Mercury Reaches Its Greatest Eastern Elongation in January 2021

Sylas Boesten, Unsplash
Sylas Boesten, Unsplash / Sylas Boesten, Unsplash

Mercury is hard to spot most nights. This isn't necessarily due to its distance from Earth: The planet orbits close to the sun, which means its light is usually washed out in the evening sky. On January 23, 2021, that won't be the case. That night, the innermost planet of the solar system will reach its greatest eastern elongation, EarthSky reports.

What Is Mercury at Greatest Eastern Elongation?

An elongation occurs when a planet reaches its farthest distance from the sun in the sky. When the planet falls to west of the sun when viewed from Earth, it's a western elongation, and when it's on the sun's eastern side, it's an eastern elongation.

On the evening of Saturday, January 23, Mercury will appear 18.6° to the east of the setting sun. That's the maximum angular distance the two bodies can reach. With that distance between them, Mercury will appear especially bright above the horizon.

How to See Mercury at Greatest Eastern Elongation

The best time to catch the elongation of Mercury in January is at twilight. When the sun sets that night, the planet will be visible 16° above the horizon in the southwestern sky. It will be around for only a brief period, setting about 90 minutes after the sun does.

If you miss Mercury's greatest elongation on January 23, don't worry: Mercury will still be far enough from the sun to observe in the weeks to follow. And it's just one of many celestial events to look forward to in 2021.

[h/t EarthSky]