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Fill the inputs and press Calculate to see corrections.
Calculator
Example data table
| Observed altitude (deg) | Pressure (hPa) | Temp (C) | RH (%) | Wavelength (nm) | Refraction (arcmin) | True altitude (deg) |
|---|---|---|---|---|---|---|
| 10 | 1013 | 15 | 50 | 550 | ~5.3 | ~9.912 |
| 2 | 1000 | 5 | 30 | 550 | ~18-20 | ~1.66 |
| 45 | 1020 | 20 | 70 | 850 | ~1.0 | ~44.983 |
Formula used
The calculator uses a widely used empirical refraction form that depends on observed altitude h. A base refraction at standard conditions is computed, then scaled by pressure and temperature. Humidity modifies an effective pressure, and wavelength introduces a small dispersion factor.
Saemundsson model (base, arcminutes)
R0 = 1.02 / tan( (h + 10.3/(h+5.11)) * pi/180 )
Good accuracy for many observing tasks above about -1 deg.
Environmental scaling
R = R0 * (Peff/1010) * (283/(273.15+T)) * D(λ)
Effective pressure: Peff = P - 0.378 e, where e is water vapor pressure.
Notes: Near the horizon, real refraction depends on vertical temperature gradients and local weather. For critical work, compare with site-specific models or observed calibration.
How to use this calculator
- Enter the observed altitude of the object in degrees.
- Choose whether pressure is measured locally or sea-level.
- Fill temperature, humidity, and wavelength for your observation.
- Select a refraction model; Saemundsson is a solid default.
- Press Calculate to view refraction and corrected true altitude.
- Use Download CSV or Download PDF to save the results.