The solar lum**inosity************** () is**** a unit**** of**** radiant flux**** (power**** emitted in**** the form**** of**** photons)**** conventionally**** used**** by**** astronomers to**** measure the luminosity**** of**** stars,**** galaxies**** and other celestial objects in**** terms of**** the output**** of**** the Sun.**** One nominal solar luminosity is defined by the International Astronomical Union to be 3.828 W. 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.

Determination

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: where A is the unit distance (the value of the astronomical unit in metres) and k is a constant (whose value is very close to one) that reflects the fact that the mean distance from the Earth to the Sun is not exactly one astronomical unit.