wyatt@cfa.harvard.EDU (Bill Wyatt) (11/06/87)
in article <513@mtxinu.UUCP>, tim@mtxinu.UUCP (Tim Wood) says: > Summary: julian date, and others > > The date format to which you referred is called "Julian date" (i.e. > November 12, 1987 at 1:30PM == 198711121330). I don't know why it's > called "Julian". This is a handy method of representing dates for > indexing, but is inconvenient for date arithmetic. [...] I've never heard of this format called Julian Date before, although it may be distinct from the Julian Day Nmber (JDN), which is also called (perhaps inaccurately) Julian Date. The standard internaltional meaning of Julian Day Number is the number of days since GMT noon on Jan 1, 4713 B.C. Thus, the JDN for Nov. 6th, GMT noon is 2,447,107.0. The zero point is set far back in time so that essentially all JDN's are positive. Lots of times people subtract off the 2,400,000 (giving dates back to the 1850's), and call it modified JDN or MJDN. Fewer digits to carry that way. -- Bill UUCP: {seismo|ihnp4}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: 17410::wyatt (this will change, sometime)
flee@gondor.psu.edu (Felix Lee) (11/07/87)
In article <763@cfa.cfa.harvard.EDU> wyatt@cfa.harvard.EDU (Bill Wyatt) writes: >in article <513@mtxinu.UUCP>, tim@mtxinu.UUCP (Tim Wood) says: >> The date format to which you referred is called "Julian date" (i.e. >> November 12, 1987 at 1:30PM == 198711121330). > >The standard internaltional meaning of Julian Day Number is the number >of days since GMT noon on Jan 1, 4713 B.C. Curious. I thought Julian date was YYDDD, as in 87304. What is the significance of 4713 BC? The Julian calendar was instituted 46 BC. 4713 BC sounds like the creation of the world as calculated from the Bible. But what does that have to do with Julius? -- Felix Lee flee@gondor.psu.edu {cbosgd,cmcl2}!psuvax1!gondor!flee
steve@mahendo.Jpl.Nasa.Gov (3171) (11/09/87)
In article <3060@psuvax1.psu.edu>, flee@gondor.psu.edu (Felix Lee) writes: > In article <763@cfa.cfa.harvard.EDU> wyatt@cfa.harvard.EDU (Bill Wyatt) writes: > >in article <513@mtxinu.UUCP>, tim@mtxinu.UUCP (Tim Wood) says: > >The standard internaltional meaning of Julian Day Number is the number > >of days since GMT noon on Jan 1, 4713 B.C. > What is the significance of 4713 BC? The Julian calendar was instituted > 46 BC. 4713 BC sounds like the creation of the world as calculated from > the Bible. But what does that have to do with Julius? The following information is taken from the Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (Crown copyright 1961, fourth impression 1977) page 431 paragraph 3: "The chronological measure of time introduced in the sixteenth century by Josephus Justus Scaliger, under the name of the Julian period in honour of his father, strictly is a chronological cycle but practically is a continuous era. It is a period of 7980 years, the least common multiple of the 28 year solar cycle, the 19 year lunar cycle and an ancient non-astronomical cycle of 15 years known as the cycle of the indiction. The epoch is the year when all three cycles began together, which was 4713 B.C., ...", Note that it was established in the sixteenth century, and has nothing to do with the Julian calendar or with some biblical accounting of the creation of the world.
philm@astroatc.UUCP (Phil Mason) (11/10/87)
In article <116@mahendo.Jpl.Nasa.Gov> steve@mahendo.Jpl.Nasa.Gov (3171) writes: > >The following information is taken from the Explanatory Supplement to the >Astronomical Ephemeris and the American Ephemeris and Nautical Almanac >(Crown copyright 1961, fourth impression 1977) page 431 paragraph 3: > >Note that it was established in the sixteenth century, and has nothing to do >with the Julian calendar or with some biblical accounting of the creation of >the world. Ooops . . . I guess I stand corrected. It *is* true, however, that Julius Caesar did form what became (after a few revisions) our modern Western calendar. Most cultures and religons have (or had) their own calendar systems. The whole concept of 'eras' and basing your dates in the future on past events is well documented, but I guess it doesn't apply here. Thanks for setting the record straight. -- Kirk : Bones ? | Phil Mason, Astronautics Technology Center Bones : He's dead Jim. | Madison, Wisconsin - "Eat Cheese or Die!" ...seismo-uwvax-astroatc!philm | I would really like to believe that my ...ihnp4-nicmad/ | employer shares all my opinions, but . . .