Sky science
Phase Angle
Direct answer
Phase Angle is the angle at an illuminated body between directions to its light source and observer.
It sets how much of the lit side faces the observer.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Bond Albedo, Geometric Albedo, Solar Elongation, Greatest Elongation
It sets how much of the lit side faces the observer.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Phase Angle
Geometric astronomy used phase angles to model Moon and planet appearances.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Phase Angle matters
Phase Angle governs phase appearance, scattering behavior, and brightness corrections.
How to read Phase Angle
Name the vertex body and distinguish phase angle from illuminated fraction.
Phase Angle in practice: an example
A phase angle near zero corresponds to a nearly full observed disk.
Where readings of Phase Angle go wrong
Clouds, rings, and surface scattering keep brightness from following area alone.
Sources and editorial method
Research references used for this entry include:
- NASA Science: Solar System Exploration - NASA overview of planets, moons, small bodies, rings, and the structure of the Solar System.
- NASA Science: Moon Phases - NASA explanation and visualization of lunar phases and libration.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
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Common questions
What is a phase angle?
Phase Angle is the angle at an illuminated body between directions to its light source and observer.
What do people get wrong about a phase angle?
Clouds, rings, and surface scattering keep brightness from following area alone.
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