<P> 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: </P> <Dl> <Dd> L ⊙ = 4 π k I ⊙ A 2 (\ displaystyle L_ (\ odot) = 4 \ pi kI_ (\ odot) A ^ (2) \,) </Dd> </Dl> <Dd> L ⊙ = 4 π k I ⊙ A 2 (\ displaystyle L_ (\ odot) = 4 \ pi kI_ (\ odot) A ^ (2) \,) </Dd> <P> 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 . </P>

What is the sun's luminosity in watts