Sky science
Great Circle
Direct answer
Great Circle: A Great Circle is the intersection of a sphere with a plane passing through the sphere’s center.
It is the largest possible circle on a sphere.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Astronomical Nadir, Prime Vertical, Small Circle, Celestial Horizon
It is the largest possible circle on a sphere.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Great Circle
Great circles have been foundational in astronomy, geography, and navigation.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Great Circle matters
Great-circle arcs define angular separation and shortest routes on spherical models.
How to read Great Circle
spherical rather than flat-map geometry speaks to wide sky separations.
Great Circle in practice: an example
The celestial equator and every hour circle are great circles.
Where readings of Great Circle go wrong
A straight line on a chart projection may not represent a great-circle arc.
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.
- International Astronomical Union: Standards of Fundamental Astronomy - IAU-maintained standards and algorithms for astronomical reference systems and coordinate transformations.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
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Common questions
What is a Great Circle?
A Great Circle is the intersection of a sphere with a plane passing through the sphere’s center.
What do people get wrong about a Great Circle?
A straight line on a chart projection may not represent a great-circle arc.
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