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.
Timeline of historical geopolitical changes
This article uses the Common Calendar System, which was established as the new Christian Calendar by Pope Gregory XIII in 1582. The Common Calendar uses the designation Common Era (CE or AD for Anno Domini) for years starting 1 January 1 CE, and Before Common Era (BCE or BC for Before Christ) for years before that date. The Common Calendar also follows the ordinal numbers rather than the cardinal numbers, so there was no "year zero" in this format; that is, the date 1 January 1 CE immediately followed the date 31 December 1 BCE. For the same reason, while the 2000s began on Saturday, 1 January 2000 CE, the official Third Millennium began on Monday, 1 January 2001 CE.