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