The Moons of Jupiter

The five naked-eye planets were known even to the ancient people. It is no wonder therefore that we can find them on their coins, either pictured as stars or as allegoric. Discovering of the moons of planets became possible by the invention of telescope. Soon after this the moons appeared on coins and medals.

Galilei észlelése a Jupiter holdjairól

When Galileo Galilei aimed the Jupiter with his telescope in January 1610, he immediately discovered the moons of Jupiter. Their motion that were connected to Jupiter revealed to him, that they are not stars, but bodies circulating around Jupiter. These small bodies were called on different names during times. Galilei named them first to Cosmica Sidera honoring his generous patron Cosimo II de' Medici, the grand duke of Tuscany. Later he changed – suggested by the duke – to Medicea Sidera, i. e. Medici stars, meaning the duke and his three brothers (Francesco, Carlo and Lorenzo). The discovery was first published in Venice in March 1610, less then after two month of first observation. Johannes Kepler – who in the beginning doubt the existence of the moons – in August and September 1610 convinced on their validity by his own observations and in 1611 in his work Dissertatio cum Nuncio Sidereo (Prague, 1610, Frankfurt 1611), and its extension "Narratio" he mentioned “stars” that “Mr. Galilei discovered” beside the Jupiter. In this time the moons were named simply stars, or satellites (trabanten). J. D. Cassini in his book issued in 1655 named them “Medici planets”. Naming the satellites of planets to moons spread in the end of XVIIth century, Currently the four largest moons of Jupiter are commonly named Galilean moons.


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Possibly the first medal that displays the Galilean moons – and moons of the Solar System at all – can be found on a medal of Cosimo III de' Medici, Grand Duke of Tuscany, issued for emphasizing the naval power of Tuscany. On the reverse a starboard broadside view of a three-masted ship on the calm sea is visible, a haloed astrological sign of Jupiter above and four stars around. There is no need of too much imagination, that they are not only simple stars, but rather the four moons of Jupiter discovered by Galilei, and of Cosimo III – the grandson of Cosimo II – was certainly proud. Only a few monarch could take pride in having stars named on their family. The text around 'CERTA . FULGENT . SIDERA' is a citation of Horace's Ode II-XVI “Otium”. Below the name of the medalist is visible 'TRAVANVS', i. e. (Gioacchino) Francesco Travani.


Original English
Otium divos rogat in patenti
prensus Aegaeo, simul atra nubes
condidit lunam neque certa fulgent
sidera nautis;

Peace the sailor prays, caught in a storm on the open Aegean, when dark-clad clouds have hid the moon and the stars shine no longer certain;


See whole text here

On the coin extracted from its original context and left out negative it means its opposite ' stars shine certain'. The shining star must be the Jupiter and its four Medicean Stars. medici revTherefore this medal not only supports Tuscany’s naval power, but also that the Medici family and Cosimo III himself was proud of his four stars, that Galileo Galilei owing excellent diplomatic talent named after them.

On the obverse bust of Cosimo III de Medici to the left is displayed in long hair and armour, fancy mantled. Date of 1666 is visible on his shoulder. Text around: 'COSMVS . III . PRINC . AETRVR' means Cosimo III duke of Etruria, as the Medici named themselves sometimes.

Bronze medal with diameter 47.8mm, weight 45.4 gramm.

The moons of Jupiter played important part in further development of science. The first precise measurement of speed of light was performed by Ole Rømer, a dutch physicist in 1676. He observed Io, the first moon of Jupiter and discovered variance in its revolution period. Rømer got 227000 kilometer per second velocity for the light. The eclipses and other events of the moons had been intended to use in navy for the determination of longitude of ships, before the first clock that worked precisely in nautical conditions was developed by John Harrison.

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A lot of astronomers studied the motion of Jupiter’s moons, among them was the Swedish astronomer Pehr Wilhelm Wargentin. A silver medal that was issued in 1783 – a work of Carl Gustaf Fehrman – shows Wargentin's portrait to the right.

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On the reverse above the globe of Earth the Ecliptic runs from the Fish on the left to the Aquarius on the right. Close to this later the globe of Jupiter is visible. The moon Europa is placed right to it signed by '2' , while the other three moons are visible on the left: '1' is Io, '3' is Ganymede and '4' is Callisto. Text around "SUBLIMIORA IAM CURAT" means “he already cared sublime (matters)”, refers to his studies of Jupiter. Wargentin became the secretary of the Royal Swedish Academy of Sciences in 1749. He kept this position until his death in 1783, for 34 years, as it can be read from the text below.

Another medal also mentions Wargentin's research work on Jupiter, although in rather allegoric form.

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On the obverse, around the portrait to the left his position as a secretary of Academy is mentioned. On the reverse allegoric scene with winged Prometheus is pictured, holding astrolabe in his hand. The text "ADSPECTAT OLYMPUM INQUIRITQUE JOVEM" means “he observed the Olympus and studied Jupiter”. Even if he did not steal the fire from the Olympus, he monitored most secrets of Jupiter, Father of Gods and men. Below the dates of his birth and death are displayed, 1717 - 1783.

65 moons of Jupiter have been discovered in the time of writing this text. In contrary to the Galilean moons the other moons are tiny and faint, they can hardly be observed even by telescope. Although some of them are not too faint, they lost in the enormous glare of Jupiter. Thanks to this the fifth moon was only discovered in 1892. No wonder therefore that on a token that was issued in the age of Luis XIV French king and displays the known Solar System of time only the four Galilean moons are pictured (on the lower left). On this token the Moon of Earth, and five moons of Saturn are also displayed.

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Mercury and Venus has no any moon, the two tiny moons of Mars were undiscovered of the production of this token.

Modern coins and medals often display the Solar System, but from most of them the moons of planets are missing. One of the several exception is an 500 Tugrik coin of Mongolia that remembers Isaac Newton the “genius of the millennium”. The revers of the coin displays the planets of Solar System, their path around the Sun, and two comets viewed from ‘above’. Jupiter stands on the left, and four small dots around it the Galilean moons are pictured.

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