Does the brightness of the Sun change over time?
Yes, modern measurements between 1978 and 1995 show that the “brightness” or total irradiance of the Sun fluctuates by a few tenths of a precent over the 11 year solar cycle. This small fluctuation reflects stability of the solar photosphere as seen in the visible spectrum which extends from the blue at 400 nanometers (nm) to the deep red at 800nm. Observations from space show increasing variation from the ultraviolet below 400nm to the x-ray region down to .1nm. However, the bulk of the output solar energy is in the visible spectrum; therefore, its variation dominates fluctuations on the Sun’s “brightness”.
