Solar constant

Article By:

Hudson, Hugh Center for Astrophysics and Space Sciences, University of California, San Diego, California.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.633100

The total solar radiant energy flux incident upon the top of the Earth's atmosphere at a standard distance (1 astronomical unit, 1.496 × 10⁸ km or 9.3 × 10⁷ mi) from the Sun. In 1980, it was discovered that the so-called solar constant (more correctly, the total solar irradiance) actually varies with time, although only by small amounts, and recent data suggest a long-term sunspot-minimum level of about 1361 W · m⁻² (1.95 cal · cm⁻² · min⁻¹). The energy received at the Earth is more than 10¹⁰ times that from the next brightest star, and almost 10⁶ times that from the full moon. This solar energy maintains the climate on Earth. The total solar irradiance gives a good measure of the solar luminosity (the Sun's total radiant energy), although anisotropic components (for example, a dark sunspot), which produce directional solar-constant variations, must be taken into account. See also: Climatology; Earth rotation and orbital motion; Photosphere; Star

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