[Kstars-devel] [kstars] doc: Adding documentation on Epochs in astronomy to the AstroInfo project.

[Akarsh Simha]

Git commit 9f5054acdc6af2a30c43b47c222897080663f983 by Akarsh Simha.
Committed on 30/08/2012 at 08:12.
Pushed by asimha into branch 'master'.

Adding documentation on Epochs in astronomy to the AstroInfo project.

CCMAIL: kstars-devel at kde.org, kde-i18n-doc at kde.org

M  +1    -0    doc/ai-contents.docbook
M  +1    -0    doc/astroinfo.docbook
A  +35   -0    doc/epoch.docbook
M  +1    -0    doc/index.docbook
M  +1    -1    doc/julianday.docbook
M  +20   -0    doc/skycoords.docbook

http://commits.kde.org/kstars/9f5054acdc6af2a30c43b47c222897080663f983

diff --git a/doc/ai-contents.docbook b/doc/ai-contents.docbook
index f19194d..b44eb00 100644
--- a/doc/ai-contents.docbook
+++ b/doc/ai-contents.docbook
@@ -18,6 +18,7 @@
 </itemizedlist>
 
 <itemizedlist><title>Time</title>
+  <listitem><para><link linkend="ai-epoch">Epoch</link></para></listitem>
   <listitem><para><link linkend="ai-julianday">Julian Day</link></para></listitem>
   <listitem><para><link linkend="ai-leapyear">Leap Years</link></para></listitem>
   <listitem><para><link linkend="ai-sidereal">Sidereal Time</link></para></listitem>
diff --git a/doc/astroinfo.docbook b/doc/astroinfo.docbook
index 05a2d05..c7f4c8f 100644
--- a/doc/astroinfo.docbook
+++ b/doc/astroinfo.docbook
@@ -26,6 +26,7 @@ The articles sometimes also contain exercises that you can perform with
 &precession;   <!--AstroInfo: Precession-->
 &zenith;       <!--AstroInfo: The Zenith-->
 
+&epoch;        <!--AstroInfo: Epoch-->
 &julianday;    <!--AstroInfo: Julian Day-->
 &leapyear;     <!--AstroInfo: Leap Year-->
 &sidereal;     <!--AstroInfo: Sidereal Time-->
diff --git a/doc/epoch.docbook b/doc/epoch.docbook
new file mode 100644
index 0000000..2e23f8f
--- /dev/null
+++ b/doc/epoch.docbook
@@ -0,0 +1,35 @@
+<sect1 id="ai-epoch">
+<sect1info>
+<author>
+<firstname>Akarsh</firstname>
+<surname>Simha</surname>
+</author>
+</sect1info>
+<title>Epoch</title>
+<indexterm><primary>Epoch</primary></indexterm>
+<para>
+An epoch is a certain instant of time (usually past) used as a
+reference for time measurement in astronomy. In order to make
+calculations easier, astronomers usually use the number of days that
+have passed since a certain moment in time as a way of representing
+time in astronomical calculations. This arbitrarily chosen "starting
+point" reference for time is called an epoch.
+</para>
+<para>
+An epoch is usually cited along
+with <link linkend="equatorial">equatorial
+coordinates</link> in astronomical catalogs, since the equatorial
+coordinates of sky objects change (although slowly) with time. Printed
+star charts / atlases are usually marked as being valid for a certain
+epoch.
+</para>
+<para>
+The most popular epoch used currently is J2000.0,
+(the <link linkend="ai-julianday">Julian year</link> 2000.0), which
+(almost exactly) corresponds to 1st January 2000, 12:00 noon
+UTC. Other epochs include B1900 and B1950.
+</para>
+<para>
+KStars supports catalogs based on J2000.0 and B1950.0 epochs.
+</para>
+</sect1>
diff --git a/doc/index.docbook b/doc/index.docbook
index cb0cd3c..495937d 100644
--- a/doc/index.docbook
+++ b/doc/index.docbook
@@ -28,6 +28,7 @@
   <!ENTITY dumpmode SYSTEM "dumpmode.docbook">
   <!ENTITY ecliptic SYSTEM "ecliptic.docbook">
   <!ENTITY ellipgal SYSTEM "ellipticalgalaxies.docbook">
+  <!ENTITY epoch SYSTEM "epoch.docbook">
   <!ENTITY equinox SYSTEM "equinox.docbook">
   <!ENTITY faq SYSTEM "faq.docbook">
   <!ENTITY flux SYSTEM "flux.docbook">
diff --git a/doc/julianday.docbook b/doc/julianday.docbook
index 03ae53c..10ad7d7 100644
--- a/doc/julianday.docbook
+++ b/doc/julianday.docbook
@@ -46,7 +46,7 @@ hours since noon, and 3/24 = 0.125 day).  Note that the Julian Day is always
 determined from <link linkend="ai-utime">Universal Time</link>, not Local Time.
 </para><para>
 Astronomers use certain Julian Day values as important reference points, called
-<firstterm>Epochs</firstterm>.  One widely-used epoch is called J2000; it is the
+<link linkend="ai-epoch">Epochs</link>.  One widely-used epoch is called J2000; it is the
 Julian Day for 1 Jan, 2000 at 12:00 noon = <abbrev>JD</abbrev> 2451545.0.
 </para><para>
 Much more information on Julian Days is available on the internet.  A good
diff --git a/doc/skycoords.docbook b/doc/skycoords.docbook
index 1017be0..bd8c40f 100644
--- a/doc/skycoords.docbook
+++ b/doc/skycoords.docbook
@@ -67,6 +67,26 @@ linkend="ai-hourangle">Hour Angle</link>.  Since a full rotation of the sky
 takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in
 one Hour of Right Ascension.
 </para>
+<para>
+The equatorial coordinates for deep-sky objects and stars do not vary
+appreciably over short durations of time, since they are not affected
+by the <firstterm>diurnal motion</firstterm> (the daily apparent
+rotation of the sky around the earth. However, note that this takes
+<link linkend="ai-sidereal">1 sidereal day</link>, as against 1 solar
+day). They are suitable coordinates for making catalogs of stars and
+deep-sky objects (note that <firstterm>Galactic
+Coordinates</firstterm> also work well, but are cumbersome to use from
+an earth point-of-view). However, there are effects that cause the RA
+and Dec of objects to vary over time,
+namely <link linkend="ai-precession">Precession</link>
+and <firstterm>nutation</firstterm>, and <firstterm>proper
+motion</firstterm>, the latter being even less important. Equatorial
+coordinates are thus generally specified with an
+appropriate <link linkend="ai-epoch">epoch</link>, to account for
+precession. Popular epochs include J2000.0
+(<link linkend="ai-julianday">Julian Year</link> 2000) and B1950.0
+(<firstterm>Besselian Year</firstterm> 1950).
+</para>
 </sect2>
 
 <sect2 id="horizontal">