Utklipp fra Wikipedia’s entry for Galileo Galilei:
On 7 January 1610 Galileo observed with his telescope what he described at the time as “three fixed stars, totally invisible by their smallness”, all within a short distance of Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these “stars” relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days he concluded that they were orbiting Jupiter. He had discovered three of Jupiter’s four largest satellites (moons): Io, Europa, and Callisto. He discovered the fourth, Ganymede, on 13 January. Galileo named the four satellites he had discovered Medicean stars, in honour of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and Cosimo’s three brothers. Later astronomers, however, renamed them Galilean satellites in honour of Galileo himself.
The significance of the above is the following…
This was one of the earliest known astronomical observations using a telescope. The discovery that Jupiter had moons orbiting it was pivotal in displacing the Aristotelian Cosmology which held that all heavenly bodies circle the Earth. This discovery supported the Copernican model of the Solar System where the Sun and not the Earth is at the centre. Galileo’s discovery in September of the same year that Venus displayed phases like the Moon showed that Venus orbited the Sun, not the Earth.