Prosit Neujahr, OpenVMS
17. November 2017 - Prosit Neujahr, OpenVMS!
Wie viele von Ihnen, die sich schon seit Jahren mit OpenVMS beschäftigen, wissen, ist der 17. November ein spezielles Datum im OpenVMS Jahreszyklus. Wir erlauben uns daher, Ihnen einen Artikel von vmssoftware.com (VSI) anzubieten, der sich mit diesem besonderen Tag beschäftigt:
Folks who spend, or have spent, a lot of time working with, in and around VMS probably know but, HAPPY NEW YEAR! That's right! According to the internal VMS calendar today, November 17, is start of the new year. Doing math (courtesy of an email forwarded to me from 2008 originally written by Andy Goldstein) this is year 158 of about 25,000 at which point the time format will run out. Although by then I'm sure our engineers will have come up with a solution. Want to know why VMS is adamant that November 17, 1858 is the beginning of time? Keep reading! (Thanks to Matt Booth for send all this good stuff out!)
Why OpenVMS 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.
Quelle: vmssoftware, inc. Facebook Page