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.
Characters and organizations in Year Zero
The Solutions Backwards Initiative is a group of students, professors, and IT specialists who devise a method of, and succeed in, sending information back through time. The information sent back is theorized to be the websites that are connected to "Year Zero". However, the data they send back through time gets somewhat corrupted, causing the garbled appearance of all of the sites in the "Year Zero" universe.