Near-Horizon Refraction Arcmin Approximation
DeclaredSaemundssonempiricaltruealtitude refractionapproxarcmin0≤h≤15deg,P/Tscaled;actualTRUEaltitude,no precisionhorizon/weather guarantee. Actual independent model/unit/frame/epoch/calibration and applicable measurement established;not precise ephemeris,forecast,geodesy operation,identity or safety approval.
Description
Near-horizon refraction raises the apparent altitude of a celestial object under a simplified atmospheric model.
Near-horizon refraction raises the apparent altitude of a celestial object under a simplified atmospheric model.1
This calculator uses a pressure- and temperature-scaled Saemundsson expression for supplied true altitudes from 0 to 15 degrees.
When to use Near-Horizon Refraction Arcmin Approximation
Use this tool to apply the stated near-horizon refraction arcmin approximation operation with an explicit input and unit basis.
How to use Near-Horizon Refraction Arcmin Approximation and how it works
- Establish the measurement basis and units described above.
- Enter actualTrueAltitudeDegrees, actualPressureHpa, actualTemperatureC and run the tool.
- Read result.value using the output meanings below.
- If input is rejected, correct the stated type, range, ordering or alignment issue and rerun; review successful results under the declared convention.
| Input field | Meaning | Value type | Requirement |
|---|---|---|---|
| actualTrueAltitudeDegrees | True Altitude Degrees. Units, ordering and applicable branch are specified in the algorithm. | number | required; finite; further conditions in the algorithm |
| actualPressureHpa | Pressure Hpa. Units, ordering and applicable branch are specified in the algorithm. | number | required; finite; strictly positive; further conditions in the algorithm |
| actualTemperatureC | Temperature C. Units, ordering and applicable branch are specified in the algorithm. | number | required; finite; further conditions in the algorithm |
Require true altitude 0–15 degrees, positive pressure in hPa and temperature above −100 through 60 °C. Evaluate the declared angle inside the tangent in degrees and return the correction in arcminutes; atmospheric layers and horizon obstructions are not modeled.
R(arcmin)=1.02/tan((h+10.3/(h+5.11))°) × (P/1010) × (283/(T+273)) Apply the declared units, ordering and branch conditions.
| Output field | Meaning | Value type |
|---|---|---|
| result | Object containing the declared output fields. | object |
| result.value | Approximate refraction correction in arcminutes. | number |
{
"input": {
"actualTrueAltitudeDegrees": 0,
"actualPressureHpa": 1010,
"actualTemperatureC": 10
},
"output": {
"result": {
"value": 28.98192738444997
}
}
}Limitations and assumptions
The output applies the declared operation to the values you supply. Establish the measurement basis, units, coefficients and any caller-selected rules before interpreting it. The tool does not independently collect or verify observations or decide whether a model or reference applies.
Supply explicit required fields with the documented types and dense aligned arrays. Unknown fields, wrong types and nonfinite numbers are rejected. Branch-specific constraints are explained above. A returned boolean, signed difference or tied result follows the stated convention; an input error instead means the required input contract was not satisfied. Very large inputs or outputs may be rejected by shared resource limits.
Alternative or Complementary methods
For a related operation, use Rise/Set Hour Angle Formula. Rise and set geometry can be formulated as crossings of a caller-selected altitude.
For a related operation, use Civil Twilight End Hour Angle. Civil twilight conventionally uses the solar center six degrees below a geometric horizon.
References
Similar or alternative tools
- Rise/Set Hour Angle Formula
Rise and set geometry can be formulated as crossings of a caller-selected altitude.
- Civil Twilight End Hour Angle
Civil twilight conventionally uses the solar center six degrees below a geometric horizon.