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
@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
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