One nominal solar
luminosity is defined by the
International Astronomical Union to be 3.828×1026W. This does not include the
solar neutrino luminosity, which would add 0.023 L☉, or 8.8×1024 W, i.e. a total of 3.916×1026 W (the mean energy of the solar photons is 26 MeV and that of the solar neutrinos 0.59 MeV, i.e. 2.27%; the Sun emits 9.2×1037 photons and as many neutrinos each second, of which 6.5×1014 per m2 reach the Earth each second). The Sun is a weakly
variable star, and its actual luminosity therefore
fluctuates. The major fluctuation is the eleven-year
solar cycle (sunspot cycle) that causes a quasi-periodic variation of about ±0.1%. Other variations over the last 200–300 years are thought to be much smaller than this.
Solar luminosity is related to
solar irradiance (the
solar constant). Solar irradiance is responsible for the
orbital forcing that causes the
Milankovitch cycles, which determine Earthly glacial cycles. The mean irradiance at the top of the Earth's atmosphere is sometimes known as the
solar constant, I☉. Irradiance is defined as power per unit area, so the solar luminosity (total power emitted by the Sun) is the irradiance received at the Earth (solar constant) multiplied by the area of the sphere whose radius is the mean distance between the Earth and the Sun:
^Noerdlinger, Peter D. (2008). "Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System". Celestial Mechanics and Dynamical Astronomy. 801: 3807.