[tz] [PROPOSED 1/2] Update IDL commentary

Paul Eggert eggert at cs.ucla.edu
Tue Jan 24 07:21:59 UTC 2023

 australasia | 36 +++++++++++++++---------------------
 1 file changed, 15 insertions(+), 21 deletions(-)

diff --git a/australasia b/australasia
index 749b72c8..84d94b99 100644
--- a/australasia
+++ b/australasia
@@ -2199,24 +2199,18 @@ Zone	Pacific/Efate	11:13:16 -	LMT	1912 Jan 13 # Vila
 # an international standard, there are some places on the high seas where the
 # correct date is ambiguous.
-# From Wikipedia <https://en.wikipedia.org/wiki/Time_zone> (2005-08-31):
-# Before 1920, all ships kept local apparent time on the high seas by setting
-# their clocks at night or at the morning sight so that, given the ship's
-# speed and direction, it would be 12 o'clock when the Sun crossed the ship's
-# meridian (12 o'clock = local apparent noon).  During 1917, at the
-# Anglo-French Conference on Time-keeping at Sea, it was recommended that all
-# ships, both military and civilian, should adopt hourly standard time zones
-# on the high seas.  Whenever a ship was within the territorial waters of any
-# nation it would use that nation's standard time.  The captain was permitted
-# to change his ship's clocks at a time of his choice following his ship's
-# entry into another zone time - he often chose midnight.  These zones were
-# adopted by all major fleets between 1920 and 1925 but not by many
-# independent merchant ships until World War II.
-# From Paul Eggert, using references suggested by Oscar van Vlijmen
-# (2005-03-20):
-# The American Practical Navigator (2002)
-# http://pollux.nss.nima.mil/pubs/pubs_j_apn_sections.html?rid=187
-# talks only about the 180-degree meridian with respect to ships in
-# international waters; it ignores the international date line.
+# From Wikipedia <https://en.wikipedia.org/wiki/Nautical_time> (2023-01-23):
+# The nautical time zone system is analogous to the terrestrial time zone
+# system for use on high seas.  Under the system time changes are required for
+# changes of longitude in one-hour steps.  The one-hour step corresponds to a
+# time zone width of 15° longitude.  The 15° gore that is offset from GMT or
+# UT1 (not UTC) by twelve hours is bisected by the nautical date line into two
+# 7°30' gores that differ from GMT by ±12 hours.  A nautical date line is
+# implied but not explicitly drawn on time zone maps.  It follows the 180th
+# meridian except where it is interrupted by territorial waters adjacent to
+# land, forming gaps: it is a pole-to-pole dashed line.
+# From Paul Eggert (2023-01-23):
+# The American Practical Navigator <https://msi.nga.mil/Publications/APN>,
+# 2019 edition, merely says that the International Date Line
+# "coincides with the 180th meridian over most of its length."

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