2021, September 13: Mercury at Greatest Elongation


September 13, 2021:  Mercury reaches greatest elongation tonight.  It is a challenge to see in the bright twilight.

Chart Caption – 2021, September 13: Speedy planet is low in the west-southeast, 25 minutes after sunset, to the lower right of brilliant Venus.


by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:29 a.m. CDT; Sunset, 7:03 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

Speedy planet Mercury reaches its greatest elongation from the sun tonight.

The speedy planet revolves around the sun every 88 days, sprinting between Earth and the sun about every 116 days.

During the longer interval, the planet races from the morning to evening sky and heads toward the morning sky, again.

From Earth, we see the planet hurled into the eastern morning sky, appearing higher each morning, although the planet is almost always visible only during twilight from the mid-northern latitudes, and rarely in a dark sky.  Then, as if pulled by a string, it heads back into bright twilight, repeating the same cycle in the western evening sky after sunset.

When the planet appears as far from the sun as it can get, this is known as the greatest elongation.  Elongation is an angular measurement from the sun to the planet.  The values range from 0° to 180°.

When one of the outer planets is at opposition, it is 180° from the sun, its maximum elongation.  Mercury and Venus do not appear far from the sun.  Mercury’s maximum angular separation is about 27°, while Venus is about 47°.

The jargon becomes a little confusing when the direction of the elongation is included.  Maximum morning elongations are called “greatest elongation west.”  The planet is west of the sun, rising before sunrise, and appearing in the eastern morning sky.

The evening elongation is “greatest elongation east.”  Mercury is east of the sun, setting after sundown, and appearing in the western evening sky.”

This is clearly confusing, so I have started using “evening greatest elongation” and “morning greatest elongation.”  The observing facts are the same, but the jargon might be a little less confusing.

Mercury’s orbit is tilted compared to the plane of the solar system, that is defined by the plane made by the Earth and Sun.  The speedy planet can appear 7° above or below the plane.  Currently the planet is below the plane of the solar system. This means that Mercury is lower in the sky at sunset than if it were above the plane of the ecliptic.

The final factor is the tilt of the ecliptic compared to the western horizon.  When viewed against the horizon, the plane wobbles like a bent rim on a bicycle.  Sometimes the angle is shallow and other times it makes a high angle with the horizon.

During the summer season in the northern hemisphere, the ecliptic makes a shallow angle with the western horizon, so that Mercury and Venus appear low in the sky after sunset, even when having larger elongations for the sun.

If you’ve been watching Venus, since it appeared in the western sky from earlier during the year, it appears in a struggle to climb higher.  In three evenings, Venus sets after the end of evening twilight until December 26.

From our place in the solar system, any two of the observing factors – value of greatest elongation, Mercury’s location compared to the ecliptic, and the angle the solar system makes with the western horizon, can severely affect the observing prospects for the planet.

Mercury’s greatest elongation this evening is 26.8°, a very favorable factor.  The other two factors are poor.  Mercury, below the ecliptic and a shallow angle of the ecliptic with the horizon, has a very unfavorable appearance for easy observing.

To find speedy Mercury, use a binocular to search for it low in the west-southwest about 25 minutes after sunset.  The western sky is fairly bright.  At this hour Venus is visible to the unaided eye, but it might take some diligent searching to find it.  The brilliant planet is over 10° above the west-southwest horizon.  Mercury, about 4° up in the west-southwest, is 15.8° to the lower right of brilliant Venus.  That’s a little over two-binocular fields away from the brilliant planet.  Mercury is low in a bright sky, without any celestial guide to easily lead the way to it.

For those with computer-guided telescopes, use a low-power eyepiece and let the computer attempt to take you to the speedy world.  Otherwise, the search with a binocular is certainly a challenge.  If you cannot find the speedy world before it sets, just 46 minutes after sunset, stay at your observing site to see Venus easily gleam through the colors of evening twilight.  Look toward the southeast to find Jupiter and Saturn.  A bright planet spectacular and the first quarter moon shine during early evening.

Detailed Daily Note:The moon is at its First Quarter phase at 3:39 p.m. CDT.  Twenty-five minutes after sunset, Mercury is about 4° up in the west-southwest.  Use a binocular.  Forty-five minutes after sunset, Venus is about 8° up in the west-southwest, 11.8° to the lower right of Zubenelgenubi and 3.9° to the lower right of λ Vir.  The moon (7.0d, 52%) is over 21° up in the south-southwest, in Ophiuchus between Scorpius and Sagittarius.  It above a line from Antares and Kaus Borealis (λ Sgr, m = 2.8).  The lunar orb is 13.6° to the upper left of Antares and 13.0° to the lower right of Kaus Borealis.  Farther eastward, Saturn is nearly 22° above the south-southeast horizon.  Jupiter is 16.7° to the lower left of Saturn and over 17° above the southeast horizon.  Two hours after sunset. Saturn is nearly 28° up in the south-southeast, 1.1° to the lower right of υ Cap.  Jupiter, at nearly the same altitude as Saturn, is above the southeast horizon.  In the starfield, the Jovian Giant is 4.9° to the upper right of ι Aqr, 2.0° to the lower right of μ Cap, and 1.4° to the upper left of Deneb Algedi.  Mercury reaches its greatest elongation (26.8°) from the sun at 11:24 p.m. CDT.


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