0 (year) - Historical, Astronomical and ISO Year Numbering Systems - Astronomers

Astronomers

To simplify calculations, astronomers have used a defined leap year zero equal to 1 BC of the traditional Christian era since the 17th century. Modern astronomers do not use years for intervals because years do not distinguish between common years and leap years, causing the resulting interval to be inaccurate.

In astronomy, the numbering of all years labeled Anno Domini remain unchanged. However, the numerical value of years labeled Before Christ are reduced by one by the insertion of a year 0 before 1 AD. Thus, astronomical BC years and historical BC years are not equivalent. To avoid this confusion, modern astronomers label years as positive or negative, instead of BC or AD.

The current method was created by Jacques Cassini, who explained:

The year 0 is that in which one supposes that Jesus Christ was born, which several chronologists mark 1 before the birth of Jesus Christ and which we marked 0, so that the sum of the years before and after Jesus Christ gives the interval which is between these years, and where numbers divisible by 4 mark the leap years as so many before or after Jesus Christ. —Jacques Cassini, Tables astronomiques, 5, translated from French

In this quote, Cassini used "year" as both a calendar year and as an instant before a year. He identified the calendar year 0 as the year during which Jesus Christ was born (on the traditional date of 25 December), and as calendar leap years divisible by 4 (having an extra day in February). But "the sum of years before and after Jesus Christ" referred to the years between a number of instants at the beginning of those years, including the beginning of year 0, identified by Cassini as "Jesus Christ", virtually identical to Kepler's "Christi". Consider the three instants ('years') labeled 1 avant Jesus-Christ, 0, 1 après Jesus-Christ by Cassini, which modern astronomers would label −1.0, 0.0, +1.0. Cassini specified that his end years must be added, so the interval between the instants (noon 1 January) 1 avant Jesus-Christ and 1 après Jesus-Christ is 1 + 1 = 2, but modern astronomers would subtract their 'years', +1.0 − (−1.0) = 2.0, which agrees with Cassini. The calendar years between these two instants would be 2 BC and 1 BC, leaving the calendar year 1 AD beginning at +1.0 outside the interval.