Astronomers use the Julian period because it is convenient to express long time intervals in days rather than months, weeks and years. It was devised by Joseph Scaliger, in 1582, who named it after his father Julius, thus creating the confusion between the Julian (Caesar) calendar and the Julian (Scaliger) period.

I consider myself a time and date nerd and I had no idea this was the case. I thought the two Julians were one and the same.

SAO started tracking satellites with an 8K (nonvirtual) 36-bit IBM 704 in 1957 when Sputnik went into orbit. The Julian day was 2435839 on January 1, 1957. This is 11225377 octal, which was too big to fit into an 18-bit field. With only 8K of memory, the 14 bits left over by keeping the Julian date in its own 36-bit word would have been wasted. They also needed the fraction of the current day (for which 18 bits gave enough accuracy), so it was decided to keep the number of days in the left 18 bits and the fraction of a day in the right 18 bits of one word.

Eighteen bits allows the truncated Julian day (the SAO day) to grow as large as 262143, which from November 17, 1858, allowed for 7 centuries. Possibly, the date could only grow as large as 131071 (using 17 bits), but this still covers 3 centuries and leaves the possibility of representing negative time. The 1858 date preceded the oldest star catalogue in use at SAO, which also avoided having to use negative time in any of the satellite tracking calculations.

This sort of history is absolutely fascinating to me, and also makes perfect sense in retrospect, especially considering the popularity of VMS in the astronomy and physics disciplines.

I wonder which came to VMS first — the SAO Julian date epoch, or the astrophysicists?