Our sun is a variable star whose brightness waxes and wanes over an 11-year cycle. The change is subtle— about 0.1 percent— but still large enough to influence climate on Earth, says Paal Brekke, the deputy project scientist for Europe's Solar and Heliospheric Observatory satellite. Accurately measuring the sun's intensity is difficult because solar radiation degrades electronic light detectors in space within five to 10 years, so new instruments have slightly different levels of sensitivity than older ones. One key to making all the readings line up was the precision sensor Solcon, which was sent into space every few years to calibrate the other instruments. (Solcon was lost with the space shuttle Columbia.) Brekke's goal is to understand the sun's variability on all timescales: the daily rise and fall of sunspots, the 11-year activity cycle, and the longer-term changes— perhaps as large as 0.6 percent over a few centuries— inferred from tree-ring studies.