This is an excerpt from EERE Network News, a weekly electronic newsletter.
Study Finds Solar Brightness to be Remarkably Constant
Stellar evolution theories suggest that the sun has brightened by about 30 percent since it started steadily burning hydrogen about 4.5 billion years ago. But if you think your solar power system is producing more electricity this year, the sun shouldn't get the credit, according to an article in the September 14th edition of Nature. Scientists studying the sun (would these be considered "stellar scientists"?) find that most of the variation in the sun's brightness is due to the 11-year cycle of solar sunspot activity. When sunspots are most frequent, the bright "faculae" that surround them actually cause the sun to be about 0.07 percent brighter than it is during periods of low sunspot activity, such as now. That means that changes in solar brightness are unlikely to affect your solar power system, or for that matter, have any significant effect on global temperatures. The sun is currently near the solar sunspot minimum, increasing its sunspot activity on a slow ramp up to the next solar maximum, which should occur in 2011. See the press release from the National Center for Atmospheric Research.
By the way, the sun will dim imperceptibly on November 8th, as the planet Mercury takes a rare trek across the solar disk. But since the planet has an apparent diameter of only 10 arc-seconds against the 1937-arc- second diameter of the sun, we calculate that only 0.003 percent of the sunlight will be blocked by the planet. Although the last Mercury transit was only three years ago, the next won't happen for another decade. The event starts at 2:12 p.m. Eastern time and ends after sunset in the East, at 6:10 p.m. See the details on the transit from the National Aeronautics and Space Administration (NASA) and to see the event online, visit the Web sites for Kitt Peak National Observatory and NASA's SOHO (Solar and Heliospheric Observatory) satellite.