Moons of Jupiter - Io, Europa, Ganymede and Callisto

© Branko Šimunek/Alam

Jupiter and the Galilean Moons

On 7 January 1610, Galileo was the first person to train a telescope on Jupiter - and what he saw surprised him. Strung in a line beside the planet were three tiny stars, one to the left of the planet and two to the right. But when he observed the formation the next night, he saw that all three were on the same side. Over the following week he watched as the tiny stars (now joined by a fourth) changed their position, while remaining beside Jupiter. By 15 January he had worked it out: he was observing four moons orbiting the planet.

It was a discovery that would have profound implications for our understanding of the cosmos, providing evidence that Earth was not the centre of the universe around which everything rotated. Savvy as well as brilliant, Galileo named the four moons “the Medicean Stars” after his patron, Cosimo Medici. But over the years, as the influence of the Medici family waned, they became known as the Galilean Moons: Io, Europa, Ganymede and Callisto.