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Through his telescope, Italian astronomer Galileo Galilei made the first observation of Jupiter's Galilean moons: Io, Europa, Ganymede and Callisto, although he was not able to distinguish the first two until the following night.
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Galileo Galilei
Florentine physicist and astronomer (1564–1642)
Galileo di Vincenzo Bonaiuti de' Galilei, commonly referred to as Galileo Galilei or mononymously as Galileo, was an Italian astronomer, physicist, and engineer, sometimes described as a polymath. He was born in the city of Pisa, then part of the Duchy of Florence. Galileo has…Source: Wikipedia (CC BY-SA)
Galilean moons
Four largest moons of Jupiter
The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede, Callisto, Io, and Europa. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets; though their…Source: Wikipedia (CC BY-SA)
Io (moon)
Innermost of the four Galilean moons of Jupiter
Io is the innermost and second-smallest of the four Galilean moons of the planet Jupiter. Slightly larger than Earth's Moon, Io is the fourth-largest natural satellite in the Solar System, has the highest density of any natural satellite, the strongest surface gravity of any…Source: Wikipedia (CC BY-SA)
Europa (moon)
Smallest Galilean moon of Jupiter
Europa is a natural satellite (moon) of Jupiter. Being observable from Earth with common binoculars, it is one of the four Galilean moons. As such it is a planetary-mass moon; the smallest and least massive orbiting Jupiter, and slightly smaller and less massive than Earth's.…Source: Wikipedia (CC BY-SA)
Ganymede (moon)
Largest moon of Jupiter and in the Solar System
Ganymede is a natural satellite of Jupiter and the largest and most massive in the Solar System. Like Saturn's largest moon Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the…Source: Wikipedia (CC BY-SA)
Callisto (moon)
Second-largest moon of Jupiter
Callisto is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and nearly as large as the smallest planet Mercury. Callisto is, with a diameter of 4,821 km, roughly a third larger…Source: Wikipedia (CC BY-SA)