🚀 go-pugleaf

In article <15...@speedy.mcnc.org> Scott Kajihara asks: 

> Question:  anyone know why the VMS system has a clock that starts about
> November 15 in the late 1800's?  UNIX's clock is a round number like midnight
> of January 1, 1970.

Why VMS 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. 


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