The long nights of December are an astronomical delight. The early evening sky hosts Saturn as the main feature, along with Uranus and Neptune. Jupiter rises later and is visible all night, its four main satellites undergoing many notable events. The morning sky shows the best Northern Hemisphere appearance of Mercury for the year.
Let’s begin with Saturn and Neptune, 4.3° apart on Dec. 1. Saturn’s easterly motion carries it to within 3.5° of Neptune by the 31st. Saturn is easy to spot at magnitude 1.0 — the brightest object in the region except when the Moon passes by Dec. 26. Both planets stand 45° high in the southern sky as soon as it’s dark and set by midnight, so plan to observe in early evening.
Neptune shines at magnitude 7.7 and requires at least binoculars to view. Saturn’s proximity makes it easy to spot. About half a binocular field of view east of Saturn lies 5th-magnitude 27 Piscium, and Neptune is 2° north of this star.
Neptune spans 2″ through a telescope and its bluish-hued disk appears nonstellar. It lies 2.8 billion miles from Earth.
At the end of the year, Saturn is 902.6 million miles from Earth and its disk spans 17″. The edge-on rings give us a rare chance to see both hemispheres at once. The rings open slightly from 0.4° to 1.0° during the month.
Eighth-magnitude Titan, Saturn’s largest moon, wanders back and forth every week. You’ll find it near Saturn Dec. 8, 16, and 24. Titan transits Saturn twice, though neither is visible from the U.S. European observers can enjoy the first transit Dec. 8 beginning at 17:37 UT and the second Dec. 24 beginning at 16:58 UT. Each ends about six hours later. The moon reappears from occultation Dec. 16 around 21:56 UT, again visible in Europe and the U.K.
Tethys, Dione, and Rhea, each shining at 10th magnitude, orbit Saturn closer than Titan. With the rings nearly edge-on, it’s fascinating to watch these moons for close conjunctions.
For example: Dione stands 1″ due north of Tethys on the 9th, and you can see their relative motion between 10 and 11 p.m. EST. On the 15th, Dione stands 2″ due north of Titan an hour earlier. These events are close to the resolution limit of smaller telescopes and will be affected by any local atmospheric turbulence. On the 16th, Tethys and Titan stand 2″ apart near the eastern edge of the rings just after 9 p.m. EST. On the 17th, Dione and Tethys meet again near Saturn’s western limb — the pair appears to merge just before 9 p.m. EST as they approach closer than 1″, although Tethys is on the far side of the rings and is occulted by them, so it may appear fainter or disappear entirely depending on seeing conditions.
The dim rings may allow you to view magnitude 11.8 Enceladus. It remains within about 37″ of Saturn, or 15″ from the edges of the rings.
Iapetus reaches superior conjunction with Saturn Dec. 6. U.S. observers will see the 11th-magnitude moon 1.2′ due south of the planet the evening of the 5th. Its then reaches its eastern (fainter) elongation on the 25th, dipping to 12th magnitude.
Uranus spends all month in Taurus, located south of the Pleiades (M45). The planet is very easy to spot as it wanders south of a pair of 6th-magnitude stars, 13 and 14 Tauri. As December opens, Uranus is in line with the pair, standing a Moon’s width due east of 14 Tau. By Dec. 13, Uranus is 7′ due south of 14 Tau. By the 24th it’s moved farther west and is 14″ due south of 13 Tau. By year’s end, Uranus stands 21″ southwest of 13 Tau and appears to have an 8th-magnitude companion, a field star. Through a telescope, Uranus reveals a pale greenish disk spanning 4″.
A nearly Full Moon crosses in front of the Pleiades Dec. 3, heralding multiple occultations of its bright stars. While the stars do disappear at the dark limb, only brighter members of the cluster will be visible thanks to the blazing Moon, and even they will be a challenge.
Magnitude 3.7 Electra is the first major Pleiad to disappear, soon after 7 p.m. CST from Chicago. The timing varies by location, but the Moon crosses the cluster between about 7 p.m. and 9:30 p.m. CST. Use high magnification on your telescope, which will remove as much of the Moon from your field of view as possible.
