Using a telescope he had built himself, Galileo Galilei pointed his instrument at Jupiter and discovered its four largest moons on January 7, 1610. These celestial bodies, later named Ganymede, Callisto, Io, and Europa, were the first objects found to be orbiting another planet, fundamentally changing the view of the cosmos. Galileo observed the moons in motion over the following nights, noting their positions relative to Jupiter and one another. His observations were methodical, and he meticulously recorded the changes in their positions as the nights progressed.

Galileo's discovery was published in his work 'Sidereus Nuncius' (The Starry Messenger), where he detailed his observations and findings. His account included illustrations and a discussion of the profound implications of such observations, challenging the prevailing geocentric model of the universe, which posited that everything revolved around the Earth. Instead, his findings provided evidence for the heliocentric model proposed by Copernicus, where planets, including Earth, orbited the Sun.

The initial observations of these moons opened up new avenues for astronomical research. They were the first celestial bodies discovered to be in orbit around a planet, allowing for further insights into the motion of celestial bodies and fundamentally altering humanity's understanding of our place in the universe. This event not only advanced scientific knowledge but also set the stage for future explorations into planetary science and the study of the solar system.