Sky science
Argument of Periapsis
Direct answer
Argument of Periapsis is the angle within an orbital plane from the ascending node to periapsis in the direction of motion.
It orients the orbit’s nearest point within its plane.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Orbital Inclination, Longitude of Ascending Node, True Anomaly, Mean Anomaly
It orients the orbit’s nearest point within its plane.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Argument of Periapsis
The parameter emerged from classical methods for representing conic orbits.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Argument of Periapsis matters
Argument of Periapsis completes the spatial orientation of an eccentric inclined orbit.
Argument of Periapsis in practice: an example
Changing the argument rotates the ellipse inside an unchanged orbital plane.
Where readings of Argument of Periapsis go wrong
Argument of Periapsis becomes undefined for special circular or equatorial cases without alternate elements.
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: Orbits and Kepler’s Laws - NASA explanation of orbital ellipses, orbital periods, and Kepler’s laws of planetary motion.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Orient periapsis inside the orbit
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Common questions
What is Argument of Periapsis?
Argument of Periapsis is the angle within an orbital plane from the ascending node to periapsis in the direction of motion.
What do people get wrong about Argument of Periapsis?
Argument of Periapsis becomes undefined for special circular or equatorial cases without alternate elements.
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