It is measured perpendicular to the incoming sunlight.[1] Total solar irradiance (TSI), is a measure of the solar power over all wavelengths per unit area incident on the Earth's upper atmosphere. The solar constant is a conventional measure of mean TSI at a distance of one astronomical Unit (AU). Irradiance is a function of distance from the Sun, the solar cycle, and cross-cycle changes.[2] Irradiance on Earth is also measured perpendicular to the incoming sunlight.