What major celestial discovery did Galileo make when observing Jupiter?
Answer
Four bright points of light orbiting the planet.
He discovered four bright points orbiting Jupiter, which he named the Medicean stars, proving that not everything in the heavens orbited the Earth.

Related Questions
What was the approximate maximum magnification achieved by Galileo's most effective telescopes?What characteristic of the Moon did Galileo's observations reveal?What major celestial discovery did Galileo make when observing Jupiter?Why did observing the full phases of Venus support the heliocentric model?What observation about the Sun challenged its traditional celestial status?Did Galileo invent the initial telescope device?What critical element did Galileo insist upon to transition observation into modern science?How did Galileo apply quantitative analysis to the Moon's features?What was Galileo's ultimate vision for the governing structure of the universe?What famous quote summarizes Galileo's view on the scope of Scripture versus science?What did Galileo's engineering breakthrough have to precede for his cosmology to be established?