OpenVMS and the Julian Period

  Why OpenVMS regards November 17, 1858 as the beginning
  of time...

  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.

  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.

  Additional topics:

  *   Julian Period

  *   Brief History of the Gregorian Calendar

  *   Julian Period

  *   History and DECwindows Calendar