The Evolution of the Roman Calendar: From Romulus to the Gregorian System

Abstract

This study examines how the Roman calendar developed from the traditional ten-month year attributed to Romulus into the twelve-month Gregorian system now used worldwide. It asks why the names September, October, November and December, which encode the numerals seven to ten, label the ninth to twelfth months. It uses a PRISMA-ScR-informed evidence-mapping design to synthesise 55 sources: ancient literary testimony, modern monographs, astronomical reference works, and comparative and methodological literature. Quantitative treatment of month lengths and mean year lengths supplements the qualitative analysis. Three findings emerge. First, the numeral–position mismatch has a single cause. The prefixing of January and February to a year that began in March shifted every month by exactly two places, and the Julian and Gregorian reforms never revised the names. Second, the ten-month, 304-day year is better understood as a partly antiquarian reconstruction than as an observed practice, since its months do not match the lunar cycle. Third, calendar reform was repeatedly an act of political authority, from the priestly control of intercalation to the renaming of Quintilis and Sextilis for Julius Caesar and Augustus. The modern calendar is a layered artefact in which Latin vocabulary, Roman religion, Hellenistic and Egyptian astronomy, and early modern ecclesiastical computation remain visible. The paper proposes the concept of nomenclatural inertia and a simple Nomenclatural Displacement Index to describe how names outlive the systems that produced them.