Observing guide: the best planets to see in December 2019

The Sky at Night's Pete Lawrence reveals how to see the planets in the night sky this month.

Keep an eye on the location of Mars throughout December 2019. The stars (above) are shown relative to the horizon at 06:30 UT on 31 December. Credit: BBC Sky at Night Magazine
Keep an eye on the location of Mars throughout December 2019. The stars (above) are shown relative to the horizon at 06:30 UT on 31 December. Credit: BBC Sky at Night Magazine

Mars is a morning object and telescopically rather poor at the moment. The reason for this is that the planet’s distance from Earth is currently high, Mars being on a far side of its orbit from Earth.

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As a consequence, when you peer at Mars though the eyepiece of a telescope it only presents a tiny 4 arcsecond disc throughout December.

On the plus side, the planet does brighten slightly from mag. +1.7 at the start of December to +1.6 at the end, but admittedly this isn’t a huge increase.

On 12 December Mars appears to sit 16 arcseconds from Zubenelgenubi (Alpha (α) Librae) and on the morning of 23 December, mag. +1.6 Mars is joined by a slender 9%-lit waning crescent Moon 3.2° to the left of the planet as seen from the UK.

By the end of December, Mars rises 3.4 hours before the Sun.

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. Credit: Pete Lawrence
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. Credit: Pete Lawrence

With such poor prospects, you might ask why we’ve started this guide by talking so much about Mars. 

The reason is that Mars is heading for a big improvement towards the latter part of 2020 and that will start to become noticeable over the next few weeks and months.

Mars reaches opposition every 2.1 years and this is the best time to observe it through a scope. The last one occurred in 2018 and was poor for UK viewing, Mars being low down and difficult to see under steady skies.

The next opposition occurs on 13 October 2020, a time when the planet will be in Pisces and better placed for us in the UK.

At the current time it’s a useful exercise to locate Mars with the naked eye so that by the time it starts to show improvement – probably around February you’ll be ready to spring into action and observe it.

Observing Mars in the December night sky

Best time to see: 30 December, 06:30 UT

Altitude:

Location: Libra

Direction: Southeast

Recommended equipment: Naked-eye observing

How to see the other planets this month

Mercury

Best time to see: 1 December, 60 minutes before sunrise

Altitude: 7° (low)

Location: Libra

Direction: Southeast

Mercury starts the month well; shining at mag. –0.5 it rises 120 minutes before the Sun. It’s slowly moving back towards the Sun and by mid-month it stays at mag. –0.5, appearing 74 minutes before sunrise. It’ll become less well-positioned but this is offset by an increase in brightness. The end of visibility will be around 25th when it’ll be rising 1.6° from
a slender crescent Moon.

Venus

Best time to see: 29 December, 17:00 UT

Altitude: 12°

Location: Capricornus

Direction: Southwest

Venus is an evening object, visible after sunset, low in the southwest. On 1 December its mag. –3.8 dot sets 90 minutes after the Sun. A scope shows an 89%-lit gibbous disc 11 arcseconds across.

By the month’s end, it will set three hours after the Sun and show a phase reduced to 82%-lit and a disc 13 arcseconds across.

Jupiter

Best time to see: 1 December, 16:45 UT

Altitude: 3° (very low)

Location: Sagittarius

Direction: Southwest

Jupiter is now too low for serious observation but remains an interesting target for naked-eye viewing at the month’s start when, at mag. –1.7, it’s joined by mag. –3.8 Venus and +0.9 Saturn. Jupiter is in solar conjunction on the 27th.

Saturn

Best time to see: 1 December, 17:15 UT

Altitude: 9° (low)

Location: Sagittarius

Direction: Southwest

Saturn appears as a mag. +0.9, slightly yellowish dot, low above the southwest horizon as darkness falls on 1 December. Towards the month’s end it will be all but lost in the evening twilight. Before it disappears, look low towards the southwest horizon on the evening of 11 December where you should see mag. +0.9 Saturn 1.8° north of mag. –3.9 Venus.

As a final Saturnian challenge for 2019, see if you can spot the planet after sunset on the 27th when there’s a slender crescent Moon 2.7° below the mag. +0.9 planet.

Uranus

Best time to see: 1 December, 19:30 UT

Altitude: 49°

Location: Aries

Direction: South

Uranus is well placed, reaching its highest point above the horizon when due south in darkness all month long. At mag. +5.7 it may be glimpsed with the naked eye from a dark sky site. Uranus is residing in the southwest corner of Aries.

Neptune

Best time to see: 1 December, 18:30 UT

Altitude: 30.5°

Location: Aquarius

Direction: South

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Neptune is well placed at the start of December, able to reach its highest point in the sky, around 30° up, in darkness. At mag. +7.9 you’ll need at least binoculars to spot it just southwest of mag. +4.2 Phi (φ) Aquarii. By the end of December, Neptune begins to lose altitude above the south-southwest horizon as darkness falls.