“It seems to me proper to adorn the Author's thought here with its conformity to a conception of Plato's regarding the determination of the various speeds of equable motion in the celestial motions of revolution. …he said that God, after having created the movable celestial bodies, in order to assign to them those speeds with which they must be moved perpetually in equable circular motion, made them depart from rest and move through determinate spaces in that natural straight motion in which we sensibly see our moveables to be moved from the state of rest, successively accelerating. And he added that these having been made to gain that degree [of speed] which it pleased God that they should maintain forever, He turned their straight motion into circulation, the only kind [of motion] that is suitable to be conserved equably, turning always without retreat from or approach toward any pre-established goal desired by them. The conception is truly worthy of Plato, and it is to be more esteemed to the extent that its foundations, of which Plato remained silent, but which were discovered by our Author in removing their poetical mask or semblance, show it the guise of a true story.”

—  Galileo Galilei

I. Bernard Cohen's thesis: Galileo believed only circular (not straight line) motion may be conserved (perpetual), see The New Birth of Physics (1960).
Sagredo, Day Four, Stillman Drake translation (1974) pp.283-284
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)

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Galileo Galilei 70
Italian mathematician, physicist, philosopher and astronomer 1564–1642

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