At a glance
- Found in
- Astronomy & Skywatching
- Related
- Phase Angle, Solar Elongation, Astronomical Opposition, Inferior Conjunction
It marks an inner planet’s widest solar separation.
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The story behind Greatest Elongation
Geometric models and ephemerides predicted inner-planet elongations for observers.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Greatest Elongation matters
Greatest Elongation often identifies a favorable observing window in evening or morning twilight.
Greatest Elongation in practice: an example
Venus at greatest eastern elongation stands farthest east of the Sun in angle, not physical distance.
Where readings of Greatest Elongation go wrong
Best visibility can occur on a nearby date because horizon angle and season also matter.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Astronomical Almanac Glossary - Definitions for astronomical coordinates, time, reference systems, and observable phenomena.
- NASA Science: Skywatching FAQ - NASA guidance for observing stars, planets, constellations, and changing sky events.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Plan an inner-planet apparition
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More in Astronomy & Skywatching
- Lunar Libration
- Parabolic Orbit
- Radio Telescope
- Solar Corona
- Telescope Focal Length
- Aphelion
- Averted Vision
- Eccentric Anomaly
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Common questions
What is Greatest Elongation?
Greatest Elongation is the maximum apparent angular separation of Mercury or Venus from the Sun during one apparition.
What do people get wrong about Greatest Elongation?
Best visibility can occur on a nearby date because horizon angle and season also matter.
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