[Info-vax] Why is starting epoch 17 Nov 1858?

[Jim DeCamp]

On Monday, November 27, 1995 at 3:00:00 AM UTC-5, zrep... at cuppa.curtin.edu.au wrote:
> In Article <01HXR4XI5... at kopc.hhs.dk>
> Arne Vajhoej <AR... at ko.hhs.dk> writes:
> >> Does anybody know why the epoch time in VMS runs from midnight,
> >> 17 November 1858?
> >"Introduction to VMS System Services" section 9.1 says:
> > The Smithsonian base date and time for the astronomical calendar
> And the TOPS-10 group could remove the line that
> said " Just for the 10 at the Smithsonian " One less
> monitor feature :)
  
It's called Smithsonian time, and yes it is the epoch for the Modified Julian Day, which is the Julian Day minus 2,400,000.5 days.  Back in the days of Sputnik, the Smithsonian Astrophysical Observatory, headquartered in Cambridge, Massachusetts, had a 36 bit computer. They wanted to track Sputnik, and other satellites. The decision was made to represent time as an integer since that date, with the upper 18 bits representing the number of days, and the lower 18 bits the fraction of a day.  The weight of the LSB would then be  1/262144 days or 86400/262144  ~ .3296 seconds.  Digital, based in Maynard, Massachusetts recruited heavily from Cambridge, so somehow, they adopted the same epoch, but not the same representation.