A measure of the length of the path through the atmosphere to sea level traversed by light rays from a celestial body, expressed as a multiple of the pathlength for a light source at the zenith. It is approximately equal to the secant of the zenith distance of the given celestial body for zenith distances up to about 70°. Bemporad's formula must be used for more accurate determination. To get a representative value at high elevation, the above values must be multiplied by the ratio of the actual atmospheric pressure to the sea level pressure.