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diff --git a/doc/kstars/skycoords.docbook b/doc/kstars/skycoords.docbook new file mode 100644 index 00000000..1017be06 --- /dev/null +++ b/doc/kstars/skycoords.docbook @@ -0,0 +1,152 @@ +<sect1 id="ai-skycoords"> +<sect1info> +<author> +<firstname>Jason</firstname> +<surname>Harris</surname> +</author> +</sect1info> +<title>Celestial Coordinate Systems</title> +<para> +<indexterm><primary>Celestial Coordinate Systems</primary> +<secondary>Overview</secondary></indexterm> +A basic requirement for studying the heavens is determining where in the +sky things are. To specify sky positions, astronomers have developed +several <firstterm>coordinate systems</firstterm>. Each uses a coordinate grid +projected on the <link linkend="ai-csphere">Celestial Sphere</link>, in +analogy to the <link linkend="ai-geocoords">Geographic coordinate +system</link> used on the surface of the Earth. The coordinate systems +differ only in their choice of the <firstterm>fundamental plane</firstterm>, +which divides the sky into two equal hemispheres along a <link +linkend="ai-greatcircle">great circle</link>. (the fundamental plane of the +geographic system is the Earth's equator). Each coordinate system is named for +its choice of fundamental plane. +</para> + +<sect2 id="equatorial"> +<title>The Equatorial Coordinate System</title> +<indexterm><primary>Celestial Coordinate Systems</primary> +<secondary>Equatorial Coordinates</secondary> +<seealso>Celestial Equator</seealso> +<seealso>Celestial Poles</seealso> +<seealso>Geographic Coordinate System</seealso> +</indexterm> +<indexterm><primary>Right Ascension</primary><see>Equatorial Coordinates</see></indexterm> +<indexterm><primary>Declination</primary><see>Equatorial Coordinates</see></indexterm> + +<para> +The <firstterm>Equatorial coordinate system</firstterm> is probably the most +widely used celestial coordinate system. It is also the most closely related +to the <link linkend="ai-geocoords">Geographic coordinate system</link>, because +they use the same fundamental plane, and the same poles. The projection of the +Earth's equator onto the celestial sphere is called the +<link linkend="ai-cequator">Celestial Equator</link>. +Similarly, projecting the geographic Poles onto the celestial sphere defines the +North and South <link linkend="ai-cpoles">Celestial Poles</link>. +</para><para> +However, there is an important difference between the equatorial and +geographic coordinate systems: the geographic system is fixed to the +Earth; it rotates as the Earth does. The Equatorial system is +fixed to the stars<footnote id="fn-precess"><para>actually, the equatorial +coordinates are not quite fixed to the stars. See <link +linkend="ai-precession">precession</link>. Also, if <link +linkend="ai-hourangle">Hour Angle</link> is used in place of Right +Ascension, then the Equatorial system is fixed to the Earth, not to the +stars.</para></footnote>, so it appears to rotate across the sky with the stars, +but of course it is really the Earth rotating under the fixed sky. +</para><para> +The <firstterm>latitudinal</firstterm> (latitude-like) angle of the Equatorial +system is called <firstterm>Declination</firstterm> (Dec for short). It +measures the angle of an object above or below the Celestial Equator. The +<firstterm>longitudinal</firstterm> angle is called the <firstterm>Right +Ascension</firstterm> (<acronym>RA</acronym> for short). It measures the angle of an object East +of the <link linkend="ai-equinox">Vernal Equinox</link>. Unlike longitude, +Right Ascension is usually measured in hours instead of degrees, because the +apparent rotation of the Equatorial coordinate system is closely related to +<link linkend="ai-sidereal">Sidereal Time</link> and <link +linkend="ai-hourangle">Hour Angle</link>. Since a full rotation of the sky +takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in +one Hour of Right Ascension. +</para> +</sect2> + +<sect2 id="horizontal"> +<title>The Horizontal Coordinate System</title> + +<indexterm><primary>Celestial Coordinate Systems</primary> +<secondary>Horizontal Coordinates</secondary> +<seealso>Horizon</seealso> +<seealso>Zenith</seealso> +</indexterm> +<indexterm><primary>Azimuth</primary><see>Horizontal Coordinates</see></indexterm> +<indexterm><primary>Altitude</primary><see>Horizontal Coordinates</see></indexterm> +<para> +The Horizontal coordinate system uses the observer's local <link +linkend="ai-horizon">horizon</link> as the Fundamental Plane. This conveniently +divides the sky into the upper hemisphere that you can see, and the lower +hemisphere that you can't (because the Earth is in the way). The pole of the +upper hemisphere is called the <link linkend="ai-zenith">Zenith</link>. The +pole of the lower hemisphere is called the <firstterm>nadir</firstterm>. The +angle of an object above or below the horizon is called the +<firstterm>Altitude</firstterm> (Alt for short). The angle of an object around +the horizon (measured from the North point, toward the East) is called the +<firstterm>Azimuth</firstterm>. The Horizontal Coordinate System is sometimes +also called the Alt/Az Coordinate System. +</para><para> +The Horizontal Coordinate System is fixed to the Earth, not the Stars. +Therefore, the Altitude and Azimuth of an object changes with time, as the +object appears to drift across the sky. In addition, because the Horizontal +system is defined by your local horizon, the same object viewed from different +locations on Earth at the same time will have different values of Altitude and +Azimuth. +</para><para> +Horizontal coordinates are very useful for determining the Rise and Set times of +an object in the sky. When an object has Altitude=0 degrees, it is either +Rising (if its Azimuth is < 180 degrees) or Setting (if its Azimuth is > +180 degrees). +</para> +</sect2> + +<sect2 id="ecliptic"> +<title>The Ecliptic Coordinate System</title> + +<indexterm><primary>Celestial Coordinate Systems</primary> +<secondary>Ecliptic Coordinates</secondary> +<seealso>Ecliptic</seealso> +</indexterm> +<para> +The Ecliptic coordinate system uses the <link +linkend="ai-ecliptic">Ecliptic</link> for its Fundamental Plane. The +Ecliptic is the path that the Sun appears to follow across the sky over +the course of a year. It is also the projection of the Earth's +orbital plane onto the Celestial Sphere. The latitudinal angle is +called the <firstterm>Ecliptic Latitude</firstterm>, and the longitudinal angle +is called the <firstterm>Ecliptic Longitude</firstterm>. Like Right Ascension +in the Equatorial system, the zeropoint of the Ecliptic Longitude is the <link +linkend="ai-equinox">Vernal Equinox</link>. +</para><para> +What do you think such a coordinate system would be useful for? If you +guessed charting solar system objects, you are right! Each of the +planets (except Pluto) orbits the Sun in roughly the same plane, so they always +appear to be somewhere near the Ecliptic (&ie;, they always have small ecliptic +latitudes). +</para> +</sect2> + +<sect2 id="galactic"> +<title>The Galactic Coordinate System</title> + +<indexterm><primary>Celestial Coordinate Systems</primary> +<secondary>Galactic Coordinates</secondary> +</indexterm> +<para> +<indexterm><primary>Milky Way</primary></indexterm> +The Galactic coordinate system uses the <firstterm>Milky Way</firstterm> as its +Fundamental Plane. The latitudinal angle is called the <firstterm>Galactic +Latitude</firstterm>, and the longitudinal angle is called the +<firstterm>Galactic Longitude</firstterm>. This coordinate system is useful for +studying the Galaxy itself. For example, you might want to know how the density +of stars changes as a function of Galactic Latitude, to how much the disk of the +Milky Way is flattened. +</para> +</sect2> +</sect1> |