U.S. solar physicists say the sun is experiencing the least sunspot activity since 1913 and activity is becoming less frequent.

National Aeronautics and Space Administration solar physicist Dean Pesnell at the Goddard Space Flight Center in Greenbelt, Md., said during 2008 there were no sunspots observed on 266 of the year's 366 days -- 73 percent of the year. To find a year with more blank suns, you have to go to 1913, which had 311 spotless days.

That has led some observers to suggest the solar cycle hit bottom last year. But Pesnell says that might not be the case, since, there were no sunspots on 78 of this year's first 90 days -- 87 percent of that period.

In addition, measurements by the Ulysses spacecraft reveal a 20 percent drop in solar wind since the mid-1990s -- the lowest since such measurements began in the 1960s. And NASA says the sun's brightness has dimmed 0.02 percent at visible wavelengths and 6 percent at extreme UV wavelengths since the solar minimum of 1996.

Competing models created by solar physicists disagree on when the solar minimum will end and NASA says that great uncertainty stems from one simple fact: No one fully understands the underlying physics of the sunspot cycle.