Here's a quote and its source with regard to the thread topic!
From: http://www.john-daly.com/solar.htmOur Variable Star
The Sun appears to us as a steady brilliant light in the sky. Its temperature is nearly 6,000 deg.C and its radiation of light and heat across the vast distances of space weakens in proportion to the inverse square of that distance. By the time solar energy reaches the Earth, the radiation amounts to 1,368 watts per square metre (w/m2).
This figure is known by solar physicists as the "Solar Constant"
Q. - When is a constant not a constant?
A. - When it's the Solar Constant !
The solar `constant' is actually a variable. Contradictory as this may sound, it is only fairly recently that the radiation from the Sun has been proved to vary according to a highly predictable cycle. The variations so far observed from satellites are not much - a mere 0.2% change in mean radiation (or about 2 w/m2).
Extreme short-term variations have been recorded of up to 0.4%. The Sun, in effect, behaves more like a flickering candle, and has been accurately measured doing this by various satellites, including the ill-fated "Solar Max" satellite whose 9-year sample of solar radiation from 1980 to 1989 proved conclusively that the Sun was indeed a variable star.
("Solar Max" became a victim of the very thing it was meant to observe. It was placed in a low orbit just outside the Earth's atmosphere. This was its undoing, as the build-up of solar radiation in 1989, due to the approaching Solar Maximum, caused the outer atmosphere to heat up and expand, and capture the satellite. Solar Max was then slowed by friction and burned up in the atmosphere.)
Further evidence of the Sun's variability came from the Shanghai observatory of the Chinese Academy of Sciences in April 1990, when they reported that the Sun's radius had shrunk 410 kilometres from 1715 to 1987, based on solar eclipse studies. A shrinking sun since 1715 would result in a hotter solar temperature, and increased solar radiation, consistent with a warming of the Earth as claimed by the IPCC.