Introduction
Why OpenVMS regards November 17, 1858 as the beginning of time…
Details
The modified Julian date, adopted by SAO (Smithsonian Astrophysical Observatory) for satellite tracking, is Julian Day 2400000, which turns out to be November 17, 1858.
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.
Julian Day 1 began at 12:00 noon, January 1, 4713 B.C. This date was thought by some to correspond approximately to the beginning of the universe. Certainly it predated any astronomical events known in the 16th century without resorting to negative times. Scaliger decided on the actual date on the grounds that it was the most recent coincidence of three major chronological cycles:
- The 28-year solar cycle, after which dates in the Julian calendar (for example September 27) return to the same days of the week (for example Tuesday).
- The 19-year lunar cycle, after which phases of the moon return to the same dates of the year.
- The 15-year indiction cycle, used in ancient Rome for tax regulation.
It takes 7980 years to complete the cycle. Noon of January 1, 1988, marks the beginning of Julian Day 2447161.
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.