2024, September 4:  Mercury at Greatest Elongation

September 4, 2024: Mercury is farthest from the sun before sunrise, known as the greatest elongation. Look for it in the eastern sky during morning twilight.

Mercury’s September Morning Appearance
Chart Caption – 2024, September 4: A space view of Mercury’s greatest elongation.

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by Jeffrey L. Hunt

Chicago, Illinois: Sunrise, 6:20 a.m. CDT; Sunset, 7:18 p.m. CDT.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Venus, Mercury, Regulus, September 11, 2023
Photo Caption – Venus, Mercury, Regulus, September 11, 2023

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Mercury West of Sun

Mercury is seen farthest from the sun, known as the greatest elongation.  Geometrically, the Sun-Earth-Mercury angle is at its largest. The planet is west of the sun.  To distinguish this event from the greatest elongation occurring during the evening, today’s is known as greatest elongation west, indicating it occurs before sunrise.

Mercury’s September Morning Appearance
Chart Caption – 2024, September 4: At 45 minutes before sunrise, Mercury, at greatest elongation, is low in the east-northeast.

Unlike the poor view of the solar system during late summer evenings, the view is near its best during the morning hours.  The plane of the solar system makes a high angle with the horizon.  While Mercury is only 18° from the sun, it stands over 8° above the eastern horizon at 45 minutes before daybreak.

Mercury’s September Morning Appearance
Chart Caption – 2024, September 4: A theoretical view of Mercury’s greatest elongation, showing its orbital path and direction.

As the planet passes its greatest elongation, it begins to retreat into brighter sunlight, but it brightens on the way.  It is lower in the sky each morning, but it brightens as it heads toward superior conjunction on the far side of the sun at month’s end.

The planets resemble stars.  From a place with a clear horizon toward the east, initially use a binocular to find Mercury.  It is easily visible without the binocular’s assist, shining through morning twilight.

Nightly Planet Parade

Mercury ends the nightly planet parade of five bright planets and two dim worlds.  The parade begins with Venus’ appearance in the west after sunset along with the crescent moon.  Saturn, nearing opposition, rises shortly after sunset and becomes visible in the east-southeast as the sky darkens and Venus sets.

Neptune follows Saturn across the horizon, although nearly impossible to see in a dim Pisces’ starfield.

Uranus, Jupiter, and Mars rise around midnight.  Bright Jupiter is easy to locate, especially as the new day approaches.  It is high in the east-southeast, while dimmer Mars is to its lower left.

Uranus is visible near the Pleiades star cluster through a binocular.

Mercury rises during morning twilight to make its appearance as described earlier, ending the nightly parade.

Mercury is in the parade for about another week.

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