How to see the planets in the night sky, December 2020

Read our astronomy guide to find out what planets are in the night sky tonight and throughout the month of December 2020.

In this guide we’ll look at which planets are visible in the night sky throughout December 2020, which are best-placed for observing, and what you need to know to spot them.

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This month Neptune is well placed at the start of December, but it loses altitude against the evening twilight by the end of the month.

Neptune is located in a triangle formed by Phi (ϕ) Aquarii, 96 Aquarii and HIP 115257
Neptune is located in a triangle formed by Phi (ϕ) Aquarii, 96 Aquarii and HIP 115257. Credit: Pete Lawrence

Neptune is the outermost of the main planets. Having reached opposition on 11 September, it is now an evening object. For a nearby planet such as Mars, the period around opposition represents a time when the planet appears significantly larger and brighter than at other times.

However, for more distant worlds such as Uranus and Neptune, the difference in brightness and size is fairly marginal. As Mars approached its best UK opposition for some time in the middle of October, it took priority in terms of amateur observing.

At mag. +7.9 you’ll need at least binoculars to see the planet, but these will only reveal it as a star-like point. A small telescope will show the planet’s blue colour, with at least 150mm of aperture being required to reveal Neptune’s tiny 2 arcsecond disc.

At present, Neptune is located east-northeast of mag. +4.2 Phi (ψ) Aquarii, within the confines of an isosceles triangle formed from Phi Aquarii, mag. +5.5 96 Aquarii and +6.2 HIP 115257 (see diagram above).

The planet appears to move away from the apex of this triangle (Phi Aquarii) towards the triangle’s base throughout the month.

Try and spot Triton, Neptune’s largest moon, visible as a faint dot near the planet. Credit: Pete Lawrence
Try and spot Triton, Neptune’s largest moon, visible as a faint dot near the planet. Credit: Pete Lawrence

Although it’s possible to see Neptune as a tiny disc, revealing any variation in its appearance remains the domain of large aperture telescopes or high-resolution planetary imaging setups.

A less demanding target is Neptune’s largest moon, Triton, which is currently shining at mag. +13.5. Use a large enough image scale and boost imaging exposure times to just over-expose the main planet, and Triton should become visible as a faint dot near the planet. This is best attempted when the seeing is fairly stable.

If you’re feeling confident in your astrophotography, read our guide on how to photograph Neptune and its moon Triton.

The phase and relative sizes of the planets in December 2020. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence
The phase and relative sizes of the planets in December 2020. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence

How to see the planets in the night sky, December 2020

Neptune

  • Best time to see 1 December, 18:30 UT
  • Altitude 31˚
  • Location Aquarius
  • Direction South
  • Features Small bluish disc, Triton
  • Recommended equipment 150mm or larger

Mercury

  • Best time to see 1 December, 30 minutes before sunrise
  • Altitude 2.5˚ (very low)
  • Location Libra
  • Direction Southeast

Mercury is bright at around mag. –0.7 during the first week of December, but its apparent distance from the Sun is decreasing so it’s soon lost in the bright pre-sunrise sky.

It reaches superior conjunction on 20 December and its late month reappearance into the evening twilight isn’t very favourable.

Venus

  • Best time to see 1 December, 90 minutes before sunrise
  • Altitude 8˚ (low)
  • Location Libra
  • Direction Southeast

Venus is a prominent, albeit low, planet in the dawn twilight this month. It rises 2 hours and 40 minutes before the Sun at the start of December, shining at mag. –3.9 and showing an 88%-lit gibbous disc.

By the end of December, Venus rises 1 hour and 30 minutes before the Sun, shines at mag. –3.8 and presents a 93%-lit 10 arcsecond gibbous disc through a scope.

Mars

  • Best time to see 1 December, 20:30 UT
  • Altitude 44˚
  • Location Pisces
  • Direction South

Mars was at opposition on 13 October and is on the decline in terms of its appearance, but it’s still bright and there’s plenty to watch out for if you have a 200mm or larger scope.

It lies in Pisces and shines at mag. –1.1 on 1 December, dropping to mag. –0.2 by the 31st. Through a scope Mars looks gibbous, with a phase varying between 92% on 1 December and 89% by 31 December. Mars shows a 14 arcsecond disc on 1 December, which shrinks to 10 arcseconds by month end.

For more info, read our guide on how to see Mars.

Jupiter

  • Best time to see 21 December, from 17:00 UT
  • Altitude 8.5˚ (low)
  • Location Capricornus
  • Direction Southwest

Jupiter and Saturn reach their historic Great Conjunction on 21 December. Both planets are now compromised by evening twilight, appearing low above the southwest horizon as darkness falls.

Jupiter shines at mag. –1.8, while a waxing crescent Moon joins the scene, 5%-lit to the southwest of Jupiter on the 16th, and as an 11%-lit crescent southeast of the planet on 17 December.

Saturn

  • Best time to see 21 December, from 17:00 UT
  • Altitude 8.5˚ (low)
  • Location Capricornus
  • Direction Southwest

Saturn is partnered with Jupiter, having a close conjunction on the evenings of 20, 21 and 22 December. Both are visible from around 17:00 UT, appearing low above the southwest horizon. Saturn shines at mag. +0.9.

For more advice on the gas giants, read our guide How to observe Jupiter and Saturn.

Uranus

  • Best time to see 1 December, 21:45 UT
  • Altitude 50˚
  • Location Aries
  • Direction South

Uranus remains well-positioned all month. Currently passing through the southern part of Aries, this distant ice giant is able to attain an altitude of 50˚ as seen from the UK, which makes it our best-positioned planet. It shines at mag. +5.7 which is just within naked-eye territory, but we’d recommend binoculars to see it.

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Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night. This guide originally appeared in the December 2020 issue of BBC Sky at Night Magazine.