Mars is the only bright evening planet to stay out until after dark throughout May 2021. Although Mercury and Venus pop out into the western twilight after sunset all month long, both of these worlds will follow the sun beneath the horizon before it gets good and dark. Best to seek out modestly-bright Mars at early evening, when it’s still relatively high in the sky. Mars slowly descends westward during the evening hours, to set in the west at late evening at mid-northern latitudes, and around mid-evening at temperate latitudes in the Southern Hemisphere.
Mars remains in front of the constellation Gemini the Twins throughout the month, until the red planet moves over into the constellation Cancer the Crab on June 8, 2021. Enjoy Mars in May 2021! It’s only going to get fainter as this year progresses. In the months ahead, Mars will slowly but surely dim as – day by day – it lags farther behind Earth in the great race of the planets, and sinks closer to the setting sun.
Let the moon help guide your eye to the red planet for several nights centered around May 15.
Venus – the brightest planet – and Mercury – the innermost planet – adorn the western twilight dusk all month long. For the most part, Venus sits beneath Mercury at dusk, or until these worlds finally meet up for a conjunction in late May 2021. But don’t wait till then to seek for Mercury, for the innermost planet beams some 40 times brighter at the beginning of the month than at the month’s end.
Mercury then crossed over into the evening sky (at superior conjunction) on April 19, 2021. Although Venus entered the evening sky more than three weeks before Mercury did, the innermost planet moves much more quickly around the sun, and in Earth’s sky. Climbing upward, Mercury caught up with Venus for a conjunction in the evening sky on April 26, 2021.
Mercury will continue to travel upward, away from the afterglow of sunset, until it swings out to its greatest elongation from the sun in mid-May. Thereafter Mercury will sink sunward, to again meet up with Venus for another conjunction on May 28, 2021.
Mercury will exit the evening sky to enter the morning sky when it passes between the Earth and sun (at inferior conjunction) on June 11, 2021. (See diagram below.)
Venus, on the other hand, won’t sweep to its greatest elongation in the evening sky until October 29, 2021. Venus will remain a fixture of the evening sky for the rest of this year, to reach inferior conjunction on January 9, 2022.
This month, use dazzling Venus to find fainter yet bright Mercury, which actually shines more brilliantly than a 1st-magnitude star during the first half of May 2021. Venus ranks as the third-brightest celestial object to light up the sky, after the sun and moon, so you have a good chance of spotting Venus 1/2 hour or sooner after sundown.
Once you spot Venus, this brilliant beauty of a planet will sink downward in the darkening evening twilight. You may – or may not – spot Mercury before Venus follows the sun below the horizon. Either way, be mindful that Mercury is there, a rather short hop above Venus. Mercury may become visible to the unaided eye an hour or so after sunset. If you can’t catch Mercury otherwise, try your luck with binoculars.
For the Northern Hemisphere, May 2021 will present the best month of the year to view Mercury in the evening sky. Don’t miss out!
Setting times for Mercury and Venus (U.S. and Canada) via Old Farmer’s Almanac
Setting times for Mercury and Venus (worldwide) via TimeandDate.com
Jupiter and Saturn remain somewhat close together, yet are slowly spreading apart, in the May predawn/dawn sky. Both planets are well up before dawn’s first light, and will be easier to view in the predawn hours than they were last month.
Saturn rises first. At mid-northern latitudes, Saturn rises about 1 1/2 hours after local midnight in early May, and around local midnight at the month’s end. (By midnight, we mean midway between sunset and sunrise.) Jupiter follows Saturn into the sky about 1/2 to 3/4 hour later.
At temperate latitudes in the Southern Hemisphere, Saturn rises around midnight in early May, and by the month’s end, comes up at late evening. Jupiter follows Saturn into the sky around 1 to 1 1/2 hours later.
What do we mean by bright planet? By bright planet, we mean any solar system planet that is easily visible without an optical aid and that has been watched by our ancestors since time immemorial. In their outward order from the sun, the five bright planets are Mercury, Venus, Mars, Jupiter and Saturn. These planets actually do appear bright in our sky. They are typically as bright as – or brighter than – the brightest stars. Plus, these relatively nearby worlds tend to shine with a steadier light than the distant, twinkling stars. You can spot them, and come to know them as faithful friends, if you try.
Bottom line: May 2021 presents all five bright planets! Mars is the only bright evening planet out after dark. Mercury and Venus adorn the western sky, whereas Jupiter and Saturn are morning planets.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.