This Week's Sky at a Glance, December 22 – 31

FRIDAY, DECEMBER 22

■ The waxing gibbous Moon jumped over Jupiter in the last 24 hours and now shines at the planet's left, as shown below. This scene is drawn for early evening while it's still twilight. Later, as the Moon and Jupiter move westward, the orientation of the scene rotates clockwise — like every sky pattern in the southern side of the sky as the hours pass. So, watch Jupiter swing down to the Moon's lower right. Keep an eye out, and by about 1 a.m. Jupiter hangs straight below the Moon.

SATURDAY, DECEMBER 23

■ The Moon is farther left of Jupiter now, off the left of the scene above. It's just a few degrees to the right of the Pleiades in early evening for North America. Binoculars will help pull in the Pleiades through the moonlight.

Watch the Moon draw closer to the Pleiades hour by hour. The Moon finally passes only about 1° or 2° south of them shortly before setting in the northwest around the first light of dawn Sunday morning the 24th, as shown below.

SUNDAY, DECEMBER 24

■ And now in early evening the bright gibbous Moon, two days from full, has jumped to the lower left (east) of the Pleiades. The Moon forms a flattened, nearly isosceles triangle with the Pleiades above and Aldebaran below.

Around 11 p.m. tonight the Moon shines just about as close to the zenith as you will ever see it (from your mid-northern latitude), giving "lustre of mid-day to objects below."

Your Moon shadow at that time will be as short as you'll ever see it. Go look. It's even a little shorter than your shortest Sun shadow, which happens at midday at the opposite solstice in June. Why? Because the Sun is always on the ecliptic (by definition), but the Moon is currently 3.5° north of the ecliptic. Go look.

The Moon will do practically the same close zenith pass about an hour later each night for the next few nights. The Moon will be full on the night of the 26th.

■ Sirius and Procyon in the balance. Sirius, the Dog Star, sparkles low in the east-southeast as evening advances. Procyon, the Little Dog Star, shines in the east about two fist-widths at arm's length to Sirius's left.

But directly left? That depends. 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. Your eastern horizon tilts differently with respect to the stars depending on your latitude. Because the Earth is round.

MONDAY, DECEMBER 25

■ This evening the Moon, a day from full, forms a wide, shallow triangle with bright Capella to its left or upper left and Aldebaran to its the right or upper right. The Moon is only a degree or two from Beta Tauri (El Nath), which at magnitude 1.6 just misses being a classed as a first-magnitude star.

The Moon and Beta Tauri cross very near your zenith around 1 a.m. tonight. The time depends somewhat on your location.

TUESDAY, DECEMBER 26

■ Full Moon tonight (exact at 7:33 p.m. EST). Now shining above the feet of Gemini in the evening, the Moon forms a deeper triangle with Capella and Aldebaran than it did last night.

The Moon passes its very closest to the zenith around 1 a.m. tonight. For the record, it goes just a trace closer to straight overhead than it did last night or the night before.

WEDNESDAY, DECEMBER 27

■ As the stars come out, face due north and look high. Cassiopeia is now a flattened M shape canted at about a 45° angle (depending on where you live). Hardly more than an hour later, the M has turned horizontal! Constellations passing near the zenith appear to rotate rapidly with respect to your direction "up."

THURSDAY, DECEMBER 28

■ Orion strides upward in the east-southeast after dinnertime. Above it shines orange Aldebaran, 1st magnitude, 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 if you count just the brightest stars.

The main Hyades stars famously form a V. It's currently lying on its side. Aldebaran forms the lower tip of the V.

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

FRIDAY, DECEMBER 29

■ 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 as a small, dim gray elongated glow 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.

SATURDAY, DECEMBER 30

■ The brightest asteroid in the sky, 4 Vesta, is just past opposition and creeping toward Zeta Tauri, the dimmer horntip of Taurus near the top of Orion's club. Binoculars will show Vesta easily at magnitude 6.5. But you'll need a fine-scale finder chart to tell it from all the other faint pinpoints in the area! Use the chart with Bob King's article Vesta Sets Sail Across Orion.

Vesta will pass 0.2° from Zeta Tau on the nights of January 7th and 8th. Then it will cruise about ½° south of the dim Crab Nebula, M1, on January 11th, 12th, and 13th.

SUNDAY, DECEMBER 31

■ After the noise and celebration at the turning of midnight tonight, step outside into the silent, cold dark. The waning gibbous Moon will be shining high in the east, with the Sickle of Leo floating about a fist at arm's length above it.

In the south Sirius will be shining at its highest, with the other bright stars of Canis Major to its right and below it.

Sirius is the bottom star of the bright, equilateral Winter Triangle. The others are Betelgeuse in Orion's shoulder to Sirius's upper right, and Procyon the same distance to Sirius's upper left. The Triangle now stands upright on Sirius, just about in balance.

And look west of the zenith for Perseus now heading down. If you're in the Mountain or Pacific time zones, you'll find Algol, the prototype eclipsing variable star, near its minimum brightness of magnitude 3.4 instead of its usual 2.1. Algol takes several additional hours to fade and to rebrighten. Use our comparison-star chart to judge its brightness all through the night.

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This Week's Planet Roundup

Mercury emerges into dawn view late this week. Starting around December 29th, look for it about three fists at arm's length lower left of Venus. On that morning Mercury is still only a difficult magnitude +1.3, but it's brightening and climbing daily.

Venus, magnitude –4.0 in Libra, shines as the bright "Morning Star" in the southeast before and during dawn. It's less high every week now. Closing in toward it from below is sparkly orange Antares, magnitude +1.0.

Mars is barely peeking over the eastern horizon in bright dawn, probably out of reach even with a telescope. It will very slowly get a little higher for the next five months!

Jupiter, magnitude –2.6 in Aries, is the bright white dot dominating the high southeast to south these evenings. It stands at its highest around 7 or 8 p.m. It has shrunk a little since opposition, but it's still a good 45 arcseconds wide in a telescope.

Saturn, magnitude +0.9 in Aquarius, is getting lower in the southwest just after dark. Don't confuse it with Fomalhaut, similarly bright, nearly two fists to Saturn's lower left. Saturn sets around 9 p.m.

Uranus, magnitude 5.6 in Aries, awaits your binoculars in the darkness 14° east (left) 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 22° east (upper left) of Saturn and is still 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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