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


Comet Leonard (C/2021 A1) could be very low within the west-southwest towards the tip of night twilight; use binoculars or a wide-field telescope. You’ll have to catch the slim time window, if any, between the sky being too shiny and the comet getting too low; 60 to 75 minutes after sundown is likely to be finest relying in your latitude.

Finding the best location, at least, can be simple! Venus and Saturn information you to the right spot of sky to look at; use the finder chart at the underside of See Comet Leonard at its Best. The dates of the comet symbols there are Universal Time dates; subtract at some point to get the North American civil date within the night.

This night the comet’s location is very simple to seek out. Just look 5° under Venus! (for the mid-latitudes of North America.) That’s a bit lower than the width of a typical binocular’s area of view.

■ Orion strides up away from the east-northeast horizon quickly after darkish now. By 9 p.m. he is excessive within the southeast in full wintry glory.

■ Have you ever watched a Sirius-rise? Find an open view proper right down to the east-southeast horizon. Watch there for Sirius to come back up about two fists at arm’s size under Orion’s vertical Belt. Sirius will rise someday round 8 p.m. now relying in your location.

About quarter-hour beforehand, a lesser star comes up barely to the best of there: Beta Canis Majoris or Mirzam. Its title means “the Announcer,” and what Mirzam publicizes is Sirius. You’re not prone to mistake the 2; the second-magnitude Announcer is simply a twentieth as shiny because the King of Stars about to make its entry.

When a star could be very low it tends to twinkle slowly, usually in vivid colours. Sirius is shiny sufficient to show these effects well, particularly with binoculars.

Moon passing Pleiades and Aldebaran in Taurus, Dec. 16-18, 2021
The Moon crosses Taurus because it waxes to full on the 18th.


■ Full Moon tonight (precisely 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 proven above. It’s at apogee, making this a “mini-moon,” very barely smaller than common: the other of a “supermoon” at perigee.

By midnight the Moon could be very excessive within the south, not removed from the zenith. The full Moon of the Christmas season rides greater throughout the sky at midnight than at some 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 within the sky. So, that is when the total Moon (reverse the Sun) is farthest north. In making its way throughout the evening sky, it behaves as a pale, chilly imitation of the June Sun six months in the past.


■ Venus and Jupiter this week proceed to shine brightly throughout twilight within the southwest and south-southwest, respectively. But each are getting decrease now, Venus particularly. As twilight fades, look ahead to dimmer Saturn rising between them.

Jupiter and Saturn are far previous opposition, so do not be disillusioned by the less-than-optimal telescopic views. Venus, then again, is enlarging whereas turning into a thinner, extra dramatic crescent. It will proceed to enlarge and skinny into the start of January.

Have you ever tried to discern the crescent of Venus along with your bare eyes? Just a few sharp-eyed folks can! One was apparently Edgar Allan Poe; see This Week’s Planet Roundup at the underside of this web page.


■ Around the tip of twilight, face north and look excessive. Cassiopeia is now a flattened M form canted at about a 45° angle (relying on the place you reside). Just two hours later, the M is horizontal! Constellations passing close to the zenith seem to rotate quickly with respect to your route “up.”

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


■ The solstice marks the official begin of winter at 10:59 a.m. EST (15:59 UT). Last evening and tonight are the longest nights of the year within 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 decrease within the west because the season turns, and Altair is the primary of its stars to go (as seen from mid-northern latitudes). Start by recognizing shiny Vega, magnitude zero, the brightest star within the northwest proper after darkish. The brightest one above it’s Deneb. The Triangle’s third star, Altair, is farther to Vega’s left or decrease left. How late into the evening, and into the advancing season, can you retain Altair in view?


■ This is the time of year when M31, the Andromeda Galaxy, passes your zenith quickly after darkish (should you stay within the mid-northern latitudes). The precise time will depend on your longitude. Binoculars will present M31 simply off the knee of the Andromeda constellation’s stick determine; see the massive night constellation chart within the heart of Sky & Telescope.