Jupiter rises about 8 p.m. local time Dec. 1 and a little over two hours earlier by the 31st. It’s a spectacular object, passing more than 70° high around local midnight, dominating the stars of Gemini. Just one month from opposition in January, Jupiter reaches magnitude –2.7 by month’s end. The disk spans 44″ at the start of December and grows to 46″ by the New Year. A waning gibbous Moon stands 8° northwest of Jupiter the evening of Dec. 6.
A full rotation of the planet (less than 10 hours) is visible in a single night. Jupiter’s fast rotation causes atmospheric features to move noticeably across the disk in 10 to 15 minutes. There is a treasure trove of delicate cloud features to follow. High-speed video capture at regular intervals of a few minutes can produce a time-lapse of Jupiter’s rotation, recording brief moments of good seeing.
The four Galilean moons are bright enough to see in any telescope, and even in tripod-mounted binoculars. As they orbit they cross in front of the planet and cast shadows, as well as hide behind Jupiter in occultations.
On Dec. 1/2, after midnight for the eastern half of the U.S., Ganymede casts its shadow across the planet’s face beginning around 1:07 a.m. EST. The shadow is so large that it takes about 10 minutes to fully appear. Three hours later, at 4:18 a.m. EST, the shadow begins to leave the western limb, just as Ganymede itself approaches the eastern side. Ganymede’s transit begins at 4:44 a.m. EST. It takes more than three hours to cross the disk, leaving as dawn begins in the Mountain Time Zone.
Io approaches the eastern limb of the gas giant Dec. 4/5, the moon’s shadow appearing at 12:19 a.m. EST (the 5th in EST only). Io joins 49 minutes later, following until the shadow leaves at 2:34 a.m. EST. Io departs 50 minutes later. The sequence repeats later in the month with Io and its shadow closer together, as we are approaching opposition.
On Dec. 20, Io’s shadow appears at 10:35 p.m. EST and the moon follows 30 minutes later. Both cross the disk in just over two hours. On Dec. 27/28, shadow ingress is at 12:28 a.m. EST (the 28th in EST only) and Io follows 20 minutes later. At opposition next month, the time difference drops to zero.
On Dec. 6/7, Europa’s shadow appears at 11:54 p.m. EST, leaving just under three hours later. The moon follows, transiting from 1:31 a.m. EST (Dec. 7 in the eastern U.S.) through 4:22 a.m. EST.
Callisto transits on the evening of the 7th, beginning at 9:17 p.m. EST and ending four hours later.
Try to spot Ganymede’s reappearance Dec. 12/13 around 1 a.m. EST (the 13th in the eastern U.S.). How early do you spot the small bulge at the northeastern limb of Jupiter? Ganymede takes about seven minutes to fully reappear from behind the disk.
The year ends with a transit of Europa and its shadow. The shadow is first at 9:01 p.m. EST, followed by the moon 28 minutes later. The transit takes nearly three hours, ending shortly before the New Year rolls into the Central Time Zone.
Mercury is now at its best for the year in the morning sky for Northern Hemisphere observers. On the 1st it rises around 5:30 a.m. local time, some 90 minutes before sunrise. It quickly achieves greatest elongation west of the Sun Dec. 7. On that day, Mercury stands 30° southeast of 1st-magnitude Spica. The star is visible about 30° high in the southeast at 6:30 a.m. local time, with Mercury 10° high.
The small planet starts December at magnitude 0.1 and reaches magnitude –0.5 by the 10th, remaining that way for nearly three weeks before brightening even more. There’s an attractive scene on the 17th as a thin waning crescent Moon joins Mercury; the two are 10° apart. By 6:45 a.m. local time, they are 9° high. As the sky brightens, be on the lookout for Antares in Scorpius, which rises shortly before 6:30 a.m. and sits 6.5° below Mercury.
The Moon slides to sit 1° below Antares on the 18th and rises 55 minutes before sunrise, a nice challenge for those with a clear eastern horizon. Look for Mercury, which should be easy. The Moon and Antares lie below and slightly to the right of the planet. By Dec. 28, Mercury is magnitude –0.6 but it is only visible in a bright twilit sky, rising an hour before the Sun. See if you can follow Mercury into the morning on New Year’s Eve, when it stands less than 3° high 30 minutes before sunrise — definitely a challenge.
