Astronomy Essentials

Mercury – up before sunrise – greatest elongation September 22

Circles showing Earth and Mercury orbits around the sun and 2 red lines from Earth to Mercury and the sun.
At greatest elongation, Mercury appears on one side of the sun as seen from Earth. It’s at its greatest distance from the sun on our sky’s dome. Mercury reaches greatest morning elongation from the sun on September 22, 2023. At that time, it’s 18 degrees from the sunrise. Easy for the Northern Hemisphere. Tougher for the Southern Hemisphere. Chart via John Jardine Goss/ EarthSky.

The innermost planet Mercury orbits the sun every 88 days. And Earth is moving, too. So Mercury goes between us and the sun pretty often, about every 116 days. It did this last on September 6, 2023, reaching the point astronomers call inferior conjunction. Since then, Mercury has been speeding ahead of Earth in orbit. And it is now re-emerging into our dawn sky, near the sunrise, below brilliant Venus. Look for it soon! Mercury will reach its greatest morning elongation – its greatest apparent distance from the rising sun – on September 22.

Because the ecliptic – path of the sun, moon and planets – makes a steep angle to our eastern horizon before dawn at this time of the year, this September, 2023, Mercury elongation is best viewed from the Northern Hemisphere.

The innermost planet – named for the fleet-footed messenger god of the ancient Romans – will be visible well into October.

Mercury greatest elongation September 2023

When to watch: Mercury becomes visible in the mid-September 2023 morning sky. It’ll be especially easy to find if you are in the Northern Hemisphere, where the ecliptic makes a steep angle with the eastern predawn horizon. Greatest elongation is – when Mercury will be farthest from the sunrise on our sky’s dome – on September 22. But the planet will continue getting brighter after that. So, in late September, although it’ll be edging back toward the sunrise, Mercury will be easier to spot in the morning twilight.
Where to look: Look in the sunrise direction, as the sky is getting lighter.
Greatest elongation is on September 22 at 13 UTC (8 a.m. CDT). Mercury is shining at a -0.3 magnitude that morning. And it is 18 degrees from the sun.
Through a telescope on and around September 22, Mercury appears 51% illuminated, in a gibbous phase, and 7.1 arcseconds across.
Note: Once you spot it, notice that Mercury brightens quickly in September, reaching a magnitude of around -1.3 before slipping away in the morning glare in October.

Mid-September mornings: Venus and Mercury Northern Hemisphere

3 white dots in a line. Mercury is at the bottom, close to the horizon. Regulus is higher and Venus is at the top.
Around mid-September, for viewers in the Northern Hemisphere, Mercury will rise in the east shortly before sunrise. It will lie in the bright morning twilight near the similarly bright star Regulus and far below brilliant Venus. Mercury will reach greatest elongation from the sun on September 22, 2023. Mercury will brighten from magnitude -0.3 to -1.0 during the month. This is the best morning apparition for Mercury in 2023 for the Northern Hemisphere. Around greatest elongation. Mercury rises about an hour before sunrise. Chart via John Jardine Goss/ EarthSky.

Mid-September mornings: Venus and Mercury Southern Hemisphere

White dots for Mercury, Regulus and Venus, plus a green ecliptic line for Southern Hemisphere viewers.
Around mid-September, Mercury will rise shortly before sunrise and will lie in the bright morning twilight near the similarly bright star Regulus. It’ll be to the lower right of Venus as well. Mercury will reach greatest elongation from the sun on September 22, 2023. Mercury will brighten from magnitude -0.3 to -1.0 during the month. Chart via John Jardine Goss/ EarthSky.

Late-September mornings: Venus and Mercury Northern Hemisphere

White dots for Mercury, Regulus and Venus, plus a green ecliptic line for Northern Hemisphere viewers.
In late-September, for viewers in the Northern Hemisphere, Mercury will rise in the east about an hour sunrise in late September. It will lie far below brilliant Venus, and below the bright star Regulus. Mercury will shine at magnitude -1.0 at month’s end. Mercury reached greatest elongation from the sun on September 22, 2023. For the Southern Hemisphere, Mercury will be lost in the morning twilight. Chart via John Jardine Goss/ EarthSky.

