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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> 0080