solar constant - History

The total radiation falling on the Earth from the Sun; symbol S, value 1365 W/m² (watts per metre squared). A better terminology is total solar irradiance, more fully the sum over the entire spectrum of the Sun's irradiance at all wavelengths incident on top of the Earth's atmosphere at an Earth–Sun distance of 1 Astronomical Unit. The total solar irradiance continually changes by as much as 1% and is therefore not constant, though the term solar constant remains in use by meteorologists and climatologists.

The solar constant is the amount of incoming solar radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays. The solar constant is not quite constant;

The solar constant includes all types of solar radiation, not just the visible light. (See electromagnetic spectrum for more details) It is linked to the apparent magnitude of the Sun, −26.8, in that the solar constant and the magnitude of the sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun.

The angular diameter of Earth seen from the sun is ca.

The solar constant is relatively constant, but varies according to sunspot activity.

History

In 1884 Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney in California, and (by taking readings at different times of day) attempted to remove atmospheric absorption effects.