Mercury in the west after sunset

Starting on September 26, 2020, Mercury – the innermost planet – swings out to an elongation of over 25 degrees east of the sun, placing Mercury in the western sky after sunset. Moreover, Mercury remains at better than 25 degrees east of the sun until October 7, 2020. On October 1, 2020 – on the same date as the full moon – Mercury sweeps to its greatest elongation of 25.8 degrees east of the sun.

When a greatest evening elongation of an inferior planet, such as Mercury, closely coincides with the spring equinox, we can anticipate on a fine evening apparition of Mercury. Since the recent September equinox counts as the Southern Hemisphere’s spring equinox, the Southern Hemisphere can expect a favorable evening view of Mercury during the next couple of weeks.

Northern Hemisphere view of Mercury in evening sky.

The shallow tilt of the ecliptic at mid-northern latitudes submerges the planet Mercury and the star Spica in the afterglow of sunset. See the feature chart at top for temperate latitudes in the Southern Hemisphere.

Yet, when a greatest evening elongation of Mercury closely coincides with the autumn equinox, it results in a poor apparition of Mercury in the evening sky. Given that the September equinox is the Northern Hemisphere’s autumn equinox, mid-northern latitudes will have a hard time teasing Mercury out of the afterglow of sunset, even with binoculars.

In short, Mercury sets closer to sunset at more northerly latitudes, but stays out longer after sunset at more southerly latitudes. We give Mercury’s setting time relative to sunset, and the time for nightfall (end of astronomical twilight).

For October 1, 2020:

40 degrees north latitude
Mercury sets 50 minutes (5/6 hour) after sunset
Nightfall: 90 minutes (1 1/2 hours) after sunset

Equator (0 degrees latitude)
Mercury sets 90 minutes (1 1/2 hours) after sunset
Nightfall: 70 minutes (1 1/6 hours) after sunset

35 degrees south latitude
Mercury sets over 120 minutes (2 hours) after sunset
Nightfall: 86 minutes (nearly 1 1/2 hours) after sunset

Want more specific info? Click here for a sky almanac

In either the Northern or Southern Hemisphere, a greatest eastern (evening) elongation of an inferior planet (Mercury or Venus) is most favorable when it coincides with the spring equinox, and the least so when a greatest evening elongation coincides with the autumn equinox. That’s because the ecliptic – the pathway of the planets – hits the horizon at its steepest angle for the year at sunset on the spring equinox, yet at its shallowest angle for the year at sunset on the autumn equinox. Hence, the Southern Hemisphere enjoys a ringside seat to this present evening apparition of Mercury whereas northerly latitudes must be relegated to the bleachers.

At sunset on the spring equinox, the ecliptic – pathway of the sun. moon and planets – hits the sunset horizon at its steepest angle of the year. But at sunset on the autumn equinox, the ecliptic hits the sunset horizon at its shallowest angle. Therefore, when an inferior planet at its greatest evening elongation coincides closely with the spring equinox, it stays out for a maximum time after sunset. Image via Dominick Ford.

Strictly speaking, the ecliptic is the Earth’s orbital plane projected onto the constellations of the zodiac. The ecliptic also depicts the sun’s annual eastward path in front of the backdrop stars, as viewed from Earth. (However, the sun’s apparent annual motion is really a reflection of Earth orbiting the sun.) Because the planets orbit the sun on the nearly the same planet that Earth orbits the sun, you’ll always find the planets of the solar system on or near the ecliptic.

If you live in the Southern Hemisphere, take advantage of your golden opportunity to view Mercury in the evening sky during the last week of September and first week of October.

Bruce McClure