Large spots on the face of the sun are sometimes visible with the naked eye when the sun is viewed through smoked glass or special light filters (though even when using smoked glass to look at the sun, the viewer risks permanent eye damage). But until the invention of the telescope such spots could be made out only with difficulty.

In 1611, Galileo turned his telescope on the sun and became the first to see the spots in detail, concluding that they were blemishes on the sun. By observing them over time as they seemed to cross the sun's surface, Galileo was able to determine the sun's rotational rate. But with no reason to believe that sunspots were significant, few bothered to follow up on Galileo's observations.

Information gathered in the 1800s showed that sunspots did indeed have properties worth studying. It had been known that occasional cycles of...