For precise sun and Mercury rising times at your location:

Old Farmer’s Almanac (U.S. and Canada)
timeanddate.com (worldwide)
Stellarium (online planetarium program)

Mercury events in 2023 and 2024

Note: Dates are listed based on UTC times

September 6, 2023: Inferior conjunction (races between Earth and sun)
September 22, 2023: Greatest elongation (morning)
October 20, 2023: Superior conjunction (passes behind sun from Earth)
December 4, 2023: Greatest elongation (evening)
December 22, 2023: Inferior conjunction (races between Earth and sun)
Jan 12, 2024: Greatest elongation (morning)
Feb 28, 2024: Superior conjunction (passes behind sun from Earth)
Mar 24, 2024: Greatest elongation (evening)
Apr 11, 2024: Inferior conjunction (races between Earth and sun)
May 9, 2024: Greatest elongation (morning)
Jun 14, 2024: Superior conjunction (passes behind sun from Earth)
Jul 22, 2024: Greatest elongation (evening)
Aug 19, 2024: Inferior conjunction (races between Earth and sun)
Sep 5, 2024: Greatest elongation (morning)
Sep 30, 2024: Superior conjunction (passes behind sun from Earth)
Nov 16, 2024: Greatest elongation (evening)
Dec 5, 2024: Inferior conjunction (races between Earth and sun)
Dec 25, 2024: Greatest elongation (morning)

Mercury charts from Guy Ottewell

Sky chart with labeled constellations and objects, and positions of Mercury at elongations marked.
View larger. | Mercury’s greatest morning elongations in 2023 – from the Northern Hemisphere – as viewed through a powerful telescope. The planet images are at the 1st, 11th and 21st of each month. Here, dots show the actual positions of the planet for every day. Chart via Guy Ottewell.
Sky chart with labeled constellations and objects, and rows of dots for elongations of Mercury.
View larger. | Mercury’s greatest morning elongations in 2023 from the Southern Hemisphere, as viewed through a powerful telescope. The planet images are at the 1st, 11th and 21st of each month. Here, dots show the actual positions of the planet for every day. Chart via Guy Ottewell.

Heliocentric solar system, September 2023

Circle with sun at center, planets around, and zodiac names on outer edge.
View larger. | Heliocentric view of solar system, September 2023. Chart via Guy Ottewell.

A comparison of elongations

Not all equal

Mercury’s greatest elongations are not created equal. Some are “greater” than others. For one thing, the distance of Mercury from the sun on our sky’s dome varies from about 28 degrees (maximum) to 18 degrees (minimum).

Also, Mercury elongations are better or worse depending on the time of the year they occur and your location on Earth. For both hemispheres, spring evenings and autumn mornings are best.

The chart below – from a Northern Hemisphere perspective – might help you visualize these differences.

Chart with row of steep, alternating light blue and gray arcs, each with a date and height in degrees.
View larger. | Mercury elongations compared. Here, gray areas represent evening apparitions (eastward elongation). Blue areas represent morning apparitions (westward elongation). The top figures are the maximum elongations, reached at the top 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 line). Maxima are reached at the parenthesized dates below (40 degrees north bold). Chart via Guy Ottewell’s 2023 Astronomical Calendar.

Seasons make a difference

So, in the autumn for either hemisphere, the ecliptic – or path of the sun, moon and planets – makes a narrow angle to the horizon in the evening. Conversely, it makes a steep slant, nearly perpendicular, in the morning. So – in autumn from either hemisphere – morning elongations of Mercury are best. Then, Mercury appears higher above the horizon and farther from the glow of the sun. Evening elongations in autumn are, however, harder to see.

On the other hand, in the spring for either hemisphere, the situation reverses. The ecliptic and the horizon meet at a sharper angle on spring evenings and at a narrower angle on spring mornings. So, in springtime for either hemisphere, evening elongations of Mercury are best. Meanwhile, morning elongations in springtime are harder to see.

Bottom line: Mercury reaches its greatest elongation – greatest distance from the sunrise – on September 22. Look east at dawn. It’ll brighten through early October.

Posted 
September 11, 2023
 in 
Astronomy Essentials

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