Catch Mercury in the west after sunset

No matter where you live on Earth, mid to late June is an excellent time to look for the planet Mercury in your western sky after sunset. On June 23, 2019, Mercury reaches a milestone the evening sky, as this world swings out to its greatest elongation of 25 degrees east of the setting sun. Mercury, the innermost planet of the solar system, is often lost in the sun’s glare. Yet practiced sky watchers know the best chance of catching Mercury after sunset is generally around the time of Mercury’s greatest eastern elongation. That’s because Mercury is now setting its maximum time after sunset.

From most of the world, Mercury now stays out better than 1 1/2 hours after the sun. To spot Mercury, find an unobstructed horizon in the direction of sunset. Then, starting an hour or so after sundown, watch for Mercury to pop out rather low in the western sky and near the sunset point on the horizon.

Diagram showing sun, Earth and Mercury. Dotted lines between Earth and Mercury's two elongations.

Not to scale. We’re looking down from the north side of the solar system. From this vantage point, Mercury and Earth circle the sun in a counterclockwise direction. 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.

Remember that binoculars always come in handy for any Mercury quest. Although Mercury is as bright as a 1st-magnitude star, its luster will be dimmed by the sunset afterglow and the murkiness of the thickened atmosphere near your horizon.

If your sky is less than crystal clear, try your luck with binoculars. Scan with them for a bright “star” near the sunset point.

With binoculars, you might also catch the red planet Mars taking the stage with Mercury in a single binocular field around this time. Mars is a solid three times fainter than Mercury, so it’s doubtful that you’ll spot the red planet without an optical aid. See their respective positions in our sky – as viewed from the Northern Hemisphere – on the chart above. The chart below shows their positions relative to one another in orbit around the sun:

Diagram of orbits of Mercury, Mars, Earth in thin green and blue lines on black.

A bird’s-eye view of the north side of the inner solar system (Mercury, Venus, Earth and Mars) on June 23, 2019, the date of Mercury’s greatest elongation. Notice that, as seen from Earth, Mercury and Mars are nearly aligned on the same line of sight. Image via Solar System Live.

Mercury’s reign in the evening sky started on May 21, 2019, and will end on July 21, 2019. After today, Mercury will fall sunward, or in the direction of sunset.

What’s more, Mercury’s waning phase is causing this planet to dim day by day. By early July, the fading planet will be easier to spot from the Southern Hemisphere than at mid-northern latitudes.

Graph with tall gray and blue parabolas representing visual distance of Mercury from sun.

View larger. | Here are the year’s apparitions of Mercury compared: 3 swings out from the neighborhood of the sun into the evening sky (gray) and 3 into the morning sky (blue). The top figures are the maximum elongations – maximum apparent distance from the sun – reached at the top dates given beneath. Curving lines show the altitude of the planet above the horizon at sunrise or sunset, for latitude 40 degrees north (thick line) and 35 degrees south (thin), with maxima reached at the parenthesized dates below (40 degrees north bold). Chart via Guy Ottewell.

Bottom line: While the opportunity is at hand, try to spot Mercury, the solar system’s innermost planet, in late June 2019.

Bruce McClure

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