This Week’s Sky at a Glance, December 25 – January 2

Comet Catalina (C/2013 US10) remains about 6th magnitude very high in the eastern sky before the first light of dawn. It's unlikely to brighten much further. On the morning of January 1st it passes barely ½° west of Arcturus, some 250 times brighter. See the December Sky & Telescope, page 45, or Bob King's article online. Both provide finder charts.


Friday, December 25

• This evening the Christmas Moon, a day past full, hangs in Gemini. Look for Castor and Pollux, the heads of the Twins, to its left. Very high above Castor and Pollux shines the constellation Auriga, featuring bright Capella.

• Merry Sol Invictus! In late Roman times this date was celebrated as Dies Natalis Solis Invicti, the Birthday of the Unconquered Sun — when the Sun, after bottoming out at the solstice, began to recover from its long decline with the hopeful promise, in the cold and the dark, of a new spring and summer to come.

Mercury in twilight, Dec. 26, 2015

This week Mercury sparkles in nice view low in the west-southwest in twilight. Higher Altair helps guide the way. They're about 30° apart: three fists at arm's length. (The visibility of the fainter stars in twilight is exaggerated.)

Saturday, December 26

• If you haven't caught Mercury yet this apparition, look now. It has become quite visible low in the west-southwest in mid-twilight, as shown here. It remains in fine view all this week.

• Tonight the Moon, moving ever eastward, shines roughly between Castor and Pollux to its upper left and Procyon to its lower right.

Sunday, December 27

• 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 upraised knee of the Andromeda constellation's stick figure; see the big evening constellation chart in the center of Sky & Telescope. The Moon this evening rises about an hour after the galaxy culminates.

Moon and Jupiter at dawn, Dec. 29-31, 2015

If you're up in the early dawn, watch the Moon's eastward march toward Jupiter beneath Leo.

Monday, December 28

• The waning gibbous Moon is up by 9 or 10 tonight, awaiting your telescope. Lower left of the Moon twinkles Regulus. The Sickle of Leo extends upper left from Regulus. By early dawn they're high in the southwest and turned around with the Sickle over the Moon, as shown below.

Tuesday, December 29

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

If you live around latitude 30° (Tijuana, New Orleans, Jacksonville), these 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.

Wednesday, December 30

• The Moon, nearly last quarter, rises due east by about 11 tonight. Jupiter follows it up about 25 minutes later (for the Americas). They climb higher through the rest of the night, gradually drawing closer together. They stand high in the south under Denebola before the first light of dawn on the 31st, as shown above.

Thursday, December 31

• After the noise and cheering at midnight tonight, step outside into the silent dark. The last-quarter Moon will be newly risen in the cold east — or if you're near the western edge of your time zone, it may still be about to rise. Jupiter shines above it.

Higher in the south, Sirius will be shining brightly. It's now the balance point at the bottom of the Winter Triangle. The triangle's other two stars are Betelgeuse in Orion's shoulder to its upper right, and Procyon equally far to its upper left.

• Before dawn on the morning of January 1st, Comet Catalina passes barely ½° west of Arcturus. The comet is about 6th magnitude; Arcturus is some 250 times brighter. See the December Sky & Telescope, page 45, or Bob King's article online. Both provide finder charts.

Friday, January 1

• By midevening at this time of year, the Great Square of Pegasus balances on one corner high in the west. The vast Andromeda-Pegasus constellation complex runs all the way from near the zenith (Andromeda's foot) down through the Great Square (Pegasus's body) to somewhat low in the west (Pegasus's nose).

Saturday, January 2

• Dawn and sunrise happen at their latest for the year this week (in the mid-northern latitudes). Look low in the southeast early in the dawn for bright Venus. It heads up up a triangle with Saturn to its lower left and twinkly Antares lower down. And on Sunday morning the 3rd, the Moon high to its upper right shines with Mars and Spica.

• Earth is at perihelion, its closest to the Sun for the year: only 1 part in 30 closer than at aphelion in July.


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 essential guide to astronomy.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown is the new Jumbo Edition for easier reading in the night. Click image for larger view.

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 new 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. Next up, once you know your way around, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide 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). 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."

This Week's Planet Roundup

Venus, Saturn, and Antares at dawn, Dec. 31, 2015.

The Venus-Saturn Antares triangle at dawn. Watch it change shape from one morning to the next. . .

Moon, Mars, and Spica at dawn, Jan 3, 2015

. . .and catch the Moon between Spica and Mars early in the dawn of Sunday the 3rd.

Mercury (magnitude –0.6) remains low in the southwest as twilight fades. Look for it about 30 to 60 minutes after sunset.

Saturn, Venus, Mars, and Jupiter form a huge diagonal line in early dawn, running upward from the low southeast to high in the south-southwest, in that order.

Brilliant Venus and high Jupiter are now separated by almost 70°. Fainter Mars is between them. Spot Spica 6° to Mars's right. They're similar in brightness but different in color.

Most difficult is Saturn, to the lower left of Venus: 15° from it on the morning of the 26th, 8° from it by the morning of January 2nd. If you can see Saturn, look for twinkly Antares to its lower right more directly below Venus (as seen from mid-northern latitudes). See the picture at right. Venus and Saturn are on their way to a close conjunction January 9th.

Brightnesses: Saturn shines at magnitude +0.5, Venus –4.1, Jupiter –2.1, Mars +1.3, and Spica +1.0.

Uranus (magnitude +5.8, in Pisces) is still in fine view high in the south right after dark.

Neptune (magnitude +7.9, in Aquarius) is getting lower in the southwest immediately after dark. Finder charts for Uranus and Neptune.


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 that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.


"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."

— Isaac Newton, 1642–1727

(From the Memoirs of the Life, Writings, and Discoveries of Sir Isaac Newton, David Brewster, 1855)