Up for a big observing challenge? Try to spot the planets Mercury and Venus coupling up on the sky’s dome at dusk on April 24, 25 and 26, 2021. We give you fair warning. The quest won’t be easy, even with binoculars. You’ll need an unobstructed horizon in the direction of sunset and crystal-clear skies to view these close-knit worlds in the glow of evening twilight. The real sky will appear brighter than it does on our sky chart, and you probably won’t see the constellation Orion until after Mercury and Venus have already set.
The Northern Hemisphere has the advantage because these two worlds stay out longer after sunset at more northerly latitudes, At southerly latitudes, the planetary twosome sets sooner after the sun. We give you the approximate setting times for the next few days at various latitudes:
60 degrees north latitude: Mercury-Venus set about 50 minutes after the sun
40 degrees north latitude: Mercury-Venus set about 35 minutes after the sun
Equator (0 degrees latitude): Mercury-Venus set about 30 minutes after the sun
35 degrees south latitude: Mercury-Venus set about 20 minutes after the sun
The good news is that both of these planets are brilliant. Venus ranks as the third-brightest celestial object to light up the heavens, after the sun and moon, respectively. Although Mercury is some 8 times dimmer than Venus, Mercury at present shines as brilliantly as Sirius, the brightest star in the nighttime sky. Unlike Sirius, though, Mercury and Venus will set before nightfall, and Sirius probably won’t pop out until after Mercury and Venus set.
Don’t worry if you miss these hard-to-find worlds over the next several days. Day by day, both Mercury and Venus will climb up higher in the sky at sunset and will stay out longer after sundown. Mercury, the faster-moving planet, will soar above Venus in the evening twilight for the next month or so, or until Mercury reaches its greatest elongation – maximum angular distance from the setting sun – on May 17, 2021. After that, Mercury will descend downward, toward the sunset, as Venus continues its climb upward, away from the setting sun.
Mercury and Venus will meet up again for their final conjunction for the year on May 29, 2021 (from North America: evening of May 28). This time around, their rendezvous will find them higher up in the sky at sunset, to say out longer after sundown.
Shortly after sunset – April 24, 25 and 26, 2021 – try to spot the furtive rendezvous of the two inferior planets, Mercury and Venus, at evening dusk.