[PATCH] doc: correct history of Julian dates

Julian dates as a count of days were introduced into astronomy by
Herschel; Scaliger's Julian Period was a way to number years, not
days.

Explain how the Julian period was derived and used by Scaliger,
partly because it's interesting, and partly to elucidate how it
relates to the Julian calendar.

Expunge a widespread error regarding the choice of name: Scaliger
wrote in 1582, "Iulianam vocavimus: quia ad annum Iulianum
duntaxat accomodata est" - "I have called the period Julian
because it is laid out for the Julian year only." (The Gregorian
reform was the same year.)

https://www.researchgate.net/publication/234215919_The_Julian_and_Modified_Julian_Dates
---
 doc/src/sgml/datetime.sgml | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml
index 5905f5fa550..94a123193f5 100644
--- a/doc/src/sgml/datetime.sgml
+++ b/doc/src/sgml/datetime.sgml
@@ -908,13 +908,32 @@ $ <userinput>cal 9 1752</userinput>

   <para>
    The <firstterm>Julian Date</firstterm> system is a method for
-   numbering days.  It is
-   unrelated to the Julian calendar, though it is confusingly
-   named similarly to that calendar.
-   The Julian Date system was invented by the French scholar
-   Joseph Justus Scaliger (1540–1609)
-   and probably takes its name from Scaliger's father,
-   the Italian scholar Julius Caesar Scaliger (1484–1558).
+   numbering days developed by the astronomer John Herschel
+   (1792–1871) in his Outlines of Astronomy (1849). It is based
+   on the Julian Period of 7980 years proposed by the French scholar
+   Joseph Justus Scaliger (1540–1609). The Julian period is
+   derived from three cycles in the Julian calendar:
+
+   <simplelist>
+    <member>
+     the solar cycle of 28 years, arising from the Julian calendar's 4
+     year cycle of leap years and its 7 day week;
+    </member>
+    <member>
+     the lunar or Metonic cycle of 19 years, which is almost the same
+     as 235 lunar months, and used for calculating the date of Easter;
+    </member>
+    <member>
+     and the indiction cycle of 15 years, which was a taxation period
+     in the Roman and Byzantine empires, frequently used when dating
+     documents.
+    </member>
+   </simplelist>
+
+   Scaliger used the cycles of the Julian period to disambiguate the
+   years of events when records were unclear; astronomers used the
+   chronologies of scholars such as Scaliger to date historical
+   observations.
   </para>

   <para>
-- 
2.49.0