This Week's Sky at a Glance, December 20 – 29

FRIDAY, DECEMBER 20

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

But directly left? Well, that depends. If you live around latitude 30° (Tijuana, Austin, New Orleans, Jacksonville), the two dog 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.

Why? Your eastern horizon tilts differently with respect to the stars depending on your latitude.

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

SATURDAY, DECEMBER 21

■ This is the shortest day of the year in the Northern Hemisphere; the longest day in the Southern Hemisphere. The solstice occurs at 4:20 a.m. EST, when the Sun reaches its farthest south declination and begins its six-month return northward. At sunrise I will be with a crowd of fellow oddballs from church singing up the Sun by a lakeside, just so it doesn't forget to begin its return. You can thank us for the coming spring and summer. Works every year.

SUNDAY, DECEMBER 22

■ How well do you really know the Orion's Belt region in binoculars? Can you piece out Orion's S, which begins and ends with the belt's two westernmost stars?

The Belt marks the rough center portion of the Orion OB1 Association, called OB1b. Are you aware of Orion OB1a and OB1c on either side of it? Binoculars are all you need, along with Matt Wedel's Binocular Highlight column and map in the January Sky & Telescope, page 43.

■ Last-quarter Moon (exact at 5:18 p.m. EST). The Moon rises around midnight, under the hind feet of Leo.

MONDAY, DECEMBER 23

■ The little Pleiades cluster shines very high in the southeast after dinnertime, no bigger than your fingertip at arm's length. How many Pleiads can you count with your unaided eye? Take your time and keep looking. Most people can count 6. With extra-sharp eyesight, a good dark sky, and a steady gaze, you may be able to make out 8 or 9.

TUESDAY, DECEMBER 24

■ Right around the end of twilight, face north and look very high. Cassiopeia is now a flattened M canted at an angle, with its left side highest (depending on where you live). Just two hours later, the M is horizontal! Constellations passing near your zenith appear to rotate rapidly with respect to your direction "up."

WEDNESDAY, DECEMBER 25

■ Tonight at 8:48 p.m. EST, be watching Jupiter in your telescope. Jupiter's moon Io will slowly reappear out of eclipse from Jupiter's shadow, just to the planet's east.

Europa looks on from farther east, while Ganymede and Callisto hang out on Jupiter's other side.

THURSDAY, DECEMBER 26

■ To the right of bright Jupiter shines orange Aldebaran, which comes with the large, loose Hyades cluster in its background. Binoculars are the ideal instrument for this cluster given its size: its brightest stars (magnitude 3.5 to 5) 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 form a V, lying on its side these evenings. 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 easy binocular double stars that form an equilateral triangle, with each pair facing the others. The brightest pair is Theta¹ and Theta² Tauri (the only members of the House that appear on the chart below). You may find that you can resolve the Theta pair with your unaided eyes.

FRIDAY, DECEMBER 27

■ Jupiter is nearly a month past its opposition. That means it's already fairly high in the east when you first catch sight of it through the fading twilight. How much later in twilight will you first see Aldebaran 6° to its right? And then the 3rd and 4th-magnitude stars of the Hyades V?

■ Right after dark, spot bright Venus in the southwest. Look just 1.1° to its left of for the 3rd-magnitude star Delta Capricorni. Binoculars will help. Although 3rd magnitude sounds easy naked-eye, Delta Cap is less than a thousandth as bright as Venus and less than a finger-width at arm's length from its glare!

■ As dawn brightens tomorrow morning, look very low in the southeast for the thin waning crescent Moon. Antares is only about 1° to the Moon's left, and brighter Mercury is 8° to its left, as shown below. Binoculars help.

Antares is magnitude +1.0, while Mercury is magnitude –0.3, three times brighter.

SATURDAY, DECEMBER 28

■ Now Delta Capricorni has moved to glimmer 1.1° below or lower left of Venus right after dark.

■ Unique lineup of Jupiter's satellites. If you're in the Eastern time zone, early this evening Jupiter's satellites will look seriously out of kilter. We're used to seeing them line up on either side of Jupiter in a more or less straight line with it, like beads on a wire. But at 6:34 p.m. EST, you can catch Callisto, Europa, and Io forming a very straight line that's canted way out of whack, aiming away from Jupiter entirely!

Such things can happen because the plane of Jupiter's orbit is currently tipped slightly to our line of sight, allowing unusual perspectives to be presented to us. See Bob King's More Unusual Jovian Satellite Lineups.

