Hudson, Hugh Center for Astrophysics and Space Sciences, University of California, San Diego, California.
- Causes of variability
- Related Primary Literature
- Additional Reading
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 ċ m−2 (1.95 cal ċ cm−2 ċ 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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