Solar Constant

The solar constant is the total solar irradiance reaching the top of Earth’s atmosphere on a surface perpendicular to the Sun’s rays at the mean Earth-Sun distance, about 1361 W/m2. Spaceborne radiometers (SORCE/TIM, the TSIS missions) revised the older 1366 W/m2 value down to near 1361. It varies by roughly 0.1 percent over the 11-year solar cycle and by about 3.4 percent annually from the eccentricity of Earth’s orbit. Averaged over the rotating sphere the incoming solar flux is one quarter of this, about 340 W/m2, the top-of-atmosphere input to the planetary radiation budget that the surface energy balance ultimately draws on.