From 7a5e3e973f65c392ef86be21272fda2f4a0d1c67 Mon Sep 17 00:00:00 2001 From: Jay Belanger Date: Sat, 1 Sep 2007 04:13:52 +0000 Subject: [PATCH] (Date Conversions): Clarify definition of Julian day. (Date Forms): Clarify definition of Julian date; add some history. --- man/ChangeLog | 7 +++++++ man/calc.texi | 55 ++++++++++++++++++++++++++++++++++++++------------- 2 files changed, 48 insertions(+), 14 deletions(-) diff --git a/man/ChangeLog b/man/ChangeLog index 245b4c7e96f..8e58d905937 100644 --- a/man/ChangeLog +++ b/man/ChangeLog @@ -1,3 +1,10 @@ +2007-09-01 Jay Belanger + + * calc.texi (Date Conversions): Clarify definition of + Julian day numbering. + (Date Forms): Clarify definition of Julian day numbering; + add some history. + 2007-08-30 Carsten Dominik * org.texi: Version 5.07 diff --git a/man/calc.texi b/man/calc.texi index 194d36d8f5f..93c7123d6a4 100644 --- a/man/calc.texi +++ b/man/calc.texi @@ -11053,17 +11053,44 @@ Please note that there is no ``year 0''; the day before days 0 and @mathit{-1} respectively in Calc's internal numbering scheme. @cindex Julian day counting -Another day counting system in common use is, confusingly, also -called ``Julian.'' It was invented in 1583 by Joseph Justus -Scaliger, who named it in honor of his father Julius Caesar -Scaliger. For obscure reasons he chose to start his day -numbering on Jan 1, 4713 BC at noon, which in Calc's scheme -is @mathit{-1721423.5} (recall that Calc starts at midnight instead -of noon). Thus to convert a Calc date code obtained by -unpacking a date form into a Julian day number, simply add -1721423.5. The Julian code for @samp{6:00am Jan 9, 1991} -is 2448265.75. The built-in @kbd{t J} command performs -this conversion for you. +Another day counting system in common use is, confusingly, also called +``Julian.'' The Julian day number is the numbers of days since +12:00 noon (GMT) on Jan 1, 4713 BC, which in Calc's scheme (in GMT) +is @mathit{-1721423.5} (recall that Calc starts at midnight instead +of noon). Thus to convert a Calc date code obtained by unpacking a +date form into a Julian day number, simply add 1721423.5 after +compensating for the time zone difference. The built-in @kbd{t J} +command performs this conversion for you. + +The Julian day number is based on the Julian cycle, which was invented +in 1583 by Joseph Justus Scaliger. Scaliger named it the Julian cycle +since it is involves the Julian calendar, but some have suggested that +Scaliger named it in honor of his father, Julius Caesar Scaliger. The +Julian cycle is based it on three other cycles: the indiction cycle, +the Metonic cycle, and the solar cycle. The indiction cycle is a 15 +year cycle originally used by the Romans for tax purposes but later +used to date medieval documents. The Metonic cycle is a 19 year +cycle; 19 years is close to being a common multiple of a solar year +and a lunar month, and so every 19 years the phases of the moon will +occur on the same days of the year. The solar cycle is a 28 year +cycle; the Julian calendar repeats itself every 28 years. The +smallest time period which contains multiples of all three cycles is +the least common multiple of 15 years, 19 years and 28 years, which +(since they're pairwise relatively prime) is +@texline @math{15\times 19\times 28 = 7980} years. +@infoline 15*19*28 = 7980 years. +This is the length of a Julian cycle. Working backwards, the previous +year in which all three cycles began was 4713 BC, and so Scalinger +chose that year as the beginning of a Julian cycle. Since at the time +there were no historical records from before 4713 BC, using this year +as a starting point had the advantage of avoiding negative year +numbers. In 1849, the astronomer John Herschel (son of William +Herschel) suggested using the number of days since the beginning of +the Julian cycle as an astronomical dating system; this idea was taken +up by other astronomers. (At the time, noon was the start of the +astronomical day. Herschel originally suggested counting the days +since Jan 1, 4713 BC at noon Alexandria time; this was later amended to +noon GMT.) Julian day numbering is largely used in astronomy. @cindex Unix time format The Unix operating system measures time as an integer number of @@ -16656,9 +16683,9 @@ The last two arguments default to zero if omitted. @cindex Julian day counts, conversions The @kbd{t J} (@code{calc-julian}) [@code{julian}] command converts a date form into a Julian day count, which is the number of days -since noon on Jan 1, 4713 BC. A pure date is converted to an integer -Julian count representing noon of that day. A date/time form is -converted to an exact floating-point Julian count, adjusted to +since noon (GMT) on Jan 1, 4713 BC. A pure date is converted to an +integer Julian count representing noon of that day. A date/time form +is converted to an exact floating-point Julian count, adjusted to interpret the date form in the current time zone but the Julian day count in Greenwich Mean Time. A numeric prefix argument allows you to specify the time zone; @pxref{Time Zones}. Use a prefix of -- 2.39.5