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diff --git a/doc/kstars/sidereal.docbook b/doc/kstars/sidereal.docbook new file mode 100644 index 00000000..a60ab44d --- /dev/null +++ b/doc/kstars/sidereal.docbook @@ -0,0 +1,80 @@ +<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> + |