Article 1295 of alt.sys.pdp10: Path: nntp1.ba.best.com!inwap From: inwap@best.com (Joe Smith) Newsgroups: alt.sys.pdp10 Subject: Re: PDP10 screen log with 'SET W V F'. Date: 23 Sep 2000 08:51:24 GMT Organization: Chez Inwap Lines: 129 Message-ID: <8qhquc$19qd$1@nntp1.ba.best.com> References: <8q7k8s$cmk$1@bob.news.rcn.net> <8q9fuc$2u70$1@nntp1.ba.best.com> NNTP-Posting-Host: shell3.ba.best.com X-Trace: nntp1.ba.best.com 969699084 42829 206.184.139.134 (23 Sep 2000 08:51:24 GMT) X-Complaints-To: abuse@best.com NNTP-Posting-Date: 23 Sep 2000 08:51:24 GMT Xref: nntp1.ba.best.com alt.sys.pdp10:1295 In article , Richard M. Alderson III wrote: >In article <8q9fuc$2u70$1@nntp1.ba.best.com> inwap@best.com (Joe Smith) writes: > >> The previous table entry %CNDTM [53..11] has the date/time in a format that >> is not affected by Y2K. (Days since 15-Nov-1858.) > >Should that be 17 Nov 1858? Tops-20 counts time since midnight GMT on that >date--time 0 is 16:00 16-Nov-1858 PST. Yep, I misremembered. I found a web page that describes why that particular date was chosen. http://www.magnet.ch/serendipity/hermetic/cal_stud/jdn.htm describes how astronomers decided that 17-Nov-1858 is day 0 for the Modified Julian Day Number. (They subtracted 2,400,000 from the full Julian Day Number, allowing dates up to the year 2130 can be specified as a 5 digit number, instead of 7 digits.) -Joe Julian Day Numbers by Peter Meyer HISTORY The Julian Day number system was invented by Joseph Justus Scaliger (born 1540-08-05 J in Agen, France, died 1609-01-21 J in Leiden, Holland), who during his life immersed himself in Greek, Latin, Persian and Jewish literature. His invention (in 1583) of the Julian day number system is considered by some the foundation of the science of chronology. Although the term Julian Calendar derives from the name of Julius Caesar, the term Julian day number probably does not. Most say that this system was named, not after Julius Caesar, but after its inventor's father, Julius Caesar Scaliger (1484-1558). Perhaps it was simply named after the Julian Calendar. The younger Scaliger combined three traditionally recognized temporal cycles of 28, 19 and 15 years to obtain a great cycle, the Scaliger cycle, or Julian period, of 7980 years (7980 is the least common multiple of 28, 19 and 15). According to the Encyclopedia Brittanica: The length of 7,980 years was chosen as the product of 28 times 19 times 15; these, respectively, are the numbers of years in the so-called solar cycle of the Julian calendar in which dates recur on the same days of the week; the lunar or Metonic cycle, after which the phases of the Moon recur on a particular day in the solar year, or year of the seasons; and the cycle of indiction, originally a schedule of periodic taxes or government requisitions in ancient Rome. The first Scaliger cycle began with Year 1 on -4712-01-01 (Julian) and will end after 7980 years on 3267-12-31 (Julian), which is 3268-01-22 (Gregorian). 3268-01-01 J is the first day of Year 1 of the next Scaliger cycle. _______________________________________________________________ ASTRONOMICAL SYTEM Little mention seems to be made as to whether Joseph Scaliger regarded -4712-01-01 J as day 0 or as day 1 in the first Julian period. Astronomers adopted this system and adapted it to their own purposes, and they took noon GMT -4712-01-01 as their zero point. For astronomers a day begins at noon and runs until the next noon (so that the nighttime falls conveniently within one "day"). Thus they defined the Julian day number of a day as the number of days (or part of a day) elapsed since noon GMT (or more exactly, UTC) on January 1st, 4713 B.C., in the Proleptic Julian Calendar. Thus the Julian day number of noon GMT on -4712-01-01 (Julian), or more casually, the Julian day number of -4712-01-01 itself, is 0. (Note that 4713 B.C. is the year -4712 according to the astronomical year numbering.) The Julian day number of 1996-03-31 is 2,450,174 -- meaning that on 1996-03-31 2,450,174 days had elapsed since -4712-01-01 (or more exactly, that at noon on 1996-03-31 2,450,174 days had elapsed since noon on -4712-01-01). Scaliger preceded the astronomers in introducing the notion of decimal times, designating midnight as .00, 6 a.m. as .25, midday as .50 and 6 p.m. as .75, thus allowing easier calculation involving dates and times. Astronomers, as noted above, preferred to use .00 to mean midday and .50 to mean midnight. _______________________________________________________________ MODIFIED JULIAN DAY NUMBER This was not to the liking of all scholars using the Julian day number system, in particular, historians. For chronologists who start "days" at midnight, the zero point for the Julian day number system is 00:00 at the start of -4712-01-01 J, and this is day 0. This means that 2000-01-01 G is 2,451,545 JD. Since most days within about 150 years of the present have Julian day numbers beginning with "24", Julian day numbers within this 300-odd-year period can be abbreviated. In 1975 the convention of the modified Julian day number was adopted: Given a Julian day number JD, the modified Julian day number MJD is defined as MJD = JD - 2,400,000.5. This has two purposes: 1. Days begin at midnight rather than noon. 2. For dates in the period from 1859 to about 2130 only five digits need to be used to specify the date rather than seven. MJD 0 thus corresponds to JD 2,400,000.5, which is twelve hours after noon on JD 2,400,000 = 1858-11-16. Thus MJD 0 designates the midnight of November 16th/17th, 1858, so day 0 in the system of modified Julian day numbers is the day 1858-11-17. -- See http://www.inwap.com/ for PDP-10 and "ReBoot" pages.