Astronomy Essentials

Mercury greatest elongation September 14, best for S. Hemisphere

Nearly vertical ecliptic line with Venus, Spica, and Mercury close to it.
Here’s Mercury around the time of its greatest elongation on September 14, 2021, as seen from Earth’s Southern Hemisphere. Note the steepness of the ecliptic, or sun’s path. This steep angle – plus Mercury’s large distance from the sun at greatest elongation (27 degrees) – lets the innermost planet stay out until after dark as seen from the southern half of Earth’s globe. Northern Hemisphere? No such luck. See below.

Mercury from Southern Hemisphere

Mercury reaches its greatest distance from the setting sun on the sky’s dome – a whopping 27 degrees – on September 14, 2021. Astronomers call it a greatest elongation of Mercury. The innermost planet has been staging its best evening apparition of 2021 for the Southern Hemisphere since late August. It’ll be visible at south temperate latitudes through the rest of September. If you’re on the southern part of Earth’s globe, you’ll find Mercury shining brightly now in the west until after the end of evening twilight. Venus is up there, too, blazing away more brightly than Mercury at nearly -4 magnitude. But Mercury is bright, too, brighter than most stars. It’s shining at around magnitude 0 when at greatest elongation.

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Observers in the Southern Hemisphere (and the northern tropics) can easily see Mercury with the eye alone now. At temperate latitudes in the Southern Hemisphere, around the time of greatest elongation, Mercury will stay out for a long time – about 45 minutes – after nightfall. Meanwhile, at mid-northern latitudes, Mercury is difficult to see, immersed in the afterglow of sunset. If you’re in the Northern Hemisphere, you might need binoculars to find Mercury. At mid-northern latitudes, Mercury sets only about 45 minutes after sunset.

There’s a little star – fainter than Venus, fainter than Mercury – near the two planets. It’s Spica in the constellation Virgo the Maiden. Spica is one of our sky’s brightest stars. But it’ll appear diminished in the evening twilight glow. If you’re in the Southern Hemisphere, watch for Mercury-Spica conjunctions around September 23 and again around September 30.

Find the setting time of Mercury in your sky via timeanddate.com (worldwide) and Old Farmer’s Almanac (U.S. and Canada)

Chart with quarter moon, Antares, Venus, Spica, with Mercury close to horizon, all nearly along shallow ecliptic line.
Here’s Mercury from mid-northern latitudes, around the time of its greatest elongation on September 14, 2021. Although Mercury’s evening elongation is nearly 27 degrees on this date, the shallow tilt of the ecliptic from northerly latitudes keeps Mercury buried in the afterglow of sunset.
Nearly vertical ecliptic line with Venus and, below but not at horizon, Spica and Mercury close together.
Here’s the first Mercury-Spica conjunction of this month – around September 23 – as seen from the Southern Hemisphere. The nearly vertical angle of the ecliptic to the evening horizon – from the Southern Hemisphere in spring (September, October, November) – provides those at southerly latitudes with a ringside seat. Mercury and Spica will have a 2nd conjunction before the month ends.
Very slanted ecliptic line with Venus, and Mercury next to horizon very close to a star.
Here’s the Mercury-Spica conjunction – around September 23 – as seen from the Northern Hemisphere. The shallow tilt of the ecliptic, or sun’s path, from the Northern Hemisphere in autumn will make both Mercury and Spica hard to spot from northerly latitudes.

Mercury’s greatest evening elongation

We refer you to the illustration below. Mercury, an inferior planet, first entered the evening sky (at superior conjunction) on August 1, 2021. It’ll reach its greatest eastern (evening) elongation on September 14, 2021, and then sweep between the sun and Earth (at inferior conjunction) on October 9, 2021.

Afterwards, Mercury will emerge in the east before dawn, to stage 2021’s best morning apparition of Mercury for the Northern Hemisphere. Read more about Mercury on October mornings.

Diagram showing orbits with lines of sight to inferior planet in different positions around its orbit.
Not to scale. Mercury’s highly elliptical orbit places the innermost planet anywhere from 0.31 to 0.47 the Earth’s distance from the sun (0.31 to 0.47 of an astronomical unit). Mercury swung to the far side of the sun at superior conjunction on August 1, 2021. It’ll reach its greatest eastern (evening) elongation on September 14, 2021, and then sweep between the sun and Earth at inferior conjunction on October 9, 2021.

Seasonal advantage smiles on Southern Hemisphere

Far and away, the Southern Hemisphere enjoys the best evening apparition of Mercury for the year. For the Northern Hemisphere, this evening showing of Mercury is the poorest. You might wonder why the stark contrast between the two hemispheres.

Remember this rule. A greatest eastern (evening) elongation of an inferior planet (Mercury and Venus) is most favorable when it happens at or near the spring equinox. Conversely, a greatest eastern (evening) elongation is least favorable around the autumn equinox. This rule applies to both the Northern and Southern Hemispheres.

This year, Mercury’s greatest elongation comes on September 13 or 14, 2021 (depending on time zone). That’s in close vicinity to the September 22 equinox.

The September equinox counts as the Southern Hemisphere’s spring equinox, yet the Northern Hemisphere’s autumn equinox. That’s why the evening apparition of Mercury in August-September 2021 so greatly favors the Southern Hemisphere. If you live in the Southern Hemisphere, take advantage of your golden opportunity. Let your eyes feast upon Mercury at its finest in your evening sky!

A colorful sunset with a thin crescent moon, overlooking a small city nestled in a valley, together with two bright planets.
View at EarthSky Community Photos. | Earlier this year – May 14, 2021 – Andreas Vasssiliou in Artemis, Attica, Greece, took this photo of the moon (top), Venus (bottom) and Mercury (between the moon and Venus) at dusk. In August-September 2021, however, Mercury lurks below Venus, instead of above. Thank you, Andreas!
Graph of Mercury's appearances in the sky in 2021 as a series of steep arcs.
View larger. | This graph compares Mercury’s 3 morning and 3 1/2 evening appearances of 2021. Gray for evening and blue for morning. The top figures are the maximum elongations (dates shown beneath). Curves 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). See how September 2021 is a good bet for Mercury from Southern Hemisphere? Meanwhile, September 2021 is poor for northerly latitudes. Image via Guy Ottewell.

Bottom line: Watch for Mercury from the Southern Hemisphere around the time of its greatest elongation on September 14, 2021. Meanwhile, from northerly latitudes, bring binoculars!

Posted 
September 13, 2021
 in 
Astronomy Essentials

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