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Refraction
While the light from celestial object is passes throgh the
Earth atmosphere
it is refracted by the air, the refraction depends on the
light path distance, temperature, atmospheric pressure, etc.
The net change of the refraction is to lower the apparent
altitude
of the celestial objects (relative to its
geometric position).
The refraction of an object observed at the
zenith is 0, while
the refraction of an object observed near the
horizon is about
34 arcmin.
Refraction correction formulae
approximate formulae for refraction correction for objects
with zenith distance
of up to 75o (altitude > 15o) :
R = 0o.00452 P / ((273 + T) tan a)
and for object below altitude of 15o:
R = P (0.1594 + 0.0196a + 0.00002a2)/[(273+T)(1 + 0.505a + 0.0845a2)]
Where P is the barometric pressure in millibars,
T is the temperature in oC, a is the
altitude
measured in degrees and R is thr amount of refraction measured in degrees.
Refraction calculator
Constructed: November 1998, by: Eran Ofek