How to see the planets, May 2021

What planets can you see tonight? Find out which Solar System worlds will be visible in the night sky throughout May 2021 with our observing guide.

Mercury and Venus will be good planetary targets to observe during May 2021. Credit: Pete Lawrence. during May

Mercury is a good planet to see in May 2021, shining away in the evening twilight near its Solar System neighbour, mag. –3.8 Venus. It’s a difficult call to pick a date which is the best time to see the planet.

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A classic balance between brightness and solar separation takes place throughout the month, Mercury appearing bright at the start of May when it’s relatively near to the Sun, dimming mid-month when it’s farthest from the Sun.

It then continues to dim as it creeps closer to the Sun once more.

For more events like this, keep an eye on our regularly updated page on astronomical conjunctions and our monthly Star Diary podcast.

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Observe Mercury and Venus throughout May 2021 for some lovely conjunctions involving the Moon. A pair of 7x50 binoculars should fare you well. Credit: Pete Lawrence
Observe Mercury and Venus throughout May 2021 for some lovely conjunctions involving the Moon. A pair of 7×50 binoculars should fare you well. Credit: Pete Lawrence

On 1 May, Mercury shines at mag. –1.0 and sets 90 minutes after the Sun. Venus will be 5˚ away on this date. From the UK it’s visible below Mercury.

On 3 May, Mercury drifts 2.3˚ south of the Pleiades open cluster, M45. Shining at mag. –0.8 on this date, Mercury should be seen quite clearly through the evening twilight. The Pleiades may not fare so well though!

On 13 May, Mercury will have dimmed to mag. +0.2 but should be easy to spot as it lies 2.7˚ to the north of a 3%-lit waxing crescent Moon. Mercury sets an impressive 135 minutes after the Sun on this date.

Greatest eastern elongation occurs on 17 May when Mercury will be at mag. +0.6. After this the planet dims but remains visible thanks to a re-approach of Venus.

On 28 May, mag. +2.2 Mercury sits just 32 arcminutes from bright Venus and sets 90 minutes after the Sun.

On 31 May, little Mercury will be well on its way back towards the Sun, setting just 70 minutes after sunset. At mag. +2.9 it will be a lot harder to see than earlier in the month.

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

How to see the planets, May 2021

Mercury

  • Best time to see: 4 May, from 30 minutes after sunset Altitude: 9˚ (low)
  • Location: Taurus
  • Direction: West-northwest
  • Features: Phase, larger apertures may detect surface features
  • Recommended equipment: 75mm or larger

Venus

  • Best time to see: 31 May, from 30 minutes after sunset
  • Altitude: 7˚ (low)
  • Location: Taurus
  • Direction: Northwest

Venus is an evening planet, setting 50 minutes after the Sun on 1 May and 1.5 hours at the month’s end. A thin Moon, less than 1%-lit sits 2.1˚ southwest of Venus on 12 May. On this date Venus sets 70 minutes after the Sun. On 28 May mag. +2.2 Mercury and Venus appear separated by just 32 arcminutes.

Despite the bright evening twilight at this time of year, mag. –3.9 Venus should stand out well after the Sun has dropped below the horizon. It’s on the far side of its orbit from Earth and as a consequence, through a telescope appears small – for Venus anyway – and almost fully lit with a phase that decreases from 99%-lit on 1 May to 96%-lit on the 31st.

Mars

  • Best time to see: 1 May, from 22:45 BST (21:45 UT)
  • Altitude: 21˚
  • Location: Gemini
  • Direction: West

The apparent size of Mars drops further during May, from 4.6 arcseconds on 1 May to 4.2 arcseconds at the month’s end. Mars also struggles to keep ahead of the evening twilight. On 1 May, Mars appears north of Castor’s right foot, at Gemini’s western end. It can just be seen properly against dark skies, albeit low over the west-northwest horizon.

Mars then passes through the stick-figure bodies representing the heavenly twins, to end up 5˚ south of the southernmost of the two twin stars in Gemini, Pollux (Beta (b) Geminorum) on 31 May. On this date, the now mag. +1.7 planet is unable to be seen against dark skies.

Jupiter

  • Best time to see: 31 May, from 04:00 BST (03:00 UT)
  • Altitude: 15˚
  • Location: Aquarius
  • Direction: Southeast

Jupiter is a morning planet, reaching 14˚ altitude at sunrise on 1 May, rising 70 minutes before the Sun. A 35%-lit waning crescent Moon lies 5.9˚ south-southeast of Jupiter on 5 May. By the month’s end Jupiter’s visibility will have improved, the gas giant rising three hours before the Sun, reaching a 20˚ altitude at sunrise.

Jupiter reaches an equinox on 2 May, a time when the Sun’s centre will appear on the projection of Jupiter’s equatorial plane as seen from a Jovian perspective. From Earth this is a time when the four largest Galilean moons can appear to interact with each other in mutual events.

For more on this, read our guide to Jupiter’s equinox and mutual events.

Saturn

  • Best time to see: 31 May, from 03:45 BST (02:45 UT)
  • Altitude: 15˚
  • Location: Capricornus
  • Direction: South-southeast

Saturn appears as a mag. +0.7 morning object in the constellation of Capricornus. A 57%-lit waning gibbous Moon lies near Saturn on 3 May and as a 46%-lit crescent on 4 May. The Moon then revisits on the morning of 31 May with a larger 72%-waning gibbous phase. Saturn is able to reach an altitude of 17˚ before it succumbs to the morning twilight at May’s end.

Uranus

Not visible this month

Neptune

Not visible this month

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