Coszenithale

Coszenithale is a term encountered in solar geometry referring to the cosine of the zenith angle. The zenith angle z is defined as the angle between the local vertical (the zenith) and the direction of the Sun. The quantity cos(z) acts as a geometric projection factor that describes how much solar radiation is effectively incident on a horizontal surface. In many solar-energy calculations, the irradiance on a horizontal plane due to direct beam radiation is proportional to cos(z) times the direct-normal irradiance, assuming a clear atmosphere and a direct component.

As z changes over the day, it is 0 degrees when the Sun is overhead (cos(z) = 1) and increases toward 90 degrees at sunrise and sunset (cos(z) approaches 0). The coszenithal concept appears in various forms in the literature, and in some non-English sources it may be used as a direct translation of the French cosinus zénithal. In contemporary English technical writing, it is more common to speak of the cosine of the zenith angle or of the zenith-projection factor rather than a standalone term.

Related ideas include Lambert's cosine law, which describes how radiance from a diffuse surface scales with the angle to the surface normal, and the use of cos(z) in solar radiation models. Limitations of the simple cos(z) factor include neglecting atmospheric scattering, diffuse irradiance, and shading, which are addressed in more complete models by including direct, diffuse, and reflected components.