This Week's Sky at a Glance, December 17 – 25

FRIDAY, DECEMBER 17

■ Comet Leonard (C/2021 A1) is very low in the west-southwest toward the end of evening twilight; use binoculars or a wide-field telescope. You'll need to catch the narrow time window, if any, between the sky being too bright and the comet getting too low; 60 to 75 minutes after sunset might be best depending on your latitude.

Finding the right location, at least, will be easy! Venus and Saturn guide you to the correct spot of sky to examine; use the finder chart at the bottom of See Comet Leonard at its Best. The dates of the comet symbols there are Universal Time dates; subtract one day to get the North American civil date in the evening.

This evening the comet's location is especially easy to find. Just look 5° below Venus! (for the mid-latitudes of North America.) That's a bit less than the width of a typical binocular's field of view.

See update at December 21 below.

■ Orion strides up clear of the east-northeast horizon soon after dark now. By 9 p.m. he's high in the southeast in full wintry glory.

■ 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 will rise sometime around 8 p.m. now depending on your location.

About 15 minutes beforehand, 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 the two; the second-magnitude Announcer is only a twentieth as bright as the King of Stars about to make its 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

■ 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 making its way across the night sky, it behaves as a pale, cold imitation of the June Sun six months ago.

SUNDAY, DECEMBER 19

■ Venus and Jupiter this week continue to shine brightly during twilight in the southwest and south-southwest, respectively. But both are getting lower now, Venus especially. As twilight fades, watch for dimmer Saturn emerging 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 enlarge and thin into the beginning of January.

Have you ever tried to discern the crescent of Venus with your naked eyes? A few sharp-eyed people can! One was apparently Edgar Allan Poe; see This Week's Planet Roundup at the bottom of this page.

MONDAY, DECEMBER 20

■ Around the end of twilight, face north and look high. Cassiopeia is now a flattened M shape canted at about a 45° angle (depending on where you live). Just two hours later, the M is horizontal! Constellations passing near the zenith appear to rotate rapidly with respect to your direction "up."

■ You are remembered, Carl Sagan (November 9, 1934 – December 20, 1996).

TUESDAY, DECEMBER 21

■ Comet Leonard update: Last night, December 20, I spotted Comet Leonard pretty easily with 10x50 binoculars in late twilight from latitude 42.4° N. I got it right away at 5:21 p.m. EST, far below Saturn: a small fuzzpatch with a subtle but definite averted-vision tail a fraction of a degree long in the correct direction. The head of the comet got plainer to see in the next five minutes before going behind the distant treeline.

Those five minutes ran from 71 to 76 minutes after my sunset, as the comet's altitude went from 6° to 5.3° and the Sun's altitude below the horizon went from -11.7° to -12.5°. Some people are reporting that the comet brightened on the 20th.

■ The solstice marks the official start of winter at 10:59 a.m. EST (15:59 UT). Last night and tonight are the longest nights of the year in the Northern Hemisphere. The Sun is at its southernmost declination for the year and begins its six-month return northward.

■ 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, the brightest star in the northwest right after dark. The brightest one 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 22

■ 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). The exact time depends on your longitude. Binoculars will show M31 just off the knee of the Andromeda constellation's stick figure; see the big evening constellation chart in the center of Sky & Telescope.

Lie on your back with binocs and look straight up. M31 passes exactly through your zenith if you're at 41° north latitude.

THURSDAY, DECEMBER 23

■ 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 fainter and higher one.

FRIDAY, DECEMBER 24

■ The Jupiter-Saturn-Venus line at dusk slowly continues to evolve. Saturn is now a little closer to Venus; a month ago it was closer to Jupiter. And the whole line is sliding farther to the lower right.

And the line is now growing longer again, not shorter. If you were hoping Jupiter was on its way to a conjunction with Venus, nope. Venus is now diving away back down toward the Sun (it passes the Sun in daytime on January 8th). As for Jupiter and Saturn, their own next conjunction doesn't come until November 2, 2040.

SATURDAY, DECEMBER 25

■ High above Orion shines orange Aldebaran with the large, loose Hyades cluster in its background. Binoculars are the ideal instrument for this cluster given its size: its brightest stars (4th and 5th magnitude) span an area about 4° wide. Higher above, the Pleiades are hardly more than 1° across counting just the brightest stars.

