, it is polar day. Caveats The calculated values are approximations due to the distinction between common/geodetic latitude and geocentric latitude. However, the two values differ by less than 12 minutes of arc, which is less than the apparent angular radius of the sun. The formula also neglects the effect of atmospheric refraction. Applications Sunrise/Sunset Sunset and sunrise occur (approximately) when the zenith angle is 90°, where the hour angle h0 satisfies Precise times of sunset and sunrise occur when the upper limb of the Sun appears, as refracted by the atmosphere, to be on the horizon. Albedo A weighted daily average zenith angle, used in computing the local albedo of the Earth, is given by where Q is the instantaneous irradiance. Summary of special angles For example, the solar elevation angle is : 90° if you are on the equator, a day of equinox, at a solar hour of twelve near 0° at the sunset or at the sunrise between −90° and 0° during the night (midnight) An exact calculation is given in position of the Sun. Other approximations exist elsewhere. See also Azimuth Solar azimuth angle Horizontal coordinate system List of orbits Position of the Sun Sun path Sunrise Sunset Sun transit time References Horizontal coordinate system Sun Solar energy