SOLAR CONSTANT

the total radiation energy received from the Sun per unit of time per unit of area on a theoretical surface perpendicular to the Sun's rays and at the Earth's mean distance from the Sun. Measurements at the Earth's surface must be corrected for atmospheric effects and seasonal changes in distance from the Sun. The value of the constant is approximately 2 calories per minute per square centimetre, or about 1.8 horsepower per minute per square metre; it may change very slightly with variations in solar activity. Any substantial change would have important implications for life on Earth.