Mercury – up before sunrise – greatest elongation September 22

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

Mid-September mornings: Venus and Mercury Southern Hemisphere

Late-September mornings: Venus and Mercury Northern Hemisphere

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

Heliocentric solar system, September 2023

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.

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.