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0001 <sect1 id="ai-sidereal">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Sidereal Time</title>
0009 <indexterm><primary>Sidereal Time</primary>
0010 <seealso>Hour Angle</seealso>
0011 </indexterm>
0012 <para>
0013 <firstterm>Sidereal Time</firstterm> literally means <quote>star time</quote>.
0014 The time we are used to using in our everyday lives is Solar Time.  The
0015 fundamental unit of Solar Time is a <firstterm>Day</firstterm>: the time it
0016 takes the Sun to travel 360 degrees around the sky, due to the rotation of the
0017 Earth. Smaller units of Solar Time are just divisions of a Day:
0018 </para><para>
0019 <itemizedlist>
0020 <listitem><para>1/24 Day = 1 Hour</para></listitem>
0021 <listitem><para>1/60 Hour = 1 Minute</para></listitem>
0022 <listitem><para>1/60 Minute = 1 Second</para></listitem>
0023 </itemizedlist>
0024 </para><para>
0025 However, there is a problem with Solar Time.  The Earth does not actually
0026 spin around 360 degrees in one Solar Day.  The Earth is in orbit around the
0027 Sun, and over the course of one day, it moves about one Degree along its
0028 orbit (360 degrees/365.25 Days for a full orbit = about one Degree per
0029 Day).  So, in 24 hours, the direction toward the Sun changes by  about a
0030 Degree.  Therefore, the Earth has to spin 361 degrees to make
0031 the Sun look like it has traveled 360 degrees around the Sky.
0032 </para><para>
0033 In astronomy, we are concerned with how long it takes the Earth to spin
0034 with respect to the <quote>fixed</quote> stars, not the Sun.  So, we would like a
0035 timescale that removes the complication of Earth's orbit around the Sun,
0036 and just focuses on how long it takes the Earth to spin 360 degrees with
0037 respect to the stars.  This rotational period is called a <firstterm>Sidereal
0038 Day</firstterm>.  On average, it is 4 minutes shorter than a Solar Day, because
0039 of the extra 1 degree the Earth spins in a Solar Day.
0040 Rather than defining a Sidereal Day to be 23 hours, 56 minutes, we define
0041 Sidereal Hours, Minutes and Seconds that are the same fraction of a Day as
0042 their Solar counterparts.  Therefore, one Solar Second = 1.00278 Sidereal
0043 Seconds.
0044 </para><para>
0045 The Sidereal Time is useful for determining where the stars are at any
0046 given time.  Sidereal Time divides one full spin of the Earth into 24
0047 Sidereal Hours; similarly, the map of the sky is divided into 24 Hours
0048 of <firstterm>Right Ascension</firstterm>.  This is no
0049 coincidence; Local Sidereal Time (<acronym>LST</acronym>) indicates the Right
0050 Ascension on the sky that is currently crossing the <link
0051 linkend="ai-meridian">Local Meridian</link>.  So, if a star has a Right
0052 Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24. More
0053 generally, the difference between an object's <acronym>RA</acronym> and the Local
0054 Sidereal Time tells you how far from the Meridian the object is.  For example,
0055 the same object at LST=06:32:24 (one Sidereal Hour later), will be one Hour of
0056 Right Ascension west of your meridian, which is 15 degrees.  This angular
0057 distance from the meridian is called the object's <link
0058 linkend="ai-hourangle">Hour Angle</link>.
0059 </para>
0060 <tip>
0061 <para>
0062 The Local Sidereal Time is displayed by &kstars; in the <guilabel>Time Info
0063 Box</guilabel>, with the label <quote>ST</quote> (you have to
0064 <quote>unshade</quote> the box by double-clicking it in order to see the
0065 sidereal time).  Note that the changing sidereal seconds are not synchronized
0066 with the changing Local Time and Universal Time seconds. In fact, if you watch
0067 the clocks for a while, you will notice that the Sidereal seconds really are
0068 slightly shorter than the LT and UT seconds.
0069 </para><para>
0070 Point to the <link linkend="ai-zenith">Zenith</link> (press <keycap>Z</keycap>
0071 or select the <menuchoice><guimenu>Pointing</guimenu>
0072 <guimenuitem>Zenith</guimenuitem></menuchoice> menu item). The Zenith is the point
0073 on the sky where you are looking <quote>straight
0074 up</quote> from the ground, and it is a point on your <link
0075 linkend="ai-meridian">Local Meridian</link>.  Note the Right Ascension of the
0076 Zenith: it is exactly the same as your Local Sidereal Time.
0077 </para>
0078 </tip>
0079 </sect1>