Solar constant

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Solar irradiance spectrum at top of atmosphere.

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. It is measured by satellite to be roughly 1366 watts per square metre.^[1] Thus, for the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×10¹⁷ W. The solar constant is not quite constant; see solar variation.

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. 1/11,000 radian, so the solid angle of Earth seen from the sun is ca. 1/140,000,000 steradian. Thus, the sun emits about 2 billion times the amount of radiation that is caught by Earth, or about 3.86×10²⁶ watts.

The solar constant is relatively constant, but varies according to sunspot activity. It affects mainly long-term climates, rather than short-term weather.

[edit] 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. However he obtained the incorrect value of 2903 W/m², perhaps due to mathematical errors. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m².