Saturn is above 30° in the south at dusk in early December. But catch it early, because the ringed planet sets at 10 p.m. local time on December 1 and 8 p.m. local time on December 31. Saturn is found in eastern Capricorn and begins the month at magnitude 0.7, dropping to magnitude 0.8 on the 23rd. Its disk spans 16″ and the rings span a majestic 36″. “.
The apparent inclination of the rings relative to our line of sight falls below 14° at the end of December. A crescent Moon is located 5° southeast of the planet on December 26.
Titan, Saturn’s largest moon, has a magnitude of 8.5, an easy target for small telescopes. Find it north of Saturn on December 10 and 26, and due south on December 2 and 18. Weaker, Tethys, Dione and Rhea shine at magnitude 10 and lie within Titan’s orbit. They become increasingly difficult to spot as the planet’s altitude drops later in the evening.
Look for magnitude 11 Iapetus on December 22-23. It reaches inferior conjunction early on the 23rd and US observers on the evening of the 22nd could spot the faint moon passing in front of the rings. Iapetus will appear dark against the brighter B ring. The transit is underway as darkness falls along the eastern seaboard.
On the 23rd, Iapetus briefly passes very close to Tethys, beginning at around 6:23 p.m. EST. The pair will appear to merge into a telescope. An hour later, as darkness falls over the Midwest, they are 3 inches apart.
Shining Jupiter is a useful guide to finding Neptune in Aquarius these winter evenings. At the beginning of December, Neptune is located 6° west of Jupiter. This distance extends to 8° on the 31st, largely due to Jupiter’s eastward motion along the ecliptic. Neptune shines at magnitude 7.9, requiring at least binoculars to spot it.
Neptune is ideally placed between two magnitude 7 stars all month. The planet retraces its steps from November as it turns east on December 4. The two stars are the easternmost pair of a parallelogram of four stars each about 1° apart, located 5° northeast of Phi (ϕ) Aquarii and easily identified with binoculars.
Catch Neptune early, as it sets at 1 a.m. on Dec. 1 and shortly before 11 p.m. on the 31st. Telescopes reveal a dark bluish disk spanning 2″. The crescent moon lies 3° south of Neptune on December 28.
Jupiter is hard to miss as the brightest planet in the sky after Venus sets. It’s high in Pisces in the south after sunset. The giant planet goes from magnitude -2.6 to -2.4 during the month. It moves east, deeper into Pisces and away from Aquarius.
Jupiter’s disk grows from 43″ to 39″, but remains an impressive sight with a wealth of atmospheric detail. Start observing Jupiter at the end of twilight for the best views, when its brightness is tempered by the brighter sky and the dark pair of equatorial belts skips. Finer details appear with patient attention. Every other night or so, the Great Red Spot makes a grand appearance.
Io, Europa, Ganymede and Callisto regularly change position and pass through Jupiter, casting their accompanying shadows. Io begins the month with a transit on December 1 at 8:25 p.m. EST. His shadow follows 77 minutes later. Watch as Jupiter flows west; Io exits the disc at 10:39 p.m. EST, followed by the shadow exit at 11:54 p.m. EST. The event repeats on the evening of December 8, starting at 10:19 p.m. EST.
Io and Europa put on a show on Dec. 9, starting when Europa passes behind the planet’s western limb at 10:43 p.m. EST. After Europa disappears, look to the eastern limb to see Io exit the eclipse about 25″ from the planet at 11:01 PM EST.
Callisto skims due south of Jupiter early in the evening of December 13. Its orbital plane is such that the moon misses the transit or eclipse, but this passage is really close to a transit. Can you make out a dark space between the planet and the moon?
December 15 finds Ganymede’s huge shadow transiting Jupiter. It appears over the southern temperate belt from 7:41 p.m. EST, but the shadow is so large that you can see it start to appear a few minutes earlier. The shadow takes nearly two hours and 40 minutes to transit. And you can end the year by observing the December 29 transit of Ganymede beginning at 10:11 p.m. EST and lasting nearly three hours. The Royal Astronomical Society of Canada Observer’s Manual gives a complete list of events.
Uranus is found in southern Aries as an easy binocular target at magnitude 5.7. It is visible all night and peaks an hour or two before local midnight. Its 4″ wide blue disc is a challenge in a telescope.
Uranus is located about 6° north-northeast of Mu (μ) Ceti. The easiest way to find it is to look north of Mu for 5th magnitude Sigma (σ) Arietis, followed 3° further along the same line by Rho (ρ) Arietis. Uranus begins the month midway between these two stars, moving slowly southwest and passing 1° due north of Sigma on December 13. Both fit easily into the field of view of 7×50 binoculars or a low power telescope. That same night, Pi (≠) Arietis is located 1.5° northwest of Uranus.
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