How to see the planets in the night sky, December 2021

In this guide we’ll look at which planets are visible in the night sky throughout December 2021, which are best-placed for observing, and what you need to know to spot them.

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.

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Passing through the eastern part of the constellation of Sagittarius, the Archer, Mercury can be seen slowly approaching Venus in the evening twilight.

On 29 December, 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 29 December, 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 31 December, 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.

How to see the planets, December 2021

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

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 1 December, and 1 hour and 20 minutes after by the month’s end.

An 8%-lit waxing crescent Moon sits nearby on 6 December, and as a 16%-lit waxing crescent on 7 December. Venus appears 4.3˚ north-northeast of mag. –0.7 Mercury on 29 December, 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 3 December.

On 26 December, Mars appears 4.6˚ north of mag. +1.0 Antares (Alpha (α) Scorpii). On 31 December Mars is revisited by the waning crescent Moon, this time 8%-lit. Mars rises two hours before the Sun on 31 December.

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 8 December (25%-lit waxing crescent) and 9 December (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 south-southeast 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 15 December.

This guide originally appeared in the December 2021 issue of BBC Sky at Night Magazine.