Best month to see Mercury after sunset

Live in the Northern Hemisphere? If so, May 2021 presents the year’s best month for spotting the planet Mercury, the solar system’s innermost planet, in the evening sky. You can also catch Mercury from the Southern Hemisphere, though the view won’t be as favorable as at more northerly latitudes. From the Southern Hemisphere, the year’s best evening apparition of Mercury will be coming in September 2021.

No matter where you live worldwide, however, you’ll want to find an unobstructed horizon in the direction of sunset to maximize your chances of catching Mercury. Mercury, the innermost planet of the solar system, isn’t difficult to spot because it’s dim. Rather, Mercury often eludes detection, because sits low in the west at sunset and then follows the sun beneath the horizon before true darkness falls. In other words, you have to catch Mercury near the sunset point on the horizon as evening dusk is giving way to nightfall.

Mercury shines at its brightest for the month in early May, and then slowly dims each day thereafter. Throughout the first week of May, Mercury actually shines at a negative magnitude, or brighter than a 1st-magnitude star. (Like in golf, where negative means a better score, a negative magnitude in astronomy indicates a brighter object.) But even as Mercury dims, it climbs farther away from the sun each day, to reach its greatest elongation from the sun on May 17, 2021. At that time, Mercury’s brilliance will match that of 1st-magnitude star.

Young moon goes by the evening planets, Venus, Mercury and Mars from May 12 to 15, 2021.

The young waxing crescent moon helps to guide your eye to the evening planets, Mercury, Venus and Mars from May 12 to 15, 2021.

In other words, chances are that you may well see Mercury with the eye alone, despite the glow of evening twilight. Binoculars, though, always help out in any Mercury quest, especially if the viewing is a bit murky near the horizon. Even with ideal seeing conditions, binoculars enable you to catch Mercury all the sooner after sunset.

Given an absolutely unobstructed horizon in the direction of sunset, we give you Mercury’s approximate setting time for various latitudes:

60 degrees north latitude:
Early May: Mercury sets 1 hour and 55 minutes after sunset
Middle May: Mercury sets 2 hours and 50 minutes after sunset
Late May: Mercury sets 1 hour and 25 minutes after sunset

40 degrees north latitude:
Early May: Mercury sets 1 hour and 15 minutes after sunset
Middle May: Mercury sets 1 hour and 55 minutes after sunset
Late May: Mercury sets 1 hour and 5 minutes after sunset

Equator (0 degrees latitude):
Early May: Mercury sets 55 minutes after sunset
Middle May: Mercury sets 1 hour and 30 minutes after sunset
Late May: Mercury sets 1 hour after sunset

35 degrees south latitude:
Early May; Mercury sets 35 minutes after sunset
Middle May: Mercury sets 1 hour 10 minutes after sunset
Late may: Mercury sets 1 hour after sunset

For more specific information, go to Old Farmer’s Almanac (USA and Canada) or TimeandDate (worldwide)

Starting the last week of May 2021, Mercury will dim to 2nd-magnitude, shining about 1/8th as brilliantly as it does in early May. Even so, Venus – the brightest of all planets – will shine rather close to Mercury on the sky’s dome in last week of May. So you can aim your binoculars at Venus to gaze at Mercury taking stage with Venus in a single binocular field.

Most likely, you won’t see Venus early in the month, as this world sits rather close to the glare of sunset throughout the first week of May. Day by day, however, Venus will climb upward, away from the sunset glare. Finally, Venus will catch up with Mercury to showcase a close-knit conjunction on May 29 (evening of May 28 in the Americas).

MAY 28 CONJUNCTION CHART

Venus rates as the 3rd-brightest celestial object to light up the heavens, after the sun and moon, respectively. Because Venus beams so mightily, you can use brilliant Venus to locate faint Mercury in the latter part of the month. Mercury will become a faint 3rd-magnitude object by late May, or by the time of the Mercury-Venus conjunction on May 28 or 29 (depending on your time zone). So you’ll probably need binoculars for any chance of spotting Mercury next to Venus in late May.

Don’t tarry! Start looking for Mercury in the first few weeks of May, while this gleaming world is still bright and beautiful. Right now, in early May, Mercury shines some 40 times more brilliantly than it will by the month’s end.

Bruce McClure