[Emacs-diffs] Changes to emacs/doc/misc/calc.texi,v

[Glenn Morris]

CVSROOT:        /sources/emacs
Module name:    emacs
Changes by:     Glenn Morris <gm>       07/09/06 05:10:13

Index: calc.texi
===================================================================
RCS file: /sources/emacs/emacs/doc/misc/calc.texi,v
retrieving revision 1.1
retrieving revision 1.2
diff -u -b -r1.1 -r1.2
--- calc.texi   6 Sep 2007 04:58:46 -0000       1.1
+++ calc.texi   6 Sep 2007 05:10:12 -0000       1.2
@@ -1,7 +1,7 @@
 \input texinfo                  @c -*-texinfo-*-
 @comment %**start of header (This is for running Texinfo on a region.)
 @c smallbook
address@hidden ../info/calc
address@hidden ../../info/calc
 @c [title]
 @settitle GNU Emacs Calc 2.1 Manual
 @setchapternewpage odd
@@ -11053,44 +11053,17 @@
 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.''  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) 
+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 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 
address@hidden @math{15\times 19\times 28 = 7980} years.
address@hidden 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.
+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.
 
 @cindex Unix time format
 The Unix operating system measures time as an integer number of
@@ -16683,9 +16656,9 @@
 @cindex Julian day counts, conversions
 The @kbd{t J} (@code{calc-julian}) address@hidden command converts
 a date form into a Julian day count, which is the number of days
-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
+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
 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