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

By Pete Lawrence

Published: Tuesday, 28 September 2021 at 12:00 am

Uranus will reach opposition on Thursday 4 November 2021. Around this time the planet appears at its brightest and largest for the year, but its great distance from Earth, means such effects aren’t noticeably different to other, non-opposition dates.

One benefit of opposition for the outer planets however, is an increased period of observability. They’re visible for the entire night.

Find out what to observe in the night sky every month in our Star Diary podcast, or prepare for the dark months ahead with our guide to autumn astronomy.

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October and November sees Uranus at its brightest in 2021, between Aries and Cetus. Credit: Pete Lawrence

Uranus is currently in Aries, located in the southern part of the constellation, just to the north of the pattern that defines the head of Cetus, the Whale.

It shines at mag. +5.7, which means it should be visible to the naked eye from a location with good, dark skies.

Through binoculars, Uranus looks exactly like a mag. +5.7 star.

A small telescope is required to bring out its distinct greenish hue and reveal its tiny, 3.8 arcsecond disc.

Larger instruments may be able to show banding in the planet’s atmosphere as well as some of the brighter moons.

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View Uranus through a small telescope to catch its green hue. Credit: Pete Lawrence

Uranus has a current total of 27 officially identified satellites, five of which are big and bright enough to be seen through larger amateur instruments.

These moons are Miranda, Ariel, Umbriel, Titania and Oberon. They present a good challenge to see visually and, due to their close proximity to the planet, a challenge to image too.

It’s easy to lose Miranda in the over-exposed glare of Uranus.

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Uranus an four of its moons, imaged by Ralph Smyth, Lisburn, County Antrim. Equipment: Celestron C8 SCT, ZWO ASI224MC, ASI290MM , Skywatcher HEQ5Pro Mount

In October 2021, Uranus is the best-placed planet to observe from the UK, reaching a peak altitude of 52° from the centre of the country, when due south.

This lifts it out of the low-altitude atmospheric murk, providing a more stable view. It also increases your chances of spotting its dim dot with your naked eye.

How to see the planets in October 2021

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The phase and relative sizes of the planets in October 2021. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence

Uranus

  • Best time to see: 31 October, around midnight UT
  • Altitude: 52º
  • Location: Aries
  • Direction: South
  • Features: Colour, moons, atmosphere
  • Recommended equipment: 150mm or larger

Mercury

  • Best time to see: 25 October, from 1 hour prior to sunrise
  • Altitude: 7° (low)
  • Location: Virgo
  • Direction: East-southeast

Mercury is unlikely to be seen at October’s start as it sets at almost the same time as the Sun. Inferior solar conjunction occurs on 9 October. Mercury then rapidly re-emerges into the morning sky, brightening as it does.

First sighting is likely on 18 October, when the planet shines at mag. +0.9 and rises 90 minutes before the Sun. When greatest western elongation occurs on 25 October, Mercury will have brightened to mag. –0.5 and have risen around two hours before the Sun.

The rest of the month sees Mercury creep back towards the Sun but also grow brighter, reaching mag. –0.8 on 31 October.

Venus

  • Best time to see: 29 October, 20 minutes after sunset
  • Altitude: 6.5° (low)
  • Location: Ophiuchus
  • Direction: South-southwest

Venus reaches its greatest eastern elongation on 29 October, 47° from the Sun. But the planet’s relative position is poor and it remains low after sunset all month. On 1 October Venus sets one hour after the Sun. By the end of the month, that delay will have increased to 100 minutes.

In theory Venus reaches a 50%-illuminated phase on 28 October, but a phase anomaly effect should mean it appears half-lit a few days earlier. A 14%-lit waxing crescent Moon sits 2.2° from Venus on the evening of 9 October.

Mars

Mars is in conjunction with the Sun on 8 October and not visible this month.

Jupiter

  • Best time to see: 1 October, 22:10 BST (21:10 UT)
  • Altitude: 22°
  • Location: Capricornus
  • Direction: South

Jupiter is high in the early evening sky during October so ideal for observing. On 1 October from the centre of the UK it appears 22º up when due south at 22:10 BST (21:10 UT).

By the end of the month it reaches this position at 19:12 UT. A waxing gibbous Moon sits near the planet on the evenings of 14 and 15 October.

Saturn

  • Best time to see: 1 October, 21:00 BST (20:00 UT)
  • Altitude: 18°
  • Location: Capricornus
  • Direction: South

Shining at mag. +0.5 on 1 October, Saturn reaches an altitude of 18°, its highest point, due south around 21:00 BST (20:00 UT). While still low, this is an improvement over recent years.

A waxing gibbous Moon passes south of Saturn on 13 and 14 October. By the end of the month, Saturn will still reach its highest point in relative darkness, but there’ll be a residual of the evening twilight behind the planet.

Neptune

  • Best time to see: 1 October, 00:00 BST (23:00 UT)
  • Altitude: 32° Location: Aquarius
  • Direction: South

Neptune is well positioned in October. Currently located in eastern Aquarius near to
mag. +4.2 Phi (φ) Aquarii, Neptune shines at mag. +7.8 and is, theoretically, the only main planet that requires optical assistance to see.

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