Montage of Jupiter's four Galilean moons, in a composite image comparing their sizes and the size of Jupiter - from top to bottom: Io, Europa, Ganymede, Callisto
The four moons were discovered sometime between 1609 and 1610 when Galileo made improvements to his telescope
, which enabled him to observe celestial bodies more distinctly than had ever been possible before.
Galileo’s discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. More importantly, the incontrovertible discovery of celestial bodies orbiting something other than the Earth dealt a serious blow to the then-accepted Ptolemaic world system
, or the geocentric theory in which everything orbits around the Earth
Galileo initially named his discovery the Cosmica Sidera
's stars"), but names that eventually prevailed were chosen by Simon Marius
. Marius claimed to have discovered the moons at the same time as Galileo, and gave them their present names in his Mundus Jovialis
, published in 1614.
As a result of improvements Galileo Galilei
made to the telescope
, with a magnifying capability of 20×,
he was able to see celestial bodies more distinctly than was ever possible before. This allowed Galilei to discover sometime between December 1609 and January 1610 what came to be known as the Galilean moons.
On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs
from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.