See Jupiter at opposition, August 2021

How to see Jupiter and the other planets this month.

Jupiter reaches opposition on 19 August, the planet managing to attain a peak altitude of 23.5˚ in true darkness, due south, at 01:22 BST (00:22 UT).

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Full Moon, technically the Moon at opposition, appears 6.3˚ south-southwest of Jupiter on the night of 22 August.

In astronomy, opposition describes when a planet or other body is in the opposite part of the sky to the Sun. At such times, planets appear brighter and larger than at other non-opposition times.

Jupiter Rouzbeh Bidshahri, Dubai, 25 July 2019. Equipment: ZWO ASI 290 mono camera, Celestron C14 Schmidt-Cassegrain telescope, Losmandy Titan mount.
Jupiter by Rouzbeh Bidshahri, Dubai, 25 July 2019. Equipment: ZWO ASI 290 mono camera, Celestron C14 Schmidt-Cassegrain telescope, Losmandy Titan mount.

In Jupiter’s case, at opposition it shines at mag. –2.9 and through the eyepiece of a telescope has an apparent diameter of 49 arcseconds, large enough to present some impressive detail.

Over the past years, Jupiter has been low as seen from the UK, as it tracked along the most southerly part of the ecliptic. This is now slowly changing, and the planet is gaining altitude with each passing year.

At opposition it’s in Capricornus, but right on the constellation’s eastern border with Aquarius.

In the middle of August 2021, Jupiter passes from the constellation of Aquarius, the Water-Bearer to Capricornus, the Sea Goat. Credit: Pete Lawrence.
In the middle of August 2021, Jupiter passes from the constellation of Aquarius, the Water-Bearer to Capricornus, the Sea Goat. Credit: Pete Lawrence.

Technically, Jupiter starts August 2021 in Aquarius, tracking west slightly through the month, crossing the Aquarius–Capricornus border to be within Capricornus on 19 August.

Jupiter reached equinox in early May and we can see a consequence of this on the morning of 20 August.

At equinox, the Sun crosses the planet’s equatorial plane. The Galilean moons have orbits closely aligned to this plane, and Io will transit Jupiter’s disc on 20 August from 02:50 BST (01:50 UT) until 05:08 BST (04:08 UT).

Near opposition, Io’s shadow virtually lines up with the moon in terms of Jovian longitude.

The proximity to equinox makes the shadow virtually line up in latitude as well. A similar Io transit occurs on 21 August between 21:18 BST (20:18 UT) and 23:33 BST (22:33 UT).

How to see the planets, August 2021

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

Jupiter

  • Best time to see: 19 August, 01:22 BST (00:22 UT)
  • Altitude: 23.5˚
  • Location: Capricornus
  • Direction: South
  • Features: Detailed atmosphere, Great Red Spot, moons
  • Recommended equipment: 75mm or larger

Mercury

  • Best time to see: 15 August, 20 minutes after sunset
  • Altitude: 1.5˚ (very low)
  • Location: Leo
  • Direction: West-northwest

Observing Mercury with the naked eye is possible this month, although the feat requires you to consider a celestial balancing act. Mercury reaches superior conjunction on 1 August, after which it returns to the evening sky.

Its position is favourable and it’s bright at the month’s start. On 6 August, shining at mag. –1.4, it is visible above the west-northwest horizon for 25 minutes after sunset. On 9 August, at mag. –1.1, a 1%-lit waxing crescent Moon lies 7˚ to the east of the planet.

As we head through August, Mercury gets fainter but its time above the horizon increases slightly.

Venus

  • Best time to see: 1 August, 30 minutes after sunset
  • Altitude: 6˚ (low)
  • Location: Leo
  • Direction: West

Venus is a poorly positioned evening planet, setting 68 minutes after the Sun on 1 August and 60 minutes after on 31 August. A 5%-lit waxing crescent Moon lies nearby on 10 August and as a 12%-lit crescent on 11 August. The visibility of Venus will be helped by its brilliance. During August it shines at mag. –4.0.

Mars

  • Best time to see: 18 August, 20 minutes after sunset, near Mercury
  • Altitude: 1.5˚ (very low)
  • Location: Leo
  • Direction: West

Mars isn’t a viable target as it’s dim at mag. +1.8, small at less than 4 arcseconds across when viewed through a scope, and too low to appear against a darkened sky.

Saturn

  • Best time to see: 2 August, from 01:00 BST (00:00 UT)
  • Altitude: 18.5˚
  • Location: Capricornus
  • Direction: South

Saturn is at opposition on 2 August, a time when it’s in the opposite part of the sky to the Sun and visible all night under dark skies. Using a scope, look out for Saturn’s rings  appearing brighter than normal at opposition, dimming back to normal brightness over the following days.

On 2 August, mag. +0.4 Saturn sits due south at 01:15 BST (00:15 UT) and attains an altitude of 18.5˚.

Uranus

  • Best time to see: 31 August, 04:00 BST (03:00 UT)
  • Altitude:
  • Location: Aries
  • Direction: Just east of south

Morning planet Uranus improves in visibility this month. The mag. +5.7 planet reaches an altitude of 22˚ in darkness on 1 August, while on 31 August it manages an altitude of 49˚ at the end of darkness, falling just short of its highest point in the sky when due south.

Neptune

  • Best time to see: 31 August, from 02:00 BST (01:00 UT)
  • Altitude: 33˚
  • Location: Aquarius
  • Direction: South

A month from opposition, morning planet Neptune’s position improves over August and by mid-month it reaches its highest point in the sky, 33˚ up, under dark-sky conditions. At mag. +7.8 you’ll need binoculars to spot Neptune.

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