Atmospheric refraction

Atmospheric refraction is the bending of a body’s light as it passes through the denser layers of air near the surface, which raises the apparent altitude so the body looks higher than it truly is. The correction is therefore always subtracted from apparent altitude. It is near zero at the zenith and largest at the horizon, about 34 arcminutes at 0 degrees altitude for standard conditions, which is why a body’s true position can sit fully below the horizon when first seen rising. The Nautical Almanac altitude-correction tables apply mean refraction, with extra tables for non-standard temperature and pressure.