Sky science
Airmass
Direct answer
Airmass estimates the relative atmospheric path length light traverses compared with the path at the zenith.
It measures how much atmosphere an observation crosses.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Atmospheric Extinction, Atmospheric Refraction, Skyglow, Zodiacal Light
It measures how much atmosphere an observation crosses.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Airmass
Quantitative photometry made airmass central to atmospheric calibration.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Airmass matters
Airmass predicts extinction, dispersion, and reduced quality at low altitude.
Where readings of Airmass go wrong
Airmass is dimensionless and is not the mass of air above one object.
Sources and editorial method
Research references used for this entry include:
- European Southern Observatory: Technology for Telescopes - Observatory reference for aperture, angular resolution, atmospheric seeing, mirrors, and telescope technology.
- 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
Plan observations at lower airmass
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Common questions
What is an airmass?
Airmass estimates the relative atmospheric path length light traverses compared with the path at the zenith.
What do people get wrong about an airmass?
Airmass is dimensionless and is not the mass of air above one object.
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