Airmass Calculator
What This Calculator Tells You
Airmass measures how much atmosphere light from a celestial object must pass through to reach you. At the zenith (directly overhead), airmass is 1.0. Near the horizon, light travels through much more atmosphere, increasing airmass and degrading image quality.
Calculator
The angle above the horizon (0° = horizon, 90° = zenith/overhead).
Low airmass with minor atmospheric effects. Excellent for most observations.
Light Loss
~0.28 mag
Approximate extinction in visual band (assumes 0.2 mag/airmass)
Path Length
1.41x
Light travels 1.4x farther through atmosphere than at zenith
Common Airmass Values
| Altitude | Airmass | Quality |
|---|---|---|
| 90° | 1.00 | Zenith |
| 60° | 1.15 | Excellent |
| 45° | 1.41 | Very Good |
| 30° | 1.99 | Good |
| 20° | 2.90 | Fair |
| 10° | 5.59 | Poor |
| 5° | 10.31 | Poor |
Why Airmass Matters
Light Loss (Extinction)
More atmosphere means more absorption and scattering. Objects appear dimmer near the horizon - typically 0.2-0.3 magnitudes lost per airmass.
Atmospheric Dispersion
At high airmass, different wavelengths refract differently, creating color fringing. Planets may show red and blue edges.
Seeing Degradation
More atmosphere means more turbulence to pass through. Images become blurrier and less stable at low altitudes.
Astrophotography
Imagers often limit observations to airmass <2.0 (altitude >30°) for quality data. Airmass <1.5 is preferred.
Rules of Thumb
- Airmass 1.0 = perfect: Only achievable for objects at the zenith
- Airmass <1.5: Preferred for imaging and critical observation
- Airmass <2.0: Acceptable for most purposes (altitude >30°)
- Airmass >3.0: Generally avoid - wait for object to rise
- The "30° rule": Serious observers often don't observe below 30° altitude
Related Calculators
- Maximum Altitude of Object - Find the highest point an object reaches
- Best Time to Observe Object - Find when airmass is lowest
- Limiting Magnitude Calculator - How airmass affects what you can see