A diagram of Mercury during May 2021. | SkyNews
Mercury, May 2021 (Chris Vaughan, Starry Night Education)

Planets at a glance: May 2021

Mercury is the planet to watch this month, as it wanes and treks away from the Sun, passing Venus and the Moon.

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For those of you who like to listen to your stories, click below to hear Susan Trishel Monsøn reading Chris Vaughan’s summary of the planets movements this month.

Mercury

A diagram of Mercury during May 2021. | SkyNews
Mercury, May 2021 (Chris Vaughan, Starry Night Education)

After sunset throughout the month of May, Mercury will be easily visible by Northern Hemisphere observers while it travels east of the Sun on the high side of a nearly vertical evening ecliptic plane; however, this will be a poor apparition for Southern Hemisphere observers. Sharing the northwestern sky with much brighter Venus, the speedy planet will climb away from the Sun until mid-month. It will pass just to the left (or celestial south) of the Pleiades star cluster on May 4, but the evening twilight will all but overwhelm those Seven Sisters. Mercury will begin shining in a nicely darkened sky after the first week of May, with the best viewing window arriving between about 8:30 and 9:30 p.m. local time. On May 16, Mercury will reach greatest eastern elongation and peak visibility 22 degrees from the Sun. After that, the planet will swing sunward each evening, eventually becoming more and more difficult to see. Since Mercury will be traversing the space between Earth and the Sun, telescope views of the planet during May will show its disk doubling in size while waning in illuminated phase from 80 per cent to eight per cent. On May 28 Mercury will pass within half a degree to the left (or celestial south) of Venus — close enough for them to appear together in a telescope eyepiece. However, Venus will outshine Mercury by 280 times — making seeing Mercury a challenge. On May 13, the crescent Moon will pass several finger widths to the left (or 3 degrees to the celestial southeast) of Mercury.  

BE CAUTIOUS WITH YOUR EYES

Ensure that the Sun has completely disappeared below the horizon before using binoculars or telescopes to view Mercury.

Venus

A diagram of Venus during May 2021. | SkyNews
Venus, May 2021 (Chris Vaughan, Starry Night Education)

Venus will spend all of May parked low in the northwestern post-sunset sky in the opening stages of its lengthy 2021 apparition. During May, it will gradually increase its angle east of the Sun from 9 to 18 degrees, but Venus’ brilliant, -3.9 magnitude will allow our sister planet to be seen easily within the evening twilight. Venus will share the sky with much dimmer Mercury during May. The speedy planet will pass only 0.4 degrees to the left of Venus on May 28, close enough to appear with Venus in a telescope eyepiece. However, Venus will outshine Mercury by 280 times, so seeing Mercury will be difficult. For another challenge, try to see the very young crescent Moon when it passes a finger’s width to the left (or 1 degree to the celestial south) of Venus on May 12. Observers in much of New Zealand, Eastern Polynesia, and Easter Island can see the Moon occult Venus in the mid-day sky. Viewed in a telescope during May, Venus will show an apparent disk diameter of approximately 10 arc-seconds, and a nearly fully-illuminated disk.

BE CAUTIOUS WITH YOUR EYES

As always, ensure that the Sun has completely disappeared below the horizon before using binoculars or telescopes to view Venus.

Mars

A diagram of Mars during May 2021. | SkyNews
Mars, May 2021 (Chris Vaughan, Starry Night Education)

During May, Mars will continue to shine as a medium-bright, reddish dot in the lower third of the western evening sky. Its eastward motion through Gemini is delaying somewhat the planet’s descent into the western twilight. Due to the Earth-Mars separation growing wider, Mars will decrease in brightness from magnitude 1.56 to 1.74 during May, and its apparent disk diameter will diminish from 4.6 to 4.2 arc-seconds. Towards the end of the month, Mars will make a trio to the left of the bright stars Castor and Pollux. On May 15, the waxing crescent Moon will shine several finger widths to the lower right (or 2 degrees to the celestial northwest) of Mars.

Jupiter

Jupiter will shine brightly among the stars of western Aquarius during May. Look for it in the southeastern sky from the wee hours until dawn. On May 1, the magnitude -2.2 planet will rise at about 3:30 a.m. local time. At month-end, Jupiter will begin rising at about 1:30 a.m. local time, and it will have brightened to magnitude -2.4. Unfortunately, the shallow morning ecliptic will prevent the planet from climbing very high before the sky brightens. Telescope views of Jupiter during May will show its large, banded disk increasing in apparent diameter from 37.5 to 41.1 arc-seconds. The Great Red Spot will be visible crossing Jupiter’s disk every second or third morning. Single transits across Jupiter’s disk by the round, black shadows of its Galilean moons will be commonplace in May. Double shadow transits will be visible in different parts of the world on May 23 and 28. Saturn will widen its separation west of Jupiter from 15 to 18 degrees during the month, and the waning moon will pass 5 degrees below and between Jupiter and Saturn on May 4.

Saturn

During May, Saturn will be in the southeastern sky, travelling eastward among the stars of Capricornus and shining with an average magnitude of 0.64. Saturn will rise at about 2:45 a.m. local time on May 1 and at 12:45 a.m. at month’s end, but the low ecliptic will keep the ringed planet from climbing high enough to see very clearly in a telescope. When viewed through a telescope during May, Saturn will exhibit an apparent disk size that grows from 16.7 to 17.6 arc-seconds. Saturn will widen its separation west of Jupiter from 15 to 18 degrees during the month, and the waning moon will pass 5 degrees below and between Jupiter and Saturn on May 4, making a nice photo opportunity.

Uranus

A diagram of Uranus during May 2021. | SkyNews
Uranus, May 2021 | SkyNews

Uranus will be only one day past solar conjunction on May 1. Even though the planet will steadily increase its elongation west of the Sun throughout the month, the shallow morning ecliptic will prevent magnitude 5.9 Uranus, and the stars of Aries surrounding it, from becoming visible until the very end of the month.

Neptune

A diagram of Neptune during May 2021. | SkyNews
Neptune, May 2021 (Chris Vaughan, Starry Night Education)

Distant blue Neptune, in eastern Aquarius, will be increasing its angle from the Sun in the southeastern sky during May. On May 1 the planet will rise at about 4:30 a.m. local time. That will advance to 2:30 a.m. at month end, but planet’s faint magnitude 7.9 and the low morning ecliptic will prevent the planet from climbing high enough for observing in telescopes.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

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