The year is an approximation of the tropical year, time interval between two consecutive passages of the Sun at the spring equinox.Ī calendar is of lunar or solar type depending on whether it favors the approximation of the month or of the year. The month is the approximate length of a lunation (time interval between two new moons). How do we define calendars?Ĭalendars are based on considerations of astronomical nature. In this case, they use a dummy calendar with the same rules of construction. The Julian calendar is used by historians to ancient times and, in particular, to those prior to its creation ( -45). So, the number of years between January 1, -45 and Januis equal to : 1991 - ( -45) = 2036. Calculating the number of years between a negative year a positive year is done algebraically. The rule for divisibility by 4 of leap years is preserved: years 0, -4, -8. Compared to the notation of historians it has two advantages:ġ. This is the notation of astronomers that is used in the calculations of ephemerides. They call year 0 the year 1 BC and have noted negatively previous years as follows:ġ BC (1 before J.-C.) = year 0 (leap year)ĥ BC (5 before J.-C.) = year -4 (leap year) 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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