In any case, Galileo's interest soon turned from falling bodies to astronomy. Only a few years earlier, a remarkable -- and controversial -- discovery had been made by the Danish astronomer, Tycho Brahe,
who found a new star in the constellation Cassiopeia in 1572.
Actually, it was a supernova -- one that disappeared two years later -- but this was enough to challenge directly another tenet of Aristotle's -- namely that the heavens were perfect, immutable, unchanging.
When another nova appeared in 1604,
Galileo was persuaded to give three public lectures in Padua -- where he was then teaching. These aroused considerable public interest -- so much, in fact, that no hall was big enough to accommodate everyone who wanted to hear him. Like Tycho Brahe, Galileo used the nova as a pretext to challenge the Aristotelian idea that the heavens were unchangeable, immutable, perfect.
Five years later, rumors of an invention made by a Dutch spectacle-maker reached Venice, and these led Galileo to construct his first telescope in July of 1609.
It wasn't long before Galileo began to make a series of startling observations, including the discovery of innumerable stars never seen before, mountains on the moon,
and the four satellites of Jupiter,
the movements of which he carefully plotted from day to day.
He was soon to publish these discoveries in a book, the Siderius Nuncius (Starry Messenger) which caused an overnight sensation.
Galileo named the moons of Jupiter the "medicean planets" -- in honor of his former student Cosmo and the famous Medici family --
thanks to which he was shortly thereafter appointed Chief Mathematician and Philosopher to the Grand Duke of Tuscany in Florence.
Go to the next section: Galileo III: The Inquistion
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