Galileo Galilei discovered the first three satellites of Jupiter
Through his telescope, Italian astronomer, physicist, mathematician, and philosopher Galileo Galilei (1564-1642) discovered the first three satellites of Jupiter on January 7th, 1610.
Further observations led him to conclude that the three bodies could not be stars as their position changed. This directly challenged Aristotle's treatise "On the Heavens", whereby he states that unlike the Earth's four elements (earth, fire, wind, and water) which were prone to physical transformations because of their properties (earth and water, being elements with weight, tended to gravitate downwards to the center of the Earth, whereas wind and fire, the lighter elements, moved upwards), heavenly bodies, made of aether, were perfect, immutable objects, and as such, could only follow a perfect and eternal circular motion (as the circle was seen as the most perfect).
This finding also challenged the Ptolemaic or geocentric system, which held that the Earth was at the center of the universe. It also proved for the first time that objects could orbit a body that was not the Earth or the Sun.
Jupiter's first three satellites (moons) are: Callisto, Io, and Europa. He observed Ganymede a few days later.