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

[Rich Alderson]

Jim DeCamp <james.c.decamp at gmail.com> writes:

> 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.

I'll leave aside that you're followiung up to a post from 1995, and just point
out that DEC used precisely that 18,,18 representation for TOPS-20, on the
PDP-10 architecture, although they inherited it from BBN, who wrote TENEX,
which was the starting point for TOPS-20.

A decade before VMS came along...

-- 
Rich Alderson					  news at alderson.users.panix.com
      Audendum est, et veritas investiganda; quam etiamsi non assequamur,
	  omnino tamen proprius, quam nunc sumus, ad eam perveniemus.
									--Galen