At 02:23 PM 8/15/00 -0400, [EMAIL PROTECTED] wrote:
>Modified Julian Day 0 thus started on 17 Nov 1858 (Gregorian) at 00:00:00
>UTC.
>(somebody threw that date out, It appears to be purely
>arbitrary rather than based on some celestial event)
Not arbitrary at all. From: http://www.kgb.com/calend.html
This information comes from the original (pre-Motif) DECwindows help file
which accompanied VMS version 5.3.
by Marios Cleovoulou
http://ourworld.compuserve.com/homepages/cleovoulou/
Copyright © 1988, 1989 by Digital Equipment Corporation.
>The Julian Period
>
>Astronomers use the Julian period because it is convenient to express
>long time intervals in days rather than months, weeks and years. It
>was devised by Joseph Scaliger, in 1582, who named it after his father
>Julius, thus creating the confusion between the Julian (Caesar)
>calendar and the Julian (Scaliger) period.
>
>Julian Day 1 began at 12:00 noon, January 1, 4713 BC. This date was
>thought by some to correspond approximately to the beginning of the
>universe. Certainly it predated any known astronomical events known in
>the 16th century without resorting to negative times. Scaliger decided
>on the actual date on the grounds that it was the most recent
>coincidence of three major chronological cycles:
>
>- The 28-year solar cycle, after which dates in the Julian calendar
>(for example September 27) return to the same days of the week (for
>example Tuesday).
>
>- The 19-year lunar cycle, after which phases of the moon return to
>the same dates of the year.
>
>- The 15-year indiction cycle, used in ancient Rome for tax regulation.
>
>It takes 7980 years to complete the cycle. Noon of January 1, 1988,
>marks the beginning of Julian Day 2447161.
>
>The Julian period is also of interest because of its use as a time
>base by the VMS operating system.
>
>VMS and the Julian Period or:
>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.
Dan
--------------------------------------"it's like this"-------------------
Dan Sugalski even samurai
[EMAIL PROTECTED] have teddy bears and even
teddy bears get drunk
