Galileo published his findings on the surface of
the moon and on Jupiter's moons in 1610 in his
book Messenger of the Stars. Two years after the
discovery of Jupiter's moons, Galileo's new discovery
dealt with the subject of the sun.

Galileo observed the sun and discovered spots - darker areas on the surface of the sun. These spots moved on the surface of the sun in an almost regular way, i.e., the same spots were seen in the same area each cycle. Galileo concluded that the sun revolved on its own axis, a fact that was not known before. The fact that the sun revolves on its own axis seemed to Galileo compatible with his claim that all planets revolve about the sun. It seemed to him only natural that the sun should also revolve on its own axis.

(The SUN SPOTS picture is from - The Universe of Galileo and Newton, Bixby, William - 1966)

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