Venus and Mars are both approaching solar conjunction in early January and are not visible this month.
The winter solstice occurs Dec. 21 at 10:03 a.m. EST.
Rising Moon: Westward ho
The lava-filled Grimaldi basin is easily identified on the western limb of the Moon as the last patch of darkness west of the huge Oceanus Procellarum. But when the first rays of sunlight tickle its clumpy rim on the evening of the 3rd, only a “Mickey Mouse head” trio of craters points the way in the lunar southwest.
Fully illuminated on the 4th, the highly battered crater rim is direct evidence of its age. Several interesting sights have also appeared next to Grimaldi, notably the equally large and battered Riccioli closer to the terminator. That crater has a much rougher floor because it was a little too far from Procellarum’s lava floods to get filled up like Grimaldi.
Use a dark filter on your eyepiece to reduce the stark brightness levels of the Full Moon. If you don’t have one, just crank up the power to spread out the light and you’ll find the result a lot easier on the eye. Spend some time here to note that Grimaldi’s floor is not a uniform shade. See if you can follow the indistinct rays of lighter rock ejected from a couple of major impacts late in the Moon’s bombardment back to their source.
A bonus feature: On Dec. 1, check out the Marius Hills along the terminator just north of the equator.
Sunrise will return to Grimaldi Jan. 1.
Meteor Watch: End the year with a bang
There’s a waning crescent Moon three days past Last Quarter as the Geminid meteor shower peaks. While the Moon will subdue fainter meteors, the Geminids are well known for brilliant members and given that Gemini is very high near local midnight, meteors can be observed anywhere across the sky. The zenithal hourly rate is over 100, and the shower is regarded as the best one of the year, so even with the Moon around you should see quite a few meteors every hour.
The second major shower in December is the Ursids, active from Dec. 17–26 and peaking the 22nd. There’s no interference from the Moon, although rates are one-tenth that of the Geminids. With the radiant up all night, however, there’s a good chance that you will spot some of its shower members.
Comet Search: Not what you expected?
Comet 24P/Schaumasse rises around midnight, making it worthwhile to stay up late, especially on the weekend of the Geminids. On Friday the 12th, it shares a low-power field with NGC 3607 and 3608 (10th and 11th magnitude, respectively). With Schaumasse forecast at 9th to 12th magnitude, you can estimate its brightness easily. Use detailed charts to make sure you’re not misidentifying a background galaxy!
Somewhat hampered by moonlight will be the “tilting knife” effect from Dec. 7–11. This is when Earth passes through the plane of the comet’s elliptical orbit, giving us an edge-on view of the dust fan. Any blue ion tail remains out of sight behind it.
From country skies on the 25th, the comet is on the south side of M100 in a high-power field. Integrated magnitude can be misleading, as M100 is a large face-on spiral that is diffuse, whereas Schaumasse is compact. Compare their forms, too — the comet will have a sharply defined southeast flank where the solar wind pushes back dust. The fan-shaped tail is still steeply tilted away, and imagers should get that classic green glow around the head.
Locating Asteroids: Riding on the back of the Whale
Take a stroll in the (star) fields to follow dwarf planet 1 Ceres. Fading from magnitude 8.5 to 8.9, the 600-mile-wide world won’t be a walk in the park, yet still easy enough from the suburbs with a 3-inch scope. You might want to start just east of Saturn and nail Neptune (magnitude 7.7) to calibrate your expectations.
Gazers without motors can let Earth do the “hopping” in the first week of December. Center on magnitude 3.6 Iota (ι) Ceti, then come back eight minutes later to find Ceres sharing the field with a slightly brighter field star. Right through the 23rd, the background stars are fainter, with a reversal on the 24th when Ceres becomes component “B” of a double.
New to charts and scopes? It’s best to make a quick sketch in a logbook by jotting down four or five stars in a 1° (low-power) field. Come back the next clear night to see which one has moved. If nothing has changed, then you found the wrong field. Just try again — a bit of humble pie is normal.
Star Dome
The map below portrays the sky as seen near 35° north latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
The all-sky map shows how the sky looks at:
9 p.m. December 1
8 p.m. December 15
7 p.m. December 31
Planets are shown at midmonth
