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.

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 been called the father of observational astronomy, modern-era classical physics, the scientific method, and modern science.

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 closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope allowed astronomers to discover the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.

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 natural satellite, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System.

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. Europa is an icy moon, and, of the three icy Galilean moons, the closest orbiting Jupiter. As a result, it exhibits a relatively young surface, driven by tidal heating.