![]() Cassini (1740 ) use an algebraic notation. The rule for divisibility by 4 may no longer be applied.Īstronomers from J. Leap years succeeding every 4 years, so these are the years 1, 5, 9. Years :įor historians, since the eighteenth century, the year preceding the year 1 of the Christian era is written " 1 BC " and is a leap year. ![]() The rules adopted, which are the various calendars, differ from one civilization to another. It is this principle that defined the Julian period described below.įor reasons of tradition, but also convenience, we have been led to complicate the system by cutting in years and months. Earlier periods may possibly be identified by listing to the past days but it is easier to introduce a zero day and negative numbers for days before the origin. The simplest solution, in principle, is to choose arbitrarily one day origin and without any artifice to list successive days. This numbering system is designed to locate any event without ambiguity: it is the problem of chronology. The identification of moments requires the definition of time scales, but regardless of the choice of a scale, it is necessary to define a system for numbering days (it is easy to see a change of day, it is more difficult to see the change of year, so the day is the natural unit). Understanding > Fundamental concepts > Calendars I CHRONOLOGY, ERAS AND CALENDARS
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