October 29, 2025: Mercury reaches greatest elongation, appearing farthest from the sun in our sky. From mid-northern latitudes, it remains a difficult target, clinging to the bright evening twilight before passing between Earth and the sun on November 20th.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 7:20 a.m. CDT; Sunset, 5:48 p.m. CDT. Times are calculated by the US Naval Observatory’s MICA computer program. Check local sources for sunrise and sunset times.
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Mercury at Greatest Elongation
Mercury reaches greatest elongation, when we see it farthest from the sun. Geometrically, the Sun–Earth–Mercury angle is at its maximum. Because Mercury and Venus revolve around the sun inside Earth’s orbital path, they never stray far from the solar glare. Mercury, the innermost planet, stays close to the sun’s glare. At this greatest elongation, Mercury is 24° from the sun.
This evening elongation occurs under very unfavorable observing conditions. The ecliptic – the plane of the solar system – makes a low angle with the western horizon, and Mercury is nearly 3° below it. At sundown, the planet is less than 10° above the southwest horizon. Thirty minutes later, it is about 3° up, a difficult view even under ideal sky-watching conditions.
Mercury passes between our world and the sun on November 20th, speeding into the morning sky for its best appearance of the year on December 7th. At 45 minutes before sunrise, it stands nearly 10° up in the southeast below Zubenelgenubi (the Scorpion’s southern claw) and Zubeneschamali (northern claw). At this morning elongation, the ecliptic makes a steeper angle with the eastern horizon.
Saturn and Moon after Sunset
An hour after sundown, the slightly gibbous moon, 53% illuminated, is less than 30° (one-third of the way from the horizon to overhead) above the south direction. It is about 45° west (to the right) of Saturn, that is in the southeast. Look for Fomalhaut (the Mouth of the Southern Fish), the 13th brightest star from mid-northern latitudes, over 10° above the south-southeast horizon. A binocular helps with seeing Deneb Algedi (the Kid’s Tail), nearly 15° to the lunar orb’s upper right.
Mercury reaches its greatest elongation under less than ideal observing conditions. It is low in the southwestern sky during evening twilight. The moon begins to approach Saturn in the southern sky.
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