February 5, 2010: For some years now, an unorthodox idea has been gaining favor among astronomers. It contradicts old teachings and unsettles thoughtful observers,
especially climatologists. "The sun," explains Lika Guhathakurta of NASA headquarters in Washington DC, "is a variable star."
But it looks so constant...
That's only a limitation of the human eye. Modern telescopes and spacecraft have penetrated the sun's blinding glare and found a maelstrom of unpredictable turmoil. Solar flares explode with the power of a billion atomic bombs. Clouds of magnetized gas (CMEs) big enough to swallow planets break away from the stellar surface. Holes in the sun's atmosphere spew million mile-per-hour gusts of solar wind. And those are the things that can happen in just one day.
'Solar Constant' is an Oxymoron
Astronomers were once so convinced of the sun's constancy, they called the irradiance of the sun "the solar constant," and they set out to measure it as they would any constant of Nature. By definition, the solar constant is the amount of solar energy deposited at the top of Earth's atmosphere in units of watts per meter-squared. All wavelengths of radiation are included—radio, infrared, visible light, ultraviolet, x-rays and so on. The approximate value of the solar constant is 1361 W/m2.
Clouds, atmospheric absorption and other factors complicate measurements from Earth's surface, so NASA has taken the measuring devices to space. Today, VIRGO, ACRIM and SORCE are making measurements with precisions approaching 10 parts per million per year. Future instruments scheduled for flight on NASA's Glory and NOAA's NPOESS spacecraft aim for even higher precisions.
To the amazement of many researchers, the solar constant has turned out to be not constant.