Naming of Moons - Roman Numeral Designations

Roman Numeral Designations

The Roman numbering system for satellites 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), refusing to adopt the names proposed by his rival Simon Marius. Similar numbering schemes naturally arose with the discovery of multiple moons around Saturn, Uranus, and Mars. The numbers initially designated the moons in orbital sequence, and were re-numbered after each new discovery; for instance, before the discovery of Mimas and Enceladus in 1789, Tethys was Saturn I, Dione Saturn II, etc., but after the new moons were discovered, Mimas became Saturn I, Enceladus Saturn II, Tethys Saturn III and Dione Saturn IV.

In the middle of the 19th century, however, the numeration became fixed, and later discoveries failed to conform with the orbital sequence scheme. Amalthea, discovered in 1892, was labelled "Jupiter V" although it orbits more closely to Jupiter than does Io (Jupiter I). 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 Timeline of discovery of Solar System planets and their natural satellites); though if a large number of satellites were discovered in a short span of time, the group could be numbered in orbital sequence, or according to other principles than strictly by order of discovery. The convention has been extended to natural satellites of minor planets, such as (87) Sylvia I Romulus.

Roman numerals are usually not assigned to satellites until they are named, so many satellites that have been discovered but only have provisional designations do not have Roman numerals assigned to them. (An exception is Saturn's moon Helene, which received the Roman numberal XII in 1982, but was not named until 1988.) Since the International Astronomical Union began assigning names to all satellites in 1975, the use of Roman numeral designations has diminished, and some are very rarely used; Phobos and Deimos are rarely referred to as Mars I and Mars II, and the Moon is never referred to as "Earth I".

The thirteen named satellites of Saturn from Aegir to Surtur were named in alphabetical order corresponding to their Roman numerals.