Our celestial neighbourhood in December
PICK OF THE MONTH
Mars
Best time to see: 1 December, 00:30 UT
Altitude: 62º
Location: Taurus
Direction: South
Features: Light deserts with darker exposed rock, polar caps, weather
Recommended equipment: 75mm or larger
Mars reaches opposition on 8 December. Around this time the planet appears at its brightest and largest in terms of apparent size. Mars is closest to Earth on 1 December, when it appears as a mag. –1.9 object among the stars of Taurus. On this date Mars has an apparent diameter of 17.2 arcseconds. At this size, albedo features – features that show different reflectivity – can be seen quite easily. The most obvious are the dark, exposed rocky regions on the planet that contrast well with the surrounding lighter deserts. In addition, the planet is sideways-on to us at present, potentially giving us a view of both polar regions. The southern polar cap will probably be absent, it being summer in Mars’s southern hemisphere. The northern cap will be augmented by a large cloud shroud, known as a polar hood.
When observing Mars, it pays to be patient and allow your eyes to get used to the view. It’s quite common to look at the planet and see nothing but a bright orange-pink disc through the eyepiece of a telescope. However, as your eye becomes accustomed to the light, the subtle surface features become more obvious.
On opposition day, 8 December, Mars will be occulted by the full Moon – the lunar phase that sees the Moon also at opposition. Mars disappears behind the lunar limb at 04:57 UT, reappearing back into view one hour later at 05:57 UT. On 31 December, although it will have faded slightly to mag. –1.2, Mars will look quite resplendent against the stars of Taurus, near to the Pleiades and Hyades open clusters and 8° to the north of orange-hued Aldebaran (Alpha (α) Tauri).
Mercury
Best time to see: 29 December, 30 minutes after sunset
Altitude: 5º (low)
Location: Sagittarius
Direction: Southwest
A mag. –0.5 evening object, not well-positioned at the start of December when it sets with mag. –3.8 Venus, 30 minutes after the Sun. Greatest eastern elongation occurs on 21 December, Mercury appearing separated from the Sun by 20.1°. On this date it lies 5.4° from mag. –3.8 Venus, setting 80 minutes after sunset. On 29 December, mag. +0.6 Mercury and –3.8 Venus appear 1.5° apart, low in the southwest after sunset. On this date, Mercury sets almost 90 minutes after the Sun.
Venus
Best time to see: 31 December, 30 minutes after sunset
Altitude: 5° (low)
Location: Sagittarius
Direction: Southwest
A bright mag. –3.8 evening object, slowly pulling away from the Sun. Telescopically, it is at its least optimal, 10 arcseconds across and over 90% illuminated. Venus chases Mercury, catching up with it late in the month. A 2%-lit Moon sits below both planets on 24 December, very low just after sunset.
Jupiter
Best time to see: 1 December, 19:32 UT
Altitude: 35°
Location: Pisces
Direction: South
A magnificent evening planet. Shining at mag. –2.5 on 1 December, it is joined by a 62%-lit waxing Moon 3.5° away before they both set in the early hours of 2 December. A second lunar visit occurs on the evening of 29 December, this time from a 46%-lit Moon. It reaches its highest position in the sky, due south, under dark sky conditions for most of the month. On 31 December, it shines at mag. –2.2.
Saturn
Best time to see: 1 December, 17:15 UT
Altitude: 21°
Location: Pisces
Direction: South
A lovely 15%-lit waxing Moon sits 4.7° south of Saturn on the evening of 26 December. By the end of the month, mag. +0.9 Saturn sits 15° above the southwest horizon by the time darkness gets underway.
Uranus
Best time to see: 1 December, 22:20 UT
Altitude: 53°
Location: Aries
Direction: South Mag. +5.7
Uranus is well-placed, due south at a dark-sky peak altitude over 50°. It is occulted by a 94%-lit waxing Moon on the afternoon of 5 December. It disappears at 16:51 UT under darkening twilight and could be tricky. Reappearance should be easier, occurring under darker conditions at 17:17 UT.
Neptune
Best time to see: 1 December, 19:00 UT
Altitude: 33º
Location: Aquarius
Direction: South Mag. +7.9
Neptune manages to attain an altitude around 30° under dark-sky conditions all month. A 36%-lit Moon sits 3.7° southwest on 28 December. Mag. –2.2 Jupiter remains close to Neptune too, 8° east at the end of the month.
More ONLINE
Print out observing forms for recording planetary events
www.skyatnightmagazine.com/bonus-content/PF5NNHB
The planets in December
The phase and relative sizes of the planets this month. Each planet is shown with south at the top, to show its orientation through a telescope
Jupiter’s Moons: December
Using a small scope you can spot Jupiter’s biggest moons. Their positions change dramatically over the month, as shown on the diagram. The line by each date represents 00:00 UT.
