Copy the KDE 3.5 branch to branches/trinity for new KDE 3.5 features.

BUG:215923


git-svn-id: svn://anonsvn.kde.org/home/kde/branches/trinity/kdeedu@1054174 283d02a7-25f6-0310-bc7c-ecb5cbfe19da

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+<sect1 id="ai-sidereal">
+<sect1info>
+<author>
+<firstname>Jason</firstname>
+<surname>Harris</surname>
+</author>
+</sect1info>
+<title>Sidereal Time</title>
+<indexterm><primary>Sidereal Time</primary>
+<seealso>Hour Angle</seealso>
+</indexterm>
+<para>
+<firstterm>Sidereal Time</firstterm> literally means <quote>star time</quote>.
+The time we are used to using in our everyday lives is Solar Time.  The
+fundamental unit of Solar Time is a <firstterm>Day</firstterm>: the time it
+takes the Sun to travel 360 degrees around the sky, due to the rotation of the
+Earth. Smaller units of Solar Time are just divisions of a Day:
+</para><para>
+<itemizedlist>
+<listitem><para>1/24 Day = 1 Hour</para></listitem>
+<listitem><para>1/60 Hour = 1 Minute</para></listitem>
+<listitem><para>1/60 Minute = 1 Second</para></listitem>
+</itemizedlist>
+</para><para>
+However, there is a problem with Solar Time.  The Earth does not actually
+spin around 360 degrees in one Solar Day.  The Earth is in orbit around the
+Sun, and over the course of one day, it moves about one Degree along its
+orbit (360 degrees/365.25 Days for a full orbit = about one Degree per
+Day).  So, in 24 hours, the direction toward the Sun changes by  about a
+Degree.  Therefore, the Earth has to spin 361 degrees to make
+the Sun look like it has traveled 360 degrees around the Sky.
+</para><para>
+In astronomy, we are concerned with how long it takes the Earth to spin
+with respect to the <quote>fixed</quote> stars, not the Sun.  So, we would like a
+timescale that removes the complication of Earth's orbit around the Sun,
+and just focuses on how long it takes the Earth to spin 360 degrees with
+respect to the stars.  This rotational period is called a <firstterm>Sidereal
+Day</firstterm>.  On average, it is 4 minutes shorter than a Solar Day, because
+of the extra 1 degree the Earth spins in a Solar Day.
+Rather than defining a Sidereal Day to be 23 hours, 56 minutes, we define
+Sidereal Hours, Minutes and Seconds that are the same fraction of a Day as
+their Solar counterparts.  Therefore, one Solar Second = 1.00278 Sidereal
+Seconds.
+</para><para>
+The Sidereal Time is useful for determining where the stars are at any
+given time.  Sidereal Time divides one full spin of the Earth into 24
+Sidereal Hours; similarly, the map of the sky is divided into 24 Hours
+of <firstterm>Right Ascension</firstterm>.  This is no
+coincidence; Local Sidereal Time (<acronym>LST</acronym>) indicates the Right
+Ascension on the sky that is currently crossing the <link
+linkend="ai-meridian">Local Meridian</link>.  So, if a star has a Right
+Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24. More
+generally, the difference between an object's <acronym>RA</acronym> and the Local
+Sidereal Time tells you how far from the Meridian the object is.  For example,
+the same object at LST=06:32:24 (one Sidereal Hour later), will be one Hour of
+Right Ascension west of your meridian, which is 15 degrees.  This angular
+distance from the meridian is called the object's <link
+linkend="ai-hourangle">Hour Angle</link>.
+</para>
+<tip>
+<para>
+The Local Sidereal Time is displayed by &kstars; in the <guilabel>Time Info
+Box</guilabel>, with the label <quote>ST</quote> (you have to
+<quote>unshade</quote> the box by double-clicking it in order to see the
+sidereal time).  Note that the changing sidereal seconds are not synchronized
+with the changing Local Time and Universal Time seconds. In fact, if you watch
+the clocks for a while, you will notice that the Sidereal seconds really are
+slightly shorter than the LT and UT seconds.
+</para><para>
+Point to the <link linkend="ai-zenith">Zenith</link> (press <keycap>Z</keycap>
+or select <guimenuitem>Zenith</guimenuitem> from the
+<guimenu>Pointing</guimenu>
+menu).  The Zenith is the point on the sky where you are looking <quote>straight
+up</quote> from the ground, and it is a point on your <link
+linkend="ai-meridian">Local Meridian</link>.  Note the Right Ascension of the
+Zenith: it is exactly the same as your Local Sidereal Time.
+</para>
+</tip>
+</sect1>
+