🚀 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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|            tempora mutantur, nos et mutamur inillis           |
|---------------------------------------------------------------|
|   roc...@decwrl.dec.com       {sun,uunet}saxony!madmax!max   |
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In article <23...@bacchus.dec.com> roc...@arktos.pa.dec.com
(Max Rochlin) writes:
+---------------
| 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. 
+---------------

Well, according to some old DEC literature (TOPS-20 manual?),
November 17, 1858, is "Day Zero" because that was the date of
the first astronomical photograph, at Harvard University(?).
DEC said that CDC also used that system.


-Rob


-----
Rob Warnock, MS-9U/510		rp...@wpd.sgi.com	rp...@pei.com
Silicon Graphics, Inc.		(415)335-1673		Protocol Engines, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA  94039-7311

kaji...@mcnc.org (Lord of Sith) writes:

>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.

Astronomers use a dating system that is merely a sequential count
of the days since 1 january 4 thousand and something BC. Apparently
a lot of astronomical cycles were more or less in synch on or about
that date. And it's far enough back that they don't need negative
numbers very often.

This count is referred to as the Julian Date. As near as I can
figure VMS starts it's clock on JD 2 400 000. There are *lots*
of algorithms for dealing with Julian Dates. So they cheated...
-- 
Leonard Erickson		...!tektronix!reed!percival!bucket!leonard
CIS: [70465,203]
"I'm all in favor of keeping dangerous weapons out of the hands of fools.
Let's start with typewriters." -- Solomon Short

> Question:  anyone know why the VMS system has a clock that starts about
> November 15 in the late 1800's?

Sure, that's when we converted to the Gregorian time table (I think
that was his name).

I believe that that was the time when we last had to fiddle
with the time, to keep it up with reality, so you can not
compute days and such stuff correctly before that time.
Actually I think it was only one hour that got lost in that
tweaking.

Most systems don't bother with times so long ago, but VMS
actually keeps track of how many tenths of microseconds that
has passed since that time. It is really quite a neat way
of keeping time, since the Clock routines just has to increment
one word all the time. However, making the conversion
routines to get the right date is a bit worse... I guess
that's why many software designers decide to do it another way.