Atmospheric Refraction

This calculator finds atmospheric refraction and converts the true altitude of an object to the apparent one and back, with pressure and temperature.

R = 1′ / tan(h + 7.31 / (h + 4.4))

At the horizon refraction is about 34′, more than the apparent diameter of the Sun. At 45° it is about 1′, and at the zenith it is zero.