars of Canis Major will be to its right and below it. Sirius forms the bottom of the bright, equilateral Winter Triangle. The triangle's other stars are Betelgeuse in Orion's shoulder to Sirius's upper right, and Procyon the same distance to Sirius's upper left. The Winter Triangle now stands upright just about in balance.

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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 essential guide to astronomy.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the new Jumbo Edition for easier reading in the night. Click image for larger view.

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 new 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, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide 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."


This Week's Planet Roundup

Jupiter with Great Red Spot, Nov. 30, 2016

Jupiter's Great Red Spot side. . .

The non-Red-Spot side of Jupiter on Nov. 27, 2016

. . .and non-Red-Spot side, imaged by Christopher Go on November 30th and 27th, respectively. South is up. With Jupiter in fine telescopic view pre-dawn, it's showing a huge white zone south of the brownish South Equatorial Belt, but nothing like that in the northern hemisphere. A very small telescope should be able to show the difference.

Mercury has faded from view in the glow of sunset.

Venus (magnitude –4.4, in Capricornus) is the bright white "Evening Star" blazing in the southwest during and after twilight. In a telescope, it's a brilliant gibbous disc (about 60% sunlit) 20 arcseconds in diameter.

Mars (magnitude +0.8, in Aquarius) still glows in the south-southwest at dusk, about 15° upper left of Venus. In a telescope it's a tiny orange blob only 6 arcseconds in diameter.

Jupiter (magnitude –1.9, in Virgo) rises around 1 a.m. and shines brightly high the south-southeast by early dawn. Spot Spica 5° below it. In a telescope, Jupiter is 35 arcseconds in diameter: relatively small as Jupiter goes.

Saturn is deep in the glow of sunrise.

Uranus (magnitude 5.8, in Pisces) is high in the south after dark.

Neptune (magnitude 7.9, in Aquarius) is near Mars after dusk. They appear closest, 0.2° apart, on December 31st. For observers at mid-northern latitudes, Neptune will be above Mars that evening. Use high power to try to discern its 8th-magnitude disk, which appears only slightly nonstellar with its diameter of 2.2 arcseconds. Finder charts for Uranus and Neptune.

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

Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.

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"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson


8 thoughts on “This Week’s Sky at a Glance, December 23 – 31

  1. eric.rachut@med.va.gov

    The date of December for Christ’s birth is based on the date of the Annunciation (the reason for this can be researched on line). Christians of the first two centuries, when this became traditional, were radically opposed to any connection with paganism, as, for example, the New Testament concern about consuming food which had been sacrificed to idols. The idea that Christianity could take over pagan festivals and customs arose during the early Middle Ages and the claim that December 25th as Christmas was simply a converted Sol Invictus was popularized only after the Enlightenment.

  2. patrick-mortonpatrick-morton

    Greetings from los Robles astronomy club in Maracaibo,Venezuela, thank you, what wonderfull momemnts we spend when my students and I are reading your articles and studying your star charts, its like travelling to another world !! its a wonderfull Christmas present !! thank you ! we wish you all folks a very Merry christmas and a Happy New Yera !! keep on that good work please !! best regards. prof Patrick Morton

  3. rocksnstarsrocksnstars

    A bit surprised that the close conjunction of Mars and Neptune is not mentioned on the 31st. I’m sure it will be next Friday, but it might be the kind of thing that could use more than a one day notice.

    1. Anthony BarreiroAnthony Barreiro

      The conjunction has now been mentioned! At the moment of closest conjunction, 0700 UT January 1, Mars and Neptune will be only 1.2 arcminutes apart. As seen from the west coast of North America after dark on Saturday night December 31, 8th magnitude Neptune will be 8 arcminutes to the north-northeast of first magnitude Mars — close enough to get the pair centered in the same view of a medium power eyepiece. At only six arcseconds in diameter, Mars will look, at best, like a slightly gibbous orange blob, and 2-arcsecond Neptune will be a blue/green dot.

      1. rocksnstarsrocksnstars

        Ha ha, yes, excellent post!!! Hopefully many people make it down here to read the comments! For me (in Norway Maine), it will be dark enough to see Mars by around 5:15 PM EST, at which time the pair will be at an altitude of 33 degrees – seemingly (and conveniently) high compared to most close planetary conjunctions I’ve observed – and separated by about 15 arcmin. Three hours later they will be 9.5 degrees high and separated by 10 arcmin, when they will barely fit the 7mm Nagler FOV at 350X in my C14. However, perhaps the most interesting thing about this conjunction is that it will allow easy identification of the most distant planet, in the same field of view as the 2nd closest planet (at opposition or inferior conjunction, since both Venus and Mercury are now closer than Mars), in any optical aid, including the smallest binocs.

  4. Zubenelgenubi 61

    The thin crescent moon mentioned in the text for Friday night should be quite easy, but beautiful. I think there would be an opportunity for a super-thin moon, on the order of 15-18 hours old, from the U.S., on Thursday night. Would almost certainly need binoculars to be seen, especially from the midwest or east. From 90 degrees west, the moon would set about 42 minutes after sunset- would probably be best to look about 15-20 minutes after sunset, about 3 degrees up. This would be in the category of difficult but not impossible, if conditions are very clear. If you can see it, and it’s not too difficult, try to see it with the unaided eye. My personal best is 17:20 with binoculars and about 19 hours with the unaided eye- this is not even close to the record.

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