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.
Names for the number 0 in English
It is quite common in English to describe decades with a plural of the first year of the decade for example "the nineties" which can be written as "the 90s". There was a debate on what to call the first decade of the 21st century, as "00s" is not usually pronounced as a word. Apart from side stepping the issue and calling the decade the "two-thousands" (2000s) there is no agreed wording, although in Britain and some other countries "noughties" was also used.