Is the sun’s output constant?
Meteorologists have long talked of the “solar constant” as the amount of energy emit ted by the sun, and once considered it to be a rock-steady number. Not so. Solar out put does indeed vary, on the order of 0.1 percent. Satellite observations show that the sun’s energy output varies over the 11-year sunspot cycle, and probably on other time scales as well. Several billion years ago the sun was thought to shine with only about 75 percent of its current intensity. In the current era, while the solar energy output fluctuates slightly over time, astrophysicists expect it to keep on shining at roughly the current rate for another five billion years, which is pretty “constant” by most definitions.