Depending on where you live worldwide, the innermost planet Mercury reaches its greatest elongation from the sun on July 29 or 30, 2017. That is its farthest point from the sun on our sky’s dome. Because this is a greatest eastern elongation, Mercury will be to the east of sunset. In other words, it’ll be low in the western sky after sunset. Look for Mercury in the western twilight, starting around 45 minutes after the sun goes down.
Mercury will be pretty much on line with two much brighter celestial objects, the moon and the planet Jupiter. Since these two brilliant beauties will pop out almost immediately after sunset, let them help guide you to Mercury. You’ll need an unobstructed horizon in the direction of sunset and a clear sky to view Mercury. You might also want to bring a pair of binoculars, just in case your sky is murky near the horizon. At mid-northern latitudes, Mercury sets roughly one hour after the sun.
Click here for an almanac telling you Mercury’s setting time in your sky.
Because Mercury, the innermost planet, orbits the sun inside Earth’s orbit, this world always stays closely tethered to the sun in Earth’s sky. But on July 30, 2017, at around 5 UTC, Mercury swings to its greatest angular distance of 27o east of sun, placing Mercury in the evening sky. However, since Mercury goes around the sun in a very eccentric (oblong) orbit, its greatest elongations vary from about 18 to 28o. In fact, Mercury’s greatest elongation on July 30, 2017 counts as the greatest greatest elongation of the year:
Greatest elongations 2017:
Jan 19: 24o 08′ west (morning)
Apr 01: 19o 00′ east (evening)
May 17: 25o 47′ west (morning)
Jul 30: 27o 12′ east (evening)
Sep 12: 17o 56′ west (morning)
Nov 24: 22o 00′ east (evening)
Mercury goes a whopping 27o east of the sun in late July 2017. You’d think this should be the best apparition of Mercury in the evening sky for 2017. But no, not for us at mid-northern latitudes! For instance, at 40o north latitude (Philadelphia, Pennsylvania), Mercury sets about one hour and 9 minutes after the sun. But in the Southern Hemisphere, at 40o south latitude (Tasmania), Mercury sets some two hours and 15 minutes after the sun. The Southern Hemisphere has the big advantage, although Mercury’s elongation from the sun is the same in both hemispheres.
In either the Northern or Southern Hemisphere, the most favorable greatest elongation of an inferior planet (Mercury or Venus) is very much dependent on the season. A greatest evening elongation is the most auspicious whenever it closely coincides with the spring equinox – and the least so when it occurs near the autumn equinox.
Mercury last reached its greatest evening elongation on April 1, 2017. Although Mercury’s evening elongation was only 19o east of the sun at the time, it was a better evening elongation at mid-northern latitudes than the upcoming one on July 30, 2017. That’s because the greatest evening elongation on April 1, 2017, came at the heels of the March 20th equinox (the Northern Hemisphere’s spring equinox).
Look for Mercury in the glow of evening dusk anyway, as the moon and Jupiter are there to help guide you to Mercury’s place near the horizon. Good luck!
Bottom line: Mercury’s greatest elongation from the sun comes July 29 or 30, 2017. That is its farthest point from the sun on our sky’s dome.
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.