The discovery of the Galilean moons

Galileo gets the credit, but did he actually see Jupiter’s four largest moons first?

A fresco by Giuseppe Bertini portraying Galileo showing the Doge of Venice how to use a telescope. Credit: Giuseppe Bertini / /

In this fresco by Guiseppe Bertini from 1857, Galileo shows the Doge of Venice how to use a telescope. Image Credit:


On the title page of Galileo’s Sidereus Nuncius (Sidereal Messenger) – the first astronomical work based on telescope observations when it was published in 1610 – the author says that the book introduces the reader to “great and surpassingly wondrous sights… observed by Galileo Galilei, Florentine patrician and public mathematician at the University of Padua… in four planets revolving with remarkable speed at differing distances and periods around the star Jupiter.

“They have been known to no one up to this day, and the author was the first to discover them. He has decided to call them the Medicean planets.”

The initial naming of the moons as the Medicean planets was to acknowledge the patronage he received from Grand Duke Cosimo II de’ Medici of Tuscany (1590–1621), whom Galileo had served as mathematics tutor in 1605.

Galileo’s first idea was to name the newly discovered moons the Cosmica Sidera (Cosmian Stars) solely in honour of his patron, but ultimately chose a name that honoured all four surviving Medici brothers: Cosimo, Francesco, Carlo and Lorenzo.

Galileo also recognised that the four objects he had observed through his telescope were the first ever seen to orbit another planet, and the importance of his discovery was not lost on him.

But though he may have been the first to name them, Galileo’s claim to having been the first to see them has come under some doubt.

Possible earlier sightings

It has been suggested that the Chinese astronomer Gan De, who carried out some of the earliest known systematic observations of the planets in the 4th century BC, may have seen Ganymede.

It was while he was studying Jupiter during the summer of 365 BC that Gan De recorded what he described as a ‘small reddish star’ next to the planet.

The Chinese astronomy historian Xi Zezong (1927-2008) suggested this may have been an early sighting of Ganymede.

It is theoretically possible to see the Galilean moons with the naked eye, but it requires near perfect conditions and incredible eyesight.

In 1614, around four years after Sidereus Nuncius was published, German astronomer Simon Marius (1753-1625) published his work Mundus Iovialis (1614) in which he described the planet Jupiter and its moons.

He laid claim to having discovered them in December 1609.

This would mean that Marius had spotted the Jovian moons some time before Galileo who, according to Sidereus Nuncius, had first seen them on 7 January 1610.

However, what is certain is that Galileo was the first to publish what he saw.

It was on that date that Galileo turned his telescope towards Jupiter and noticed what he described as “three little stars… positioned near (Jupiter) – small but yet very bright” and noting the presence of a fourth ‘little star’ a few days later.

Although his first thoughts were that these were ‘fixed stars’, Galileo was sufficiently intrigued by the fact they were “arranged exactly along a straight line and parallel to the ecliptic” to continue his observations, which ultimately revealed their true nature.

An illustration showing Jupiter and its four Galilean moons, from left to right Io, Europa, Ganymede, Callisto. Credit: NASA/JPL/DLR
An illustration showing Jupiter and its four Galilean moons, from left to right Io, Europa, Ganymede, Callisto.

After announcing the moons in Sidereus Nuncius, independent verification and sightings of the newly discovered Jovian moons came from a number of sources.

These included such noteworthy observers as Johannes Kepler in Prague, English astronomer Thomas Harriot and French astronomer Joseph Gaultier de La Vallette.

Regardless of who saw them first the mythological names by which these satellites are known today are those given them by Marius (inspired by a suggestion from Johannes Kepler).

In 1614, he wrote in Mundus Iovialis, “Io, Europa, the boy Ganymede and Callisto greatly pleased lustful Jupiter.”

However, the names didn’t gain favour until the early 20th century, mainly because Galileo refused to use them.

In the meantime, they were generally referred to as Jupiter I, II, III and IV according to their closeness to Jupiter.

It would be another few centuries until Jupiter’s next moon, Amalthea, was discovered by American astronomer Edward Emerson Barnard using the 36-inch Great Lick Refractor at Lick Observatory on 9 September 1892.

Amalthea holds the distinction of being the last planetary satellite to be discovered by direct visual observation.

The sixth Jovian moon, Himalia, was revealed on 3 December 1904 using astrophotography, as have all of its many subsequently discovered moons.

Since then, the number of confirmed moons orbiting Jupiter has increased greatly, and at the time of writing the number of known satellites stands at well over 70.

A guide to the Galilean moons


Diameter: 3,660km

Mass: 0.015 Earth masses

Orbital distance: 421,700km

Io, the closest moon to Jupiter, has over 400 volcanoes across its surface. The gas giant’s constant gravitational tugging keeps its interior molten.


Diameter: 3,122km

Mass: 0.008 Earth masses

Orbital distance: 670,900km

The smallest of the moons, Europa is covered in a thick layer of ice. Beneath this is a liquid water ocean that could be hospitable to life.


Diameter: 5,268km

Mass: 0.025 Earth masses

Orbital distance: 1,070,400km

The largest moon in the Solar System. It is thought to have a large rocky core covered with layers of ice and water which show signs of tectonic activity.


Diameter: 4,821km

Mass: 0.018 Earth masses

Orbital distance: 1,882,700km

The least active of the Galilean moons. Its ancient surface is almost completely covered in craters, so every new crater erases an older one.



Brian Jones has written 17 books on astronomy for children and adults