The Sun is a weaklyvariable star, and its actual luminosity thereforefluctuates.[3] The major fluctuation is the eleven-yearsolar 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.[4]
Solar luminosity is related tosolar irradiance (thesolar 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 isorbital forcing that causes theMilankovitch 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:whereA is theunit distance (the value of theastronomical unit inmetres) andk 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.
^Noerdlinger, Peter D. (2008). "Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System".Celestial Mechanics and Dynamical Astronomy.801: 3807.arXiv:0801.3807.Bibcode:2008arXiv0801.3807N.
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