Sky science
Semimajor Axis
Direct answer
Semimajor Axis: The Semimajor Axis is half an ellipse’s longest diameter and is the principal size parameter of a bound Keplerian orbit.
It measures the characteristic size of an elliptical orbit.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Light-Time Correction, Orbital Elements, Orbital Eccentricity, Orbital Inclination
It measures the characteristic size of an elliptical orbit.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Semimajor Axis
Keplerian ellipse geometry made semimajor axis central to celestial mechanics.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Semimajor Axis matters
Semimajor Axis links orbital scale with period through Kepler’s third law.
How to read Semimajor Axis
Specify the orbit pair and units rather than calling it an instantaneous distance.
Semimajor Axis in practice: an example
Earth’s heliocentric semimajor axis defines a characteristic orbital scale near one astronomical unit.
Where readings of Semimajor Axis go wrong
An eccentric body is usually not located one semimajor axis from its focus.
Sources and editorial method
Research references used for this entry include:
- NASA Science: Orbits and Kepler’s Laws - NASA explanation of orbital ellipses, orbital periods, and Kepler’s laws of planetary motion.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Definitions for astronomical coordinates, time, reference systems, and observable phenomena.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Relate orbital size to period
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Common questions
What is the Semimajor Axis?
The Semimajor Axis is half an ellipse’s longest diameter and is the principal size parameter of a bound Keplerian orbit.
What do people get wrong about the Semimajor Axis?
An eccentric body is usually not located one semimajor axis from its focus.
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