Mercury at Greatest Elongation

​On March 8, 2025, Mercury will reach its greatest eastern elongation, appearing 18 degrees from the Sun in the evening sky. This event occurs at 05:59 GMT. Mercury will be visible in the western sky shortly after sunset, located in the constellation Pisces and shining at a magnitude of -0.4, making it brighter than most stars and visible to the naked eye under clear condition. Venus will also be visible nearby, about 6.6° above and to Mercury’s right on this date. This is one of the best opportunities to observe Mercury in the evening sky for 2025, especially for observers in the Northern Hemisphere. 
 
The brightness of Mercury changes significantly around its greatest elongation due to its orbital position and phase. At greatest elongation, Mercury appears roughly half-illuminated, similar to a quarter Moon phase, which affects its apparent magnitude:

• Before greatest elongation: Mercury is brighter because it presents a larger illuminated surface area as seen from Earth. For example, it can shine at magnitudes as bright as -0.2 when closer to perihelion.
• At greatest elongation: Mercury typically displays a half-illuminated disk and reaches a magnitude of around +0.4 to -0.4, depending on the specific elongation and its distance from the Sun.
• After greatest elongation: As Mercury moves back toward the Sun’s vicinity, its illuminated area decreases, and it fades rapidly in brightness.

This variation is due to Mercury’s elliptical orbit, which alters both its distance from Earth and the Sun’s illumination angle during each apparition.
Using an astronomy app will enable you to locate Mercury.
 
References:
Editors of EarthSky. (2025, March 4). Mercury farthest from sunset on March 7-8. EarthSky. Retrieved from https://earthsky.org/tonight/mercury-after-sunset-greatest-elongation-east/
Ford, D. ( 2025, March 8). Mercury at Greatest Elongation East. In-The-Sky.Org. Retrieved from https://in-the-sky.org/news.php?id=20250308_11_101