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Improve documentation about Julian dates; in particular, point out the

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difference between Julian and Gregorian reckoning of when JD 0 was.
This commit is contained in:
Tom Lane 2007-12-15 01:18:34 +00:00
parent 21e07f6c94
commit 164d255b49
2 changed files with 44 additions and 30 deletions
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70
doc/src/sgml/datetime.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/datetime.sgml,v 2.57 2007/12/02 22:33:20 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/datetime.sgml,v 2.58 2007/12/15 01:18:33 tgl Exp $ -->
<appendix id="datetime-appendix">
<title>Date/Time Support</title>
@ -470,20 +470,8 @@
<title>History of Units</title>
<para>
The Julian Date 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).
Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Date
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.
</para>
<para>
The <quote>Julian Date</quote> is different from the <quote>Julian
Calendar</quote>. The Julian calendar
was introduced by Julius Caesar in 45 BC. It was in common use
The Julian calendar was introduced by Julius Caesar in 45 BC.
It was in common use in the Western world
until the year 1582, when countries started changing to the Gregorian
calendar. In the Julian calendar, the tropical year is
approximated as 365 1/4 days = 365.25 days. This gives an error of
@ -529,7 +517,7 @@
4 October.
This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
countries followed shortly after, but Protestant countries were
reluctant to change, and the Greek orthodox countries didn't change
reluctant to change, and the Greek Orthodox countries didn't change
until the start of the 20th century.
The reform was observed by Great Britain and Dominions (including what is
@ -549,17 +537,17 @@ $ <userinput>cal 9 1752</userinput>
</screen>
</para>
<note>
<para>
The SQL standard states that <quote>Within the definition of a
<quote>datetime literal</quote>, the <quote>datetime
value</quote>s are constrained by the natural rules for dates and
times according to the Gregorian calendar</quote>. Dates between
1752-09-03 and 1752-09-13, although eliminated in some countries
by Papal fiat, conform to <quote>natural rules</quote> and are
hence valid dates.
</para>
</note>
<para>
The SQL standard states that <quote>Within the definition of a
<quote>datetime literal</quote>, the <quote>datetime
value</quote>s are constrained by the natural rules for dates and
times according to the Gregorian calendar</quote>. Dates between
1582-10-05 and 1582-10-14, although eliminated in some countries
by Papal fiat, conform to <quote>natural rules</quote> and are
hence valid dates. <productname>PostgreSQL</> follows the SQL
standard's lead by counting dates exclusively in the Gregorian
calendar, even for years before that calendar was in use.
</para>
<para>
Different calendars have been developed in various parts of the
@ -567,12 +555,38 @@ $ <userinput>cal 9 1752</userinput>
For example,
the beginnings of the Chinese calendar can be traced back to the 14th
century BC. Legend has it that the Emperor Huangdi invented the
century BC. Legend has it that the Emperor Huangdi invented that
calendar in 2637 BC.
The People's Republic of China uses the Gregorian calendar
for civil purposes. The Chinese calendar is used for determining
festivals.
</para>
<para>
The <quote>Julian Date</quote> is unrelated to the <quote>Julian
calendar</quote>.
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).
In the Julian Date system, each day has a sequential number, starting
from JD 0 (which is sometimes called <emphasis>the</> Julian Date).
JD 0 corresponds to 1 January 4713 BC in the Julian calendar, or
24 November 4714 BC in the Gregorian calendar. Julian Date counting
is most often used by astronomers for labeling their nightly observations,
and therefore a date runs from noon UTC to the next noon UTC, rather than
from midnight to midnight: JD 0 designates the 24 hours from noon UTC on
1 January 4713 BC to noon UTC on 2 January 4713 BC.
</para>
<para>
Although <productname>PostgreSQL</> supports Julian Date notation for
input and output of dates (and also uses them for some internal datetime
calculations), it does not observe the nicety of having dates run from
noon to noon. <productname>PostgreSQL</> treats a Julian Date as running
from midnight to midnight.
</para>
</sect1>
</appendix>

4
doc/src/sgml/func.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.417 2007/12/13 13:22:05 alvherre Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.418 2007/12/15 01:18:34 tgl Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
@ -4960,7 +4960,7 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
</row>
<row>
<entry><literal>J</literal></entry>
<entry>Julian Day (days since January 1, 4712 BC at midnight)</entry>
<entry>Julian Day (days since November 24, 4714 BC at midnight)</entry>
</row>
<row>
<entry><literal>Q</literal></entry>