Our celestial neighbourhood in December
PICK OF THE MONTH
Mercury
Best time to see: 31 December, 30 minutes after sunset
Altitude: 5˚ (low)
Location: Sagittarius
Direction: Southwest
Features: Phase, surface shadings with larger instruments
Recommended equipment: 75mm, or larger
Although it’s poorly placed at the start of December, the planet Mercury’s position improves towards the end of the month, aided by that brilliant beacon of the twilight sky, Venus. On 19 December, Mercury sets around 35 minutes after the Sun and despite shining at mag. –0.7 on this date, will be a tricky object to see. Fortunately, this brightness is maintained for the rest of the month, and coupled with an increasing apparent separation from the Sun, it will become an easier target as we head towards the end of December.
Passing through the eastern part of the constellation of Sagittarius, the Archer, Mercury can be seen slowly approaching Venus in the evening twilight. On the 29th, both planets appear separated by 4.3˚, Mercury being south of mag. –4.3 Venus on this date. Mercury sets about 1 hour and 15 minutes after the Sun on the 29th, so with a good flat southwest horizon, there’s every chance of spotting the mag. –0.7 planet from approximately 30 minutes after sunset. By the 31st, although located 6.3˚ southeast of Venus, the orientation of both planets is such that they more-or-less level out and appear to set together on this date. Both planets set 1 hour and 20 minutes after the Sun on 31 December.
If you’re able to get a telescope trained on Mercury, it will show as a tiny, almost fully lit disc mid-month, having a 97%- lit phase on 16 December. This drops to 80%-lit by 31 December. Over the same period the planet’s apparent size increases from 4.9 to 5.8 arcseconds, still rather small by any measure.
Mercury reaches greatest eastern elongation next month, on 7 January. A thin, 5%-lit waxing crescent Moon lies near to it a few days earlier on 4 January.
Venus
Best time to see: 1 December, from 30 minutes after sunset
Altitude: 10˚
Location: Sagittarius
Direction: South-southwest An evening planet, visible low above the south-southwest to southwest horizon at December’s start. It sets 2 hours and 35 minutes after the Sun on the 1st, and 1 hour and 20 minutes after by the month’s end. An 8%-lit waxing crescent Moon sits nearby on the 6th, and as a 16%-lit waxing crescent on the 7th. Venus appears 4.3˚ north-northeast of mag. –0.7 Mercury on the 29th, with both staying close for the rest of the month. At mag. –4.7, Venus is at its brightest at December’s start.
Mars
Best time to see: 31 December, 1 hour before sunrise
Altitude: 5˚ (low)
Location: Ophiuchus
Direction: Southeast Mars makes a return to the morning sky. It’s small through a scope’s eyepiece, 4.0 arcseconds across by the month’s end. A 1%-lit waning crescent Moon lies 4.5˚ east-southeast of mag. +1.6 Mars on the morning of the 3rd. On the 26th, Mars appears 4.6˚ north of mag. +1.0 Antares (Alpha (α) Scorpii). On the 31st Mars is revisited by the waning crescent Moon, this time 8%-lit. Mars rises two hours before the Sun on the 31st.
Jupiter
Best time to see: 1 December, 17:20 UT
Altitude: 23˚
Location: Capricornus
Direction: South Mag. –2.3 Jupiter reaches its highest position in the sky, 23˚ up, at December’s start, under deep twilight conditions. By the month’s end, the Sun is still up as Jupiter reaches this position, despite the planet setting 4.5 hours after the Sun. A crescent Moon appears nearby on the evenings of the 8th (25%-lit waxing crescent) and 9th (35%-lit waxing crescent).
Saturn
Best time to see: 1 December from 17:00 UT
Altitude: 18˚
Location: Capricornus
Direction: Just west of south At the start of December, mag. +0.7 Saturn no longer appears above the southern horizon in darkness, only visible west of south as the evening twilight deepens. At the month’s start, Saturn lies in the middle of a line formed by Jupiter to the east and Venus to the west.
Uranus
Best time to see: 1 December, 22:00 UT
Altitude: 52˚
Location: Aries
Direction: South Evening planet Uranus was at opposition last month and remains well placed during December, reaching a highest altitude of 52˚ in darkness all month. The mag. +5.7 planet sits 25 arcminutes southsoutheast of mag. +6.0 29 Arietis on 29 December.
Neptune
Best time to see: 1 December, 19:00 UT
Altitude: 32˚
Location: Aquarius
Direction: South Mag. +7.9 Neptune’s position declines this month, the evening planet only reaching a highest altitude of 32˚ in total darkness until the 15th.
More ONLINE
Print out observing forms for recording planetary events
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 during the month, as shown on the diagram. The line by each date represents 01:00 BST (00:00 UT).
