Page "Astronomical naming conventions" Paragraph 40

The Roman numbering system arose with the very first discovery of natural satellites other than Earth's Moon: Galileo referred to the Galilean moons as I through IV ( counting from Jupiter outward ), in part to spite his rival Simon Marius, who had proposed the names now adopted, after his own proposal to name the bodies after members of the Medici family failed to win currency.

Similar numbering schemes naturally arose with the discovery of moons around Saturn and Mars.

Although the numbers initially designated the moons in orbital sequence, new discoveries soon failed to conform with this scheme ( e. g. " Jupiter V " is Amalthea, which orbits closer to Jupiter than does Io ).

The unstated convention then became, at the close of the 19th century, that the numbers more or less reflected the order of discovery, except for prior historical exceptions ( see the Timeline of discovery of Solar System planets and their moons ).