Editing Solar luminosity (section)

== Determination ==
Solar luminosity is related to [[Irradiance|solar irradiance]] (the [[solar constant]]). Slow changes in the axial tilt of the planet and the shape of its orbit cause cyclical changes to the solar irradiance. The result is [[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, {{math|''I''<sub>☉</sub>}}. 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:
<math display="block">L_\odot = 4\pi k I_\odot A^2 </math>
where {{math|''A''}} is the [[Astronomical unit|unit distance]] (the value of the [[astronomical unit]] in [[metre]]s) and {{math|''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.