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December: change is in the air as the year draws to a close. The king of planets, Jupiter, is slowly relinquishing his celestial throne to the fiery warrior planet, Mars.

Jupiter Waning; Mars Growing

We begin with the exciting news that December 2009 contains two full Moons: on the 1st and again on the 31st. The second full Moon in a month is often called a Blue Moon, though this one might be tinted a bit red since it coincides with partial lunar eclipse. Sadly, the eclipse won't be visible from North America. Still, we will be able to ring in the new year by the light of a full moon!

Meanwhile, Mercury emerges from the solar glare during the second week of December. This will be a poor apparition for mid-northern observers since the elusive planet is currently mired in the deep southern zodiac. Mercury will reach greatest elongation (20[degrees] east of the Sun) on the evening of the 18th. The -0.5-magnitude "star" will stand 12[degrees] above the southwestern horizon at sundown and set about 80 minutes later. The bonus that night will be a 2-day-old Moon hanging several degrees above-left of Mercury. On the 21st, Mercury's disk will be exactly half-lit and 7.3" wide. One week after greatest elongation, the little planet's altitude at sunset will remain almost as high and by then its disk will have grown into an 8.2" crescent. It will be swallowed by sunlight soon after.


Don't delay if you want to observe Jupiter. As the month opens, the evening sky's brightest (magnitude -2.2) and biggest (37" diameter) planet is past the meridian at dusk and sets at around 10 p.m. Jupiter is drifting eastward through eastern Capricornus and on the 6th forms a right-angle triangle with Delta ([delta]) and Gamma ([gamma]) Capricorni, the latter marking the triangle's vertex. On the 18th, the triangle is again perfectly shaped but this time with Delta marking the vertex. During this period, Jupiter catches up to slower-moving Neptune and hangs about 1/2[degrees] directly south of the 7.9-magnitude pale-blue planet on the 19th. The pair remain within 1[degrees] of each other from the 15th to the 25th. Watch the waxing crescent Moon drift by on the 20th and 21st.

While Jupiter is on the wane, Mars is slowly but steadily growing in prominence. Its brightness grows impressively from magnitude 0 as the month opens to nearly magnitude -1 by New Year's Eve. And it's returning to the evening sky, too. At mid-month the planet clears the horizon in western Leo before 9 p.m. and is high in the south during the pre-dawn hours. On the 22nd, Mars is stationary among the stars and subsequently begins retrograding back toward Cancer. On the last night of the year, the planet rises before 8 p.m. Its nearly full, 13"-wide disk will be well placed for telescopic viewing by midnight--unless you have other activities planned for that hour. For more Martian details, see the Close-up on page 92.

On December 15th, Saturn rises shortly after midnight and approaches the meridian at dawn. The planet shines at magnitude 1.0 in western Virgo. Its oblate disk is 17" across and the rings stretch 38" from tip to tip. The rings are inclined by 4.7[degrees] at mid-month, which is slightly greater than back in mid-May. The telescopic views will certainly be enjoyable.

By the way, tiny Vesta makes another close approach to a star in Leo this month. During the pre-dawn hours of the 12th, the 7.8-magnitude asteroid will be just 1/2[degrees] south of 5.7-magnitude 46 Leonis. You'll find the pair a little more than 6[degrees] east-northeast of Regulus.

Finally, Venus is not easily visible in December. The Morning Star of recent months will reach superior conjunction with the brightest star in the sky (the Sun) on January 11, 2010.

Sampling the December Deep Sky

Bow to the sky's royal couple, Queen Cassiopeia and King Cepheus, who hold court in the northern sky near Polaris. Let's sample some of the deep-sky treasures that enrich their realm.

We begin our treasure hunt in Cassiopeia with a pair of clusters 6[degrees] apart. The first is M52 a mere 3/4[degrees] south of orangey, 5th-magnitude 4 Cassiopeiae. That star shines 6 1/4[degrees] northwest of 2.3-magnitude Beta ([beta]) Cass, the westernmost star in Cassiopeia's eye-catching W form.

Almost 5,000 light-years from Earth, M52 is detectable in binoculars and an easy target for small scopes. The 6.9-magnitude cluster is 1/4[degrees] across, slightly oblong, and clearly detached from the surrounding Milky Way. M52 contains upwards of 200 stars that cover a wide range of brightness and are mostly bluish, indicating that this is a fairly young group. The conspicuous 8th-magnitude reddish sun in M52's southwestern outskirts is not a true member.




Nearly 3[degrees] southwest of Beta is NGC 7789. This fine-textured cluster is about 70% farther away than M52 yet is listed as being a hair brighter. But the cluster's 6.7-magnitude is misleading. Although NGC 7789 packs three times as many stars as M52, most of them are faint and their collective glow is spread over a comparatively wide area. Consequently, NGC 7789 appears significantly dimmer than M52. Another difference is that the somewhat yellowish suns of NGC 7789 have been shining for a couple of billion years--much longer than those of M52.


Viewed in a dark sky with binoculars, NGC 7789 resembles a tailless comet adrift in the Milky Way. But telescopes will resolve the "comet" into a wonderfully rich veil of stars. In mid-size reflectors NGC 7789 displays about a dozen brighter stars set against a faint, uniform powder cut by several narrow voids. The outskirts blend seamlessly into the Milky Way background. This is a subtle, delicate cluster that looks best at low power. It is also one of the prettiest non-Messier clusters.

Now let's move westward into Cepheus. Cepheus is hardly a standout, especially in a city sky. Its main pattern comprises mostly 3rd- and 4th-magnitude stars in the shape of a cartoon house with a steep roof. On December evenings, you'll find the house lying on its side with its roof slightly above the North Star.

