Astronomical year numbering
Cassini gave the following reasons for using a year 0:
Fred Espanak of NASA lists 50 phases of the moon within year 0, showing that it is a full year, not an instant in time. Jean Meeus gives the following explanation:
Although he used the usual French terms "avant J.-C." (before Jesus Christ) and "après J.-C." (after Jesus Christ) to label years elsewhere in his book, the Byzantine historian Venance Grumel used negative years (identified by a minus sign, −) to label BC years and unsigned positive years to label AD years in a table. He did so possibly to save space and put no year 0 between them.
Year zero
The "anno Domini" nomenclature was not widely used in Western Europe until the 9th century, and the to historical year was not uniform throughout Western Europe until 1752. The first extensive use (hundreds of times) of 'BC' occurred in "Fasciculus Temporum" by Werner Rolevinck in 1474, alongside years of the world ("anno mundi"). The terms "anno Domini", "Dionysian era", "Christian era", "vulgar era", and "common era" were used interchangeably between the Renaissance and the 19th century, at least in Latin. But "vulgar era" was suppressed in English at the beginning of the 20th century after "vulgar" acquired the meaning of "offensively coarse", replacing its original meaning of "common" or "ordinary". Consequently, historians regard all these eras as equal.