This Week's Sky at a Glance, December 15 – 24

FRIDAY, DECEMBER 15

■ The "Summer" Triangle is sinking lower in the west, and Altair is the first of its stars to go (for mid-northern skywatchers). Start by spotting bright Vega, magnitude zero, in the northwest right after dark. The brightest star above Vega is Deneb. Altair, the Triangle's third star, 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?

If you live as far south as Miami, Altair and Vega set at the same time. Seen from farther north than that, Altair is the first to go.

SATURDAY, DECEMBER 16

■ Have you ever watched a Sirius-rise? Find an open view right down to the east-southeast horizon, and watch for Sirius to rise about two fists at arm's length below Orion's vertical belt. Sirius rises sometime around 8 p.m. now, depending on your location.

About 15 minutes before Sirius-rise, 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 them; the second-magnitude Announcer is only a twentieth as bright as the king of stars about to make his royal entry.

When a star is very low it tends to twinkle slowly, and often in vivid colors. Sirius is bright enough to show these effects well, especially with binoculars.

SUNDAY, DECEMBER 17

■ This evening, spot Saturn about 3° upper right of the Moon as shown above. The Moon is 1.3 light-seconds from us. Saturn is currently 84 light minutes away in the background, almost 4,000 times farther from us.

■ Algol dips to its minimum brightness, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 6:17 p.m. EST. Algol takes several additional hours to rebrighten. Comparison-star chart.

MONDAY, DECEMBER 18

■ This is the time of year when Orion shines in the east-southeast after dinnertime, walking upward. He's well up now, but his three-star Belt is still 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 8 p.m. to twinkle furiously.

TUESDAY, DECEMBER 19

■ First-quarter Moon (exact at 1:39 p.m. EST). Upper right of it after dinnertime is the Great Square of Pegasus, starting to tilt up on one corner. The Square's upper left edge points diagonally down nearly at the Moon.

WEDNESDAY, DECEMBER 20

■ The Moon is approaching Jupiter night by night, as shown below.

To the right of the Moon this evening (outside the frame) is the Great Square of Pegasus. The line between two opposite corners of the Great Square now points roughly at the Moon.

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

THURSDAY, DECEMBER 21

■ Now Jupiter is only about 7° left of the Moon in early evening. Watch then draw closer together through the night.

■ Tonight is the longest night of the year (in the Northern Hemisphere); the solstice is at 10:27 p.m. EST. That's when the Sun stands still in declination, then begins its six-month northward return toward the summer solstice next June 20th.

FRIDAY, DECEMBER 22

■ The Moon has passed Jupiter in the last 24 hours. The bright planet now shines to the Moon's right at dusk; upper right of it later.

SATURDAY, DECEMBER 23

■ The waxing gibbous Moon is about 4° or 5° to the right of the Pleiades in early evening (for North America). Binoculars help with the Pleiades through the moonlight.

Watch the Moon draw closer to them hour by hour, finally passing only about 1° or 2° south of them shortly before setting in the northwest around the first light of dawn Sunday morning.

SUNDAY, DECEMBER 24

■ And now the bright gibbous Moon, two days from full, has jumped to the lower left (east) of the Pleiades in early evening. It forms a flattened, nearly isosceles triangle with the Pleiades above it 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), "casting lustre of midday on objects below." Your Moon shadow at that time will be the shortest you'll ever see it. Go look.

The Moon will do very nearly the same close zenith pass about an hour later each night for the next several 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.

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This Week's Planet Roundup

Mercury is "combust," to use an old astronomical term: out of sight in the glare of the Sun.

Venus, magnitude –4.1 in Libra, shines as the bright "Morning Star" in the southeast before and during dawn. It's not as high as it was a month or two ago.

Venus rises above the east-southeast horizon about 1½ hours before dawn's first light. Watch for it to come up three or four fists at arm's length to the lower right of Arcturus, the brightest star twinkling high in the east.

Mars is out of sight behind the glare of the Sun.

Jupiter, magnitude –2.7 in Aries, is the bright white dot dominating the high southeast to south these evenings. It stands at its highest in the south around 8 p.m. It has shrunk a little since opposition, but it's still a good 46 arcseconds wide in a telescope.

Saturn, magnitude +0.9 in Aquarius, glows yellowish lower in the south-southwest just after dark. Fomalhaut, similarly bright, twinkles roughly two fists at arm's length to Saturn's lower left. Saturn declines toward the southwest as evening progresses and sets around 9 or 10 p.m. So get your telescope on it early!

Uranus, magnitude 5.6 in Aries, awaits your binoculars in the darkness 14° east 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 23° east 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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