Roman numerals, the numeral system devised and formerly used by the Romans and still used today to write names such as Elizabeth II or Henry VIII, etc.
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It is named after International Year of Astronomy of 2009 (MMIX in Roman numerals).
However, 0 had been used as a number in the medieval computus (the calculation of the date of Easter), beginning with Dionysius Exiguus in 525, without being denoted by a numeral (standard Roman numerals do not have a symbol for 0); instead nulla or nullae, genitive of nullus, the Latin word for "none", was employed to denote a 0 value.
The "I" in "Atlas I" can cause confusion, as all previous Atlas rockets were designated using letters, ending with the Atlas H, however subsequent rockets were designated using roman numerals, starting with the Atlas II.
Roman Numerals (I, II, III) were cumbersome to use and only maintained a dominant role in arithmetic until 1202, when Leonardo Fibonacci in his work Liber Abaci, demonstrated that calculations with Hindu-Arabic numerals (1, 2, 3) were far more efficient.