Galileo Galilei

Galileo Galilei was born in Pisa in 1564. He became deeply interested in mathematics, motion, measurement, and the physical world.

His career combined teaching, experimentation, instrument design, and astronomical observation during a period when long-standing ideas about the heavens were being reconsidered.

After learning about the newly developed spyglass, Galileo built improved instruments and began pointing them toward astronomical objects.

His observations revealed surface features on the Moon, objects orbiting Jupiter, changing phases of Venus, and sunspots.

In 1610, Galileo observed objects near Jupiter that changed position from night to night.

He determined that these objects were orbiting Jupiter. They are now known as Io, Europa, Ganymede, and Callisto.

Copernicus had proposed a planetary model centered on the Sun before Galileo began his telescopic work.

Galileo's observations of Venus and Jupiter provided important evidence that fit poorly with a simple Earth-centered picture of the heavens.

That creates a natural historical path from Copernicus's model to Galileo's observations and then to Kepler's mathematical description of planetary motion.