They are more commonly referred to as the Galilean moons of Jupiter owing to the fact that they were discovered by Galileo. The four Galilean moons – Io, Europa, Ganymede and Callisto – are ...

Jupiter and its four planet-size moons, called the Galilean satellites, were photographed by Voyager 1 and assembled into this collage. They are not to scale but are in their relative positions.

Four days later, he observed a fourth point of light near the planet with the same unusual behavior. By Jan. 15, Galileo correctly concluded that he had discovered four moons orbiting around Jupiter, ...

Initially believing they were distant stars, Galileo’s repeated observations over several nights and realized they were moons orbiting the gas giant. And thus, Io, Europa and Ganymede became ...

Galileo was the first person to observe the rings of Saturn. He spent hour after hour on painstaking drawings of the Moon through its phases, observed Venus and Mars, and even made sketches of ...