The main Hyades stars famously form a V. It's currently lying on its side, open to the left. Aldebaran forms the lower of the V's two tips.

With binoculars, follow the lower branch of the V to the right from Aldebaran. The first thing you come to is the House asterism: a pattern of stars like a child's drawing of a house with a peaked roof. The house is currently upright and bent to the right like it got pushed.

The House includes three binocular double stars that form an equilateral triangle, with each pair facing the center. The brightest pair is Theta¹ and Theta² Tauri. You may find that you can resolve the Theta pair with your unaided eyes.

 

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This Week's Planet Roundup

Mercury remains hidden in the sunset for much of the week. But by Christmas Eve, look for it in early twilight some 9° below Venus as shown above. Next week Mercury will get easier every day.

Venus, brilliant at magnitude –4.7, shines low in the southwest in twilight. It's a dramatic crescent now in a telescope or even good binoculars, as it draws nearer to Earth and nearer to our line of sight to the Sun. This week we see its crescent enlarge from 50 to 55 arcseconds tall, while thinning from 13% to only 8% sunlit.

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A Naked-Eye Venus Challenge! Some people can resolve the crescent of Venus with their unaided eyes when it's this large. Mere 20/20 vision probably isn't good enough; success may await the eagle-eyed with 20/15, 20/12, 0r (rare) 20/10 vision. Try early in twilight before the sky becomes too dark and Venus too glary. Look long and carefully and please report your results to Sky & Telescope's Bob King, [email protected], as told in the May 2020 issue, page 49.

You may improve your chances by sighting through a clean, round hole in a stiff piece of paper about 1 mm, 2mm, or 3mm in diameter next to your eye (try them all). It will mask out optical aberrations that are common away from the center of your eye's cornea and lens. Try each eye.

One person who apparently succeeded was Edgar Allan Poe. An amateur astronomer since boyhood, he used a naked-eye sighting of Venus's crescent as the central event in his poem "Ulalume" (1847) near the end of his life. Before dawn, a bereaved lover roams a misty October woodland accompanied by "Psyche, my soul." Ahead of them low in the east, where the constellation Leo ascends before dawn in mid-autumn, they witness the new-risen Venus, star of romantic love in Roman mythology, coming "up through the lair of the Lion." Poe refers to the planet as Astarte — the wilder, more wanton Greek version of the Romans' Venus goddess:

And now, as the night was senescent,
And star-dials pointed to morn —
As the star-dials hinted of morn —
At the end of our path a liquescent
And nebulous lustre was born,
Out of which a miraculous crescent
Arose with a duplicate horn—
Astarte's bediamonded crescent
Distinct with its duplicate horn.

Poe contrasts its passionate brilliance to cooler, more composed Dian, the horned crescent Moon, and urges Psyche forward:

Let us on by this tremulous light!
Let us bathe in this crystalline light!

But Psyche, who knows better, is terrified, and this being Poe, the adventure doesn't end well.

Poe wrote "Ulalume" in the fall of 1847. Before dawn on November 4, 1847, a crescent Venus and crescent Moon ("Dian") indeed hung near each other low in the east below Leo — in Poe's "lair of the Lion," the sky area from which the traditional Leo figure stalks upward.

Venus was there a couple weeks earlier as well, during the mid- and late "lonesome October" of that most immemorial year, as a larger, thinner, more easily resolvable crescent, though Dian at that time was absent.

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Mars, far and faint at magnitude +1.6, is very low in the dawn this week far below high Arcturus. Mars is crossing upper Scorpius. Use binoculars to see that Mars is nearly between Delta and Beta Scorpii on the mornings of December 17th and 18th, and on the 19th that it's almost on top of the fainter Omega¹ - Omega² Scorpii pair.

Marslike Antares, meanwhile, is below or lower right of the planet; their separation closes from 8° to 5° this week.

Mars is on the far side of its orbit from us, so in a telescope it's just a tiny fuzzblob 4 arcseconds wide.

Jupiter, magnitude –2.2 at the Aquarius-Capricornus border, shines in the south-southwest at dusk about three fists at arm's length upper left of Venus. Look two fists lower left of Jupiter for Fomalhaut, magnitude +1.2.

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

Saturn sets by 7 or 8 p.m. Jupiter follows it down about 1½ hours 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) is less high in the south-southwest after dark.

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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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