Comet Catalina (c/2013 US10) with two tails on Dec 6, 2015, by Brian Ottum.
Comet Catalina displayed two widely separated tails on December 6, 2015. A comet's thin blue ion (gas) tail quickly becomes locked to the ionized solar wind and blows at high speed away from the direction of the Sun. The solid grains making up the yellow dust tail are less influenced by the Sun, move away from the comet much more slowly, and therefore follow along with the comet's orbital motion for longer. These two directions may or may not be similar. Brian Ottum took this image with a 10-inch f/5 Newtonian reflector from Rancho Hidalgo, New Mexico. Click for larger view.

Comet Catalina (C/2013 US10) remains at about magnitude 6.3 as it climbs higher in the east before dawn. It's unlikely to brighten much further, but it will move much higher before dawn in the next few weeks. See the December Sky & Telescope, page 45, or our article Get a Predawn Peek of Comet Catalina. Both provide finder charts.


Friday, December 18

• First-quarter Moon (exact at 10:14 a.m. EST). This evening the Moon shines more or less under the eastern (left) side of the Great Square of Pegasus. Can you see the Moon moving with respect to this line as the hours go by?

Saturday, December 19

• 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 sharp eyesight, a good dark sky, and dark-adapted eyes, you may be able to make out 8 or 9.

Sunday, December 20

• This is the time of year when Orion shines in the east-southeast after dinnertime. 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 is about to rise and twinkle furiously.

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

Monday, December 21

• Look above the gibbous Moon this evening for the two or three leading stars of little Aries. They're lined up more or less horizontally. A slightly greater distance left of the Moon are the Pleiades.

This is the longest night of the year in the Northern Hemisphere. South of the equator, it's the longest day. Northern winter, and southern summer, begin at the solstice: 11:48 p.m. EST (4:48 December 22nd Universal Time). This is when the Sun reaches its southernmost declination and begins its six-month return northward.

Moon and stars, Dec. 22-24, 2015
On its way to full, the Moon passes the Pleiades and Aldebaran. It's full on the night of Christmas Eve, north of Orion.

Tuesday, December 22

• The Moon shines in Taurus this evening, with the Pleiades to its upper left and Aldebaran to its lower left as shown here.

• Mars is passing 3° north of Spica tonight and tomorrow night. They rise in the east-southeast around 1 or 2 a.m. (far lower left of Jupiter) and shine high in the south by dawn.

Wednesday, December 23

• The Moon, nearly full, shines near orange Aldebaran tonight. The Moon occults (covers) Aldebaran for Europe and much of northern Asia. Watch the occultation via Gianluca Masi's Virtual Telescope webcast, starting at 17:45 UT (12:45 p.m. Eastern Standard Time).

Thursday, December 24

• Full Moon tonight (exact at 6:11 a.m. Christmas morning EST). The full Moon nearest the winter solstice rides higher across the night sky than any other full Moon of the year. It's highest of all around midnight, "giving luster of midday to objects below." (Moreover, tonight's full Moon is closer to perigee than average.) The Moon will be shining north of Orion's main pattern, near the top of Orion's dim club.

Friday, December 25

• 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 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.

• This evening the Christmas Moon, just a day past full, hangs in the east 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, with bright Capella.

Mercury in twilight, Dec. 26, 2015
By now Mercury is sparkling in good view low in the west-southwest in twilight. Higher Altair helps guide the way. They're 32° apart: about 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 nicely visible low in the west-southwest in mid-twilight, as shown here.

• 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.


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

Jupiter on Dec. 9, 2015
Jupiter's Great Red Spot is strikingly brick red this season, making it easier to see in a telescope than it has been for years. Christopher Go in the Philippines shot this image on December 9th at 20:07 UT, when the GRS had just crossed the central meridian. The System II longitude on the central meridian was 240°. South here is up. Note the white turbulence downwind of the Red Spot (to the right; celestial east; following), the long-lived small white ovals in the South Temperate Belt south of it, and the continuing narrowness, redness, and raggedness of the North Equatorial Belt. Blue festoons extend from the NEB into the white Equatorial Zone.

Mercury (magnitude –0.6) glimmers low above the southwest horizon as twilight fades. Look for it about 30 to 45 minutes after sunset. It's getting a little higher and brighter every day.

Saturn, Venus, Mars, and Jupiter form a long diagonal line in early dawn, running upward from southeast to south in that order.

Brilliant Venus and high Jupiter are now separated by almost 60°. Fainter Mars is between them, closer to Venus. Spot Spica a little below or lower left of Mars.

Most difficult is Saturn, far lower left of Venus: 23° from it on the morning of the 19th, 15° from it by the morning of the 26th.

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) and Neptune (magnitude +7.9, in Aquarius) are still in good view in the south and 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)



Image of mary beth

mary beth

December 20, 2015 at 6:38 pm

Whether we wake or sleep,
Whether we carol or weep,
The Sun with his Planets in chime,
Marketh the going of Time.
–Edward FitzGerald, English poet (1809–83)

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