SUNDAY, DECEMBER 29

■ As the year nears its end, Orion has fully come into its own. He's striding up the east-southeastern sky as soon as it gets dark, with his three-star Belt is nearly vertical. Left of the Belt is orange Betelgeuse and right of the Belt is bright white Rigel, supergiants both.

The Belt points up toward Aldebaran and Jupiter and, even higher, the Pleiades. In the other direction, it points down to where Sirius rises shortly after twilight's end.

This Week's Planet Roundup

Mercury, magnitude –0.3, is having a one of its better apparitions in the dawn. Look for it low in the southeast about 60 to 45 minutes before sunrise. Late in the week, look too for twinklier orange Antares about 8° to Mercury's lower right or right. Antares is more than a magnitude fainter. The thin Moon poses close to it on the morning of December 28th, as shown in the scene above. Binoculars will help.

Venus (magnitude –4.4, in Capricornus) is the "Evening Star" shining high in the southwest in twilight. It's now high enough to remain up for a good two hours after dark before setting.

Get your telescope on it early in twilight. Venus is still a bit gibbous, about 58% sunlit, on its way to apparent dichotomy (seemingly half lit) around the turn of the new year. Venus is also growing as it swings toward us in its faster orbit; it now measures 21 arcseconds from pole to pole.

After dark on December 27th and 28th, look for 3rd-magnitude Delta Capricorni just 1.1° to Venus's left or lower left. Again, binoculars will help; Delta Cap is not quite a thousandth as bright!

Mars (about magnitude –1.0, in Cancer) rises in the east-northeast shortly after the end of twilight. Watch for it to come up below Castor and Pollux. Once it's high up, use binoculars to look for M44, the Beehive star cluster, 4° or 5° below it.

Mars shows best in a telescope when very high toward the southeast or south: by late evening or midnight. It has enlarged to 14 arcseconds in apparent diameter — practically as large and bright as it will appear (14.6 arcseconds) for the couple of weeks around its opposition January 15th.

Jupiter, not long past its own opposition, shines at a bright magnitude –2.7 in Taurus. It dominates the eastern sky during evening, with fainter orange Aldebaran to its right and the Pleiades at least twice as far above it or to its upper right.

Bright Capella shines even farther to Jupiter's left or upper left.

Jupiter is at its telescopic best when very high toward the southeast or south by 8 or 9 p.m. It's still 47 arcseconds wide.

Saturn, magnitude +1.0 in Aquarius, glows high in the south-southwest after dark, upper left of Venus and closing in on it fast. Saturn is 28° from Venus on December 20th and 21° from Venus by the 27th. Watch them continue to approach each other toward their conjunction on January 18th, when they will pass each other by 2.2°.

The two form a big triangle with Fomalhaut (the same brightness as Saturn) lower left of Saturn and directly left of Venus. Watch the triangle change shape night to night. On Christmas night the triangle will be its closest to perfectly equilateral.

Uranus (magnitude 5.6, at the Taurus-Aries border) is high in the southeast during evening, about 7° from the Pleiades. You'll need a good finder chart to tell it from the similar-looking surrounding stars; see the November Sky & Telescope, page 49.

Neptune (tougher at magnitude 7.9, under the Circlet of Pisces) is high in the south after dark, 13° east of Saturn. Again you'll need a sufficient finder chart.

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.

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. Both show all 30,000 stars to magnitude 7.6, and 1,500 deep-sky targets — star clusters, nebulae, and galaxies — to search out among them.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many, as well as many more deep-sky objects. It's currently out of print, but maybe you can find one used.

The next up, once you know your way around well, are the even larger Interstellarum atlas (201,000+ stars to magnitude 9.5, and 14,000 deep-sky objects selected to be detectable by eye in large amateur telescopes), andUranometria 2000.0 (332,000 stars to mag 9.75, and 10,300 deep-sky objects). 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. The pinnacle for total astro-geeks is the new Annals of the Deep Sky series, currently at 11 volumes as it works its way forward through the constellations alphabetically. So far it's up to H.

Can computerized telescopes replace charts? Not for beginners I don't think, and not for scopes 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 how to explore the sky. 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 and a curious mind." Without these, "the sky never becomes a friendly place."

If you do get a computerized scope, make sure that its drives can be disengaged so you can swing it around and point it readily by hand when you want to, rather than only slowly by the electric motors (which eat batteries).

However, finding faint telescopic objects the old-fashioned way with charts isn't simple either. Do learn the essential tricks at How to Use a Star Chart with a Telescope.

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.

"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