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 1367 watts per square metre. Thus, for the whole Earth, with a cross-section of 125,000,000 km&sup2, the power is 170 petawatt. 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 watt.

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

See also

• Earth's energy budget
• Solar variation