Hiding in the basement of the Cepheus house is IC 1396. More than 1[degrees] wide, this coarse assembly materializes in small scopes at low power as a few dozen stars, 6th-magnitude and fainter, thinly scattered across the field of view. At its center is the triple star Struve 2816 ([SIGMA]2816). Its 5.7-magnitude primary (the brightest member of IC 1396) is flanked by two 8th-magnitude companions, one 20" north-northwest and the other 12" southeast. Nearby is the binary Struve 2819 (2819 [pounds sterling]), whose 7.4- and 8.6-magnitude components are separated by 13". The entire cluster is embedded in extremely tenuous nebulosity that is best attempted under a pristine sky with a narrowband UHC or O III filter.

Just northwest of IC 1396 is the variable star Mu [MU] Cephei. A red supergiant, Mu is celebrated for its deep ruby hue. It was dubbed the Garnet Star by 18th century English astronomer William Herschel. Mu fluctuates erratically between 3rd- and 5th-magnitude over a period of about two years. You probably won't be aware of its wavering light output, but the color of the Garnet Star is delightfully obvious in any optics any clear night of the year.

Drifting further northwest brings us to the main floor of Cepheus. There you might spot tiny NGC 7160. Your finderscope should pick it up as two 7th-magnitude stars 10' apart. In a telescope powered up to around 50x, the southeastern member of the pair becomes another double, called South 800 (S800), with 7.1- and 7.9-magnitude components 1' apart. South 800 highlights a straggly line of seven 9th- and 10th-magnitude stars (plus several much fainter ones) spanning 6'. NGC 7160 isn't much of a cluster, but it's easy to find.

Our final treasure lies a further 2V3[degrees] from Mu. Xi ([xi]) Cephei is a splendid binary with a 4.4-magnitude white primary and a 6.4-magnitude blue secondary 8" apart. A small scope at about 40x will easily resolve this pretty pair.


Early November: 10 p.m.

Late November: 9 p.m.

Early December: 8 p.m.

Late December: 7 p.m.



Dec 4 Moon at perigee (225,855 mi).

Dec 14 Geminid meteor shower peaks during the pre-dawn hours.

Dec 18 Mercury at greatest elongation in the evening sky.

Dec 20 Moon at apogee (251,488 mi).

Dec 21 Winter solstice.

Dec 24 Pluto in conjunction with the Sun.

Dec 1 Full Moon

Dec 9 Last Quarter

Dec 16 New Moon

Dec 24 First Quarter

Dec 31 Full Moon


December Meteors

The Geminid meteor shower, nearly always powerful, will be in its glory this year since the Moon will be completely absent from the night sky. Depending on your observing conditions, you might catch several dozen Geminid meteors per hour when the shower reaches maximum strength on the night of December 13th-14th. The peak of activity occurs around 2 a.m. on the 14th, with the shower's radiant (near 1.6-magnitude Castor) well up.


M42, the Orion Nebula

The 42nd entry in the Messier catalog, the Orion Nebula has earned top billing among stargazers. Not only is it plainly visible in binoculars, M42 is one of only a handful of deep-sky objects readily visible without any optics. On December nights, the nebula is a midnight treat. To see it high in the evening sky, you have to wait until February. M42 is easy to locate: it's the fuzzy patch in the vertical row of stars marking Orion's Sword (see page 25.)

About 1,500 light-years of empty space separate us from the Orion Nebula. Actually, the space isn't completely empty--it's permeated with a tenuous mixture of atoms and molecules. In some regions of the Milky Way, hydrogen and other elements gravitate together to form giant molecular clouds. One of the nearest examples is the Orion Molecular Cloud, whose dark cloak of dust and gas spans 100 light-years in southern Orion. M42 is a luminous "blister" (about 25 light-years wide) on the side of the Orion Molecular Cloud that faces us.

The Orion Nebula is energized by a quartet of hot, young stars, called the Trapezium, situated near the center of the blister. Ultraviolet radiation produced by the Trapezium stars is heating the surrounding gas and causing it to fluoresce. These massive stars are also sculpting the nebula. Their intense light has been burning a hole in the dark fog of the Orion Molecular Cloud for tens of thousands of years. It is only because the growing cavity has broken through the edge of the cloud that astronomers can observe the wonders within.


In the early 1990s, the Hubble Space Telescope peered into this brightly lit cavern and found hundreds of young stars less than one million years old. Hubble also discovered protostars--unborn suns still incubating inside dense clumps of gas. These findings confirmed what astronomers had long suspected: that the Orion Nebula is a cradle of star formation. And best of all, you can peer into that cradle with binoculars or a small telescope.



No other world fires the imagination quite like Mars. And yet, the famed Red Planet really only looks its best a few weeks on either side of opposition, when it's closest to Earth. Alas, not all Mars oppositions are created equal; every 15 to 17 years we get a batch of two or three good ones. The last peak occurred in 2003 when Mars was only about 35 million miles away and presented a disk 25.1" wide. The next opposition, slated for January 29, 2010, is near the trough of this cycle. On opposition night, the planet's distance will be about 60 million miles and its disk will have a diameter of just 14"--admittedly small, but still big enough for a 4- or 6-inch scope, operating between 150x and 200x, to show the most conspicuous surface markings indicated on our map below.



The features available for scrutiny depend on what part of Mars is facing Earth at any given time. Syrtis Major, Sabaeus Sinus, and Meridiani Sinus, form a prominent group of dark markings near the Martian equator and will be well placed for viewing during the middle two weeks of January. And keep an eye out for ice. It will be spring in Mars's northern hemisphere. Your scope might reveal a portion of the large (though shrinking) north polar cap.
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Title Annotation:SKY DIARY
Date:Jan 1, 2009
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