Astronomical year numbering is another method of designating BC/AD years. In this system, the year 1 BC is numbered 0, the year 2 BC is numbered −1, and in general the year n BC is numbered −(n − 1). The numbers of AD years are not changed, but AD is not used, being replaced by either no sign or a positive sign. The system is so named due to its use by astronomers. It is convenient to have a year 0 because when calculating the number of years in a period that spans the epoch, the end years need only be subtracted from each other.
A zero year was first used by the eighteenth century French astronomers Phillipe de La Hire (1702) and Jacques Cassini (1740). However, both of these astronomers used the applicable BC/AD designations of Latin and French with their year zero, thus near the epoch the years were designated 2 BC, 1 BC, 0, AD 1, AD 2, etc. They did not use −/0/+. During the nineteenth century, astronomers designated years with either BC/0/AD or −/0/+. Astronomers did not exclusively use the −/0/+ system until the twentieth century.
