Today, the sunspot number is zero, which means the sun is blank, right?
Wrong. This morning, NASA’s STEREO-A spacecraft photographed sunspot 1029 seething with activity over the sun’s western horizon: Photo credit: STEREO Extreme Ultra Violet Telescope (195 A) This impressive sunspot, which rotated over the sun’s western limb three days ago, does not add to the sunspot number because it is no longer visible from Earth. Astronomers only count spots that are on the Earth-facing side of the sun. That’s how it’s been done since Rudolph Wolf invented the sunspot number in 1848. In those days, only one side of the sun was visible from Earth, so the tradition was established. Now, however, for the first time in the history of astronomy, NASA’s twin STEREO spacecraft are seeing over the sun’s horizon, tracking sunspots that officially “don’t count.” The two spacecraft are moving toward opposite sides of the sun, and by February 2011 the entire sun will be under their watchful eyes. Perhaps it is time to start thinking about a “whole sun” sunspot number. As today’s
