Mercury’s greatest evening elongation January 23-24

Mercury, the solar system’s innermost planet, isn’t hard to see because it’s faint, but because this world is so often lost in the sun’s glare. At present, Mercury is reaching the outer edge of its orbit as seen from Earth, placing Mercury optimally high in the western evening sky. What’s more, Mercury shines more brilliantly than almost any star in the starry sky.

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At present, Mercury is nearly five times brighter than the nearby star Fomalhaut, a respectable 1st-magnitude star. Most likely, if you only see one starlike object near the sunset point on the horizon as dusk ebbs into darkness, it’s Mercury. Look westward for Mercury, with either the unaided eye or binoculars, some 45 to 60 minutes after sunset.

Here’s a rough guide to Mercury’s setting time for the next several days (presuming a level horizon):

60 degrees north latitude: Mercury sets 2 hours after sunset
40 degrees north latitude: Mercury sets 1 1/2 hours after sunset
Equator (0 degrees latitude): Mercury sets 1 1/4 hours after sunset
35 degrees south latitude: Mercury sets 1 hour after sunset

Find more specific information at Old Farmer’s Almanac (U.S. and Canada) or TimeandDate (worldwide).

We refer you to the diagram below. Mercury first entered the evening sky (at superior conjunction) on December 20, 2020, and will leave the evening sky (at inferior conjunction) on February 8, 2021. Mercury swings to its greatest eastern (evening) elongation – maximum angular distance of 18.6 degrees east of the sun – on January 24, 2021, at 02:00 UTC. At U.S. time zones, that translates to January 23, at 9 p.m EST, 8 p.m. CST, 7 p.m. MST and 6 p.m. PST.

Diagram showing solar system from above, and Mercury at eastern and western elongation.

Not to scale. Mercury’s mean distance from the sun is about 0.39 times Earth’s distance from the sun. We’re looking down at the north side of the solar system plane. In this view, Mercury and Earth circle the sun in a counterclockwise direction. Earth and Mercury also rotate counterclockwise on their axes. At its greatest eastern elongation, Mercury is seen in the west after sunset; and at its greatest western elongation, Mercury is seen in the east before sunrise.

No matter where you live worldwide, however, you have to catch Mercury after sundown but before Mercury sets. Mercury will remain 18 degrees east of the sun for about a week, from January 21 to 27. Take advantage of your golden opportunity to view Mercury while the time is at hand!

By the way, Mercury and Fomalhaut are pretty much side by side as viewed from mid-northern latitudes. At more northerly latitudes, Mercury is higher up in the sky than Fomalhaut, and you may not see Fomalhaut at all, even with binoculars. At more southerly latitudes, Fomalhaut is higher up than Mercury, and this star will still be out at southerly latitudes after Mercury has set.

Right now – January 23 and 24, 2021 – presents the best time to view Mercury during its present apparition in the evening sky. Day by day, Mercury will dim bit by bit as it falls sunward throughout the next couple of weeks. By the month’s end, Mercury will still be some 15 degrees east of the sun, yet no brighter than Fomalhaut.

Bottom line: Look for Mercury in the western sky at evening the next couple of days. Mercury will be dscending closer to the sun over the next few weeks, becoming less visible because of th sun’s glare.

Bruce McClure