Lie in your again with binocs and look straight up. M31 passes precisely by way of your zenith should you’re at 41° north latitude.


■ Left of Orion after dinnertime is the constellation Gemini. Castor and Pollux, the top stars of the Gemini stick-figure twins, are at its left finish; the twins are mendacity on their sides. Castor is the fainter and better one.


■ The Jupiter-Saturn-Venus line at nightfall slowly continues to evolve. Saturn is now a little nearer to Venus; a month in the past it was nearer to Jupiter. And the entire line is sliding farther to the decrease proper.

And the road is now rising longer once more, not shorter. If you had been hoping Jupiter was on its way to a conjunction with Venus, nope. Venus is now diving away again down towards the Sun (it passes the Sun in daytime on January eighth). As for Jupiter and Saturn, their very own subsequent conjunction would not come till November 2, 2040.

Jupiter has moved some 6° to the higher left Third-magnitude Delta Capricorni, whereas Saturn has migrated almost to the middle of Capricornus’s dim boat-shape. And are you able to catch Mercury under Venus but? Look early!


■ High above Orion shines orange Aldebaran with the massive, unfastened Hyades cluster in its background. Binoculars are the perfect instrument for this cluster given its measurement: its brightest stars (4th and fifth magnitude) span an space about 4° extensive. Higher above, the Pleiades are hardly greater than 1° throughout counting simply the brightest stars.

The major Hyades stars famously kind a V. It’s presently mendacity on its aspect, open to the left. Aldebaran types the decrease of the V’s two ideas.

With binoculars, observe the decrease department of the V to the best from Aldebaran. The very first thing you come to is the House asterism: a sample of stars like a kid’s drawing of a home with a peaked roof. The home is presently upright and bent to the best prefer it received pushed.

The House consists of three binocular double stars that kind an equilateral triangle, with every pair dealing with the middle. The brightest pair is Theta1 and Theta2 Tauri. You could discover which you can resolve the Theta pair along with your unaided eyes.


This Week’s Planet Roundup

Mercury stays hidden within the sundown for a lot of the week. But by Christmas Eve, search for it in early twilight some 9° under Venus as proven above. Next week Mercury will get simpler on daily basis.

Venus, sensible at magnitude –4.7, shines low within the southwest in twilight. It’s a dramatic crescent now in a telescope and even good binoculars, because it attracts 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, whereas thinning from 13% to solely 8% sunlit.

A Naked-Eye Venus Challenge! Some folks can resolve the crescent of Venus with their unaided eyes when it is this huge. Mere 20/20 imaginative and prescient most likely is not ok; success could await the eagle-eyed with 20/15, 20/12, 0r (uncommon) 20/10 imaginative and prescient. Try early in twilight earlier than the sky turns into too darkish and Venus too glary. Look lengthy and thoroughly and please report your outcomes to Sky & Telescope‘s Bob King, [email protected], as advised within the May 2020 concern, web page 49.

You could enhance your possibilities by sighting by way of a clear, spherical hole in a stiff piece of paper about 1 mm, 2mm, or 3mm in diameter subsequent to your eye (attempt all of them). It will masks out optical aberrations which are widespread away from the middle of your eye’s cornea and lens. Try every eye.

One one who apparently succeeded was Edgar Allan Poe. An novice astronomer since boyhood, he used a naked-eye sighting of Venus’s crescent because the central occasion in his poem “Ulalume” (1847) close to the tip of his life. Before daybreak, a bereaved lover roams a misty October woodland accompanied by “Psyche, my soul.” Ahead of them low within the east, the place the constellation Leo ascends earlier than daybreak 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 back to the planet as Astarte — the wilder, extra wanton Greek model of the Romans’ Venus goddess:

And now, because the evening was senescent,
And star-dials pointed to morn —
As the star-dials hinted of morn —

At the tip 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 compares its passionate brilliance to cooler, extra composed Dian, the horned crescent Moon, and urges Psyche ahead:

Let us on by this tremulous gentle!
Let us bathe on this crystalline gentle!

