<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>Location</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>

