During the Maunder Minimum, a period of diminished solar activity between 1645 and 1715, sunspots were rare on the face of the sun, sometimes disappearing entirely for months to years. At the same time, Earth experienced a bitter cold period known as the "Little Ice Age."Were the events connected? Scientists cannot say for sure, but it's quite likely. Slowdowns in solar activity -- evidenced by reductions in sunspot numbers -- are known to coincide with decreases in the amount of energy discharged by the sun. During the Little Ice Age, though, few would have thought to track total solar irradiance (TSI), the amount of solar energy striking Earth's upper atmosphere. In fact, the scientific instrument needed to make such measurements -- a spaceborne radiometer -- was still three centuries into the future.
Modern scientists have several tools for studying TSI. Since the 1970s, scientists have relied upon a collection of radiometers on American and European spacecraft to keep a close eye on solar fluctuations from above the atmosphere, which intercepts much of the sun's radiation. When NASA launches the Glory satellite this fall (no earlier than October 2009), researchers will have a more accurate instrument for measuring TSI than they've ever had before.
The Total Irradiance Monitor (TIM) on Glory is more sophisticated, but still related in concept to the very earliest ground-based solar radiometers, which were invented in 1838. Where those radiometers used sunlight to heat water and indicate the intensity of the sun's brightness at the Earth's surface, Glory's TIM instrument will use a black-coated metallic detector to measure how much heat is produced by solar radiation as it reaches the top of the Earth's atmosphere.