But Psyche, who is aware of higher, is terrified, and this being Poe, the journey would not finish properly.

Poe’s narrator peering at a pre-dawn crescent Venus, “distinct with its duplicate horn,” as Psyche, his winged soul, begs him away. Studies by Dante Gabriel Rossetti, circa 1847–1848. The figures on the best are a preliminary sketch, adopted by the extra highly effective model on the left.

Poe wrote “Ulalume” within the fall of 1847. Before daybreak on November 4, 1847, a crescent Venus and crescent Moon (“Dian”) certainly hung close to one another low within the east under Leo — in Poe’s “lair of the Lion,” the sky space from which the normal Leo determine stalks upward.

Venus was there a couple weeks earlier as properly, in the course of the mid- and late “lonesome October” of that almost all immemorial year, as a bigger, thinner, extra simply resolvable crescent, although Dian at that point was absent.

Mars, far and faint at magnitude +1.6, could be very low within the daybreak this week far under excessive Arcturus. Mars is crossing higher Scorpius. Use binoculars to see that Mars is sort of between Delta and Beta Scorpii on the mornings of December seventeenth and 18th, and on the nineteenth that it is nearly on prime of the fainter Omega1 – Omega2 Scorpii pair.

Marslike Antares, in the meantime, is under or decrease proper of the planet; their separation closes from 8° to five° this week.

Mars is on the far aspect of its orbit from us, so in a telescope it is simply a tiny fuzzblob 4 arcseconds extensive.

Jupiter, magnitude –2.2 at the Aquarius-Capricornus border, shines within the south-southwest at nightfall about three fists at arm’s size higher left of Venus. Look two fists decrease left of Jupiter for Fomalhaut, magnitude +1.2.

Saturn, in Capricornus, is sort of halfway between Jupiter and Venus. At magnitude +0.7, Saturn is just one/15 Jupiter’s brightness.

Saturn units by 7 or 8 p.m. Jupiter follows it down about 1½ hours later.

Uranus (magnitude 5.7, in Aries north of the top of Cetus) could be very excessive within the southeast in early night. See Bob King’s story and finder chart.

Neptune (magnitude 7.9, in Aquarius) is much less excessive within the south-southwest after darkish.

All descriptions that relate to your horizon — together with the phrases up, down, proper, and left — are written for the world’s mid-northern latitudes. Descriptions and graphics that additionally rely on longitude (primarily Moon positions) are for North America.

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

Want to develop into a higher astronomer? Learn your way across the constellations. They’re the important thing to finding all the things fainter and deeper to hunt with binoculars or a telescope.

This is an outside nature passion. For an easy-to-use constellation information masking the entire night sky, use the massive month-to-month map within the heart of every concern of Sky & Telescope, the journal of the American Astronomical Society.

Once you get a telescope, to place it to good use you may want a detailed, large-scale sky atlas (set of charts). The primary commonplace is the Pocket Sky Atlas (in both the original or Jumbo Edition), which reveals stars to magnitude 7.6.

Pocket Sky Atlas cover, Jumbo edition
The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and a whole lot of telescopic galaxies, star clusters, and nebulae amongst them. Shown right here is the Jumbo Edition, which is in arduous covers and enlarged for simpler studying open air by red flashlight. Sample charts. More about the current editions.

Next up is the bigger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; almost 3 times as many. The subsequent up, as soon as you realize your way round, are the even bigger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And make sure you learn How to Use a Star Chart with a Telescope.

You’ll additionally need a good deep-sky guidebook, similar to the massive Night Sky Observer’s Guide set (2+ volumes) by Kepple and Sanner.

Can a computerized telescope exchange charts? Not for learners, I do not assume, and never on mounts and tripods which are lower than top-quality mechanically, which means heavy and costly. And as Terence Dickinson and Alan Dyer say of 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.”

Audio sky tour. Out below the night sky along with your
earbuds in place, hearken to Kelly Beatty’s month-to-month
podcast tour of the heavens above. It’s free.

“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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