What is "solar constant"

Constant \Con"stant\, n.

1. That which is not subject to change; that which is invariable.

2. (Math.) A quantity that does not change its value; -- used in countradistinction to variable.

3. (Astron.) A number whose value, when ascertained (as by observation) and substituted in a general mathematical formula expressing an astronomical law, completely determines that law and enables predictions to be made of its effect in particular cases.

4. (Physics) A number expressing some property or condition of a substance or of an instrument of precision; as, the dielectric constant of quartz; the collimation constant of a transit instrument.

5. (Computers) a data structure that does not change during the course of execution of a program. It may be a number, a string, or a more complex data structure; -- contrasted with variable.

   Aberration constant, or Constant of aberration (Astron.), a number which by substitution in the general formula for aberration enables a prediction to be made of the effect of aberration on a star anywhere situated. Its value is 20[sec].47.

   Absolute constant (Math.), one whose value is absolutely the same under all circumstances, as the number 10, or any numeral.

   Arbitrary constant, an undetermined constant in a differential equation having the same value during all changes in the values of the variables.

   Gravitation constant (Physics), the acceleration per unit of time produced by the attraction of a unit of mass at unit distance. When this is known the acceleration produced at any distance can be calculated.

   Solar constant (Astron.), the quantity of heat received by the earth from the sun in a unit of time. It is, on the C. G. S. system, 0.0417 small calories per square centimeter per second.
   --Young. [Webster 1913 Suppl.]

   Constant of integration (Math.), an undetermined constant added to every result of integration.

n. 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.

n. the rate at which radiant solar energy is received at the outer layer of the earth's atmosphere

The solar constant, a measure of flux density, is the conventional name for the mean solar electromagnetic radiation (the solar irradiance) per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) from the Sun (roughly the mean distance from the Sun to the Earth). The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite as being 1.361 kilo watts per square meter (kW/m²) at solar minimum and approximately 0.1% greater (roughly 1.362 kW/m²) at solar maximum. The solar "constant" is not a physical constant in the modern CODATA scientific sense; it varies in value, and has been called a "misconception". It has been shown to vary historically in the past 400 years over a range of less than 0.2 percent.