Solar Constant

The amount of heat from the sun that strikes the surface of the earth each day expressed as calories per square centimeter of earth surface per minute. The actual amount is 1.94 cal/ cm2/min. Recently, astronomers specializing in solar phenomena have discovered that this “constant” can and does change slightly from moment to moment, perhaps altering the energy budget for the planet and giving rise to small differences that might reflect themselves later in larger ecological change.

Links
  http://remotesensing.oma.be/solarconstant/solar.html