which scientific theory is he most well known for propounding?
The Starry Messenger
Galileo Galilei
 "The Starry Messenger" ("Sidereus Nuncius") was the first scientific treatise to be published based on observations made through a telescope. It reported his discoveries of:
what are the four biggest moons of jupiter Io, Europa, Ganymede, and Callisto. who were they first spotted by? Galileo Galilei when was he born? 15 February 1564
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Galileo Galilei [SEP] Writings
It also subscribed to the Philoponan impetus dynamics in which impetus is self-dissipating and free-fall in a vacuum would have an essential terminal speed according to specific weight after an initial period of acceleration. Galileo's 1610 "The Starry Messenger" ("Sidereus Nuncius") was the first scientific treatise to be published based on observations made through a telescope. It reported his discoveries of: