W hen Galileo turned his tele­ scope on the sun in 1610, he discovered that there were dark spots on the solar disk. Observing the motion of the spots across the disk, he deduced that the sun rotated once in about 27 days. One of his contempo­ raries, Christoph Scheiner, undertook a systematic program of observing the spots. From these observations Scheiner found that the rotation period of the...