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.
1000
Year 1000 (M) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar. In the proleptic Gregorian calendar, it was a non-leap century year starting on Wednesday (like 1800). It was also the last year of the 10th century as well as the last year of the 1st millennium of the Dionysian era ending on December 31st, but the first year of the 1000s decade.