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0001 <sect1 id="ai-skycoords"> 0002 <sect1info> 0003 <author> 0004 <firstname>Jason</firstname> 0005 <surname>Harris</surname> 0006 </author> 0007 </sect1info> 0008 <title>Celestial Coordinate Systems</title> 0009 <para> 0010 <indexterm><primary>Celestial Coordinate Systems</primary> 0011 <secondary>Overview</secondary></indexterm> 0012 A basic requirement for studying the heavens is determining where in the 0013 sky things are. To specify sky positions, astronomers have developed 0014 several <firstterm>coordinate systems</firstterm>. Each uses a coordinate grid 0015 projected on the <link linkend="ai-csphere">Celestial Sphere</link>, in 0016 analogy to the <link linkend="ai-geocoords">Geographic coordinate 0017 system</link> used on the surface of the Earth. The coordinate systems 0018 differ only in their choice of the <firstterm>fundamental plane</firstterm>, 0019 which divides the sky into two equal hemispheres along a <link 0020 linkend="ai-greatcircle">great circle</link>. (the fundamental plane of the 0021 geographic system is the Earth's equator). Each coordinate system is named for 0022 its choice of fundamental plane. 0023 </para> 0024 0025 <sect2 id="equatorial"> 0026 <title>The Equatorial Coordinate System</title> 0027 <indexterm><primary>Celestial Coordinate Systems</primary> 0028 <secondary>Equatorial Coordinates</secondary> 0029 <seealso>Celestial Equator</seealso> 0030 <seealso>Celestial Poles</seealso> 0031 <seealso>Geographic Coordinate System</seealso> 0032 </indexterm> 0033 <indexterm><primary>Right Ascension</primary><see>Equatorial Coordinates</see></indexterm> 0034 <indexterm><primary>Declination</primary><see>Equatorial Coordinates</see></indexterm> 0035 0036 <para> 0037 The <firstterm>Equatorial coordinate system</firstterm> is probably the most 0038 widely used celestial coordinate system. It is also the most closely related 0039 to the <link linkend="ai-geocoords">Geographic coordinate system</link>, because 0040 they use the same fundamental plane, and the same poles. The projection of the 0041 Earth's equator onto the celestial sphere is called the 0042 <link linkend="ai-cequator">Celestial Equator</link>. 0043 Similarly, projecting the geographic Poles onto the celestial sphere defines the 0044 North and South <link linkend="ai-cpoles">Celestial Poles</link>. 0045 </para><para> 0046 However, there is an important difference between the equatorial and 0047 geographic coordinate systems: the geographic system is fixed to the 0048 Earth; it rotates as the Earth does. The Equatorial system is 0049 fixed to the stars<footnote id="fn-precess"><para>Actually, the equatorial 0050 coordinates are not quite fixed to the stars. See <link 0051 linkend="ai-precession">precession</link>. Also, if <link 0052 linkend="ai-hourangle">Hour Angle</link> is used in place of Right 0053 Ascension, then the Equatorial system is fixed to the Earth, not to the 0054 stars.</para></footnote>, so it appears to rotate across the sky with the stars, 0055 but of course it is really the Earth rotating under the fixed sky. 0056 </para><para> 0057 The <firstterm>latitudinal</firstterm> (latitude-like) angle of the Equatorial 0058 system is called <firstterm>Declination</firstterm> (Dec for short). It 0059 measures the angle of an object above or below the Celestial Equator. The 0060 <firstterm>longitudinal</firstterm> angle is called the <firstterm>Right 0061 Ascension</firstterm> (<acronym>RA</acronym> for short). It measures the angle of an object East 0062 of the <link linkend="ai-equinox">Vernal Equinox</link>. Unlike longitude, 0063 Right Ascension is usually measured in hours instead of degrees, because the 0064 apparent rotation of the Equatorial coordinate system is closely related to 0065 <link linkend="ai-sidereal">Sidereal Time</link> and <link 0066 linkend="ai-hourangle">Hour Angle</link>. Since a full rotation of the sky 0067 takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in 0068 one Hour of Right Ascension. 0069 </para> 0070 <para> 0071 The equatorial coordinates for deep-sky objects and stars do not vary 0072 appreciably over short durations of time, since they are not affected 0073 by the <firstterm>diurnal motion</firstterm> (the daily apparent 0074 rotation of the sky around the earth. However, note that this takes 0075 <link linkend="ai-sidereal">1 sidereal day</link>, as against 1 solar 0076 day). They are suitable coordinates for making catalogs of stars and 0077 deep-sky objects (note that <firstterm>Galactic 0078 Coordinates</firstterm> also work well, but are cumbersome to use from 0079 an earth point-of-view). However, there are effects that cause the RA 0080 and Dec of objects to vary over time, 0081 namely <link linkend="ai-precession">Precession</link> 0082 and <firstterm>nutation</firstterm>, and <firstterm>proper 0083 motion</firstterm>, the latter being even less important. Equatorial 0084 coordinates are thus generally specified with an 0085 appropriate <link linkend="ai-epoch">epoch</link>, to account for 0086 precession. Popular epochs include J2000.0 0087 (<link linkend="ai-julianday">Julian Year</link> 2000) and B1950.0 0088 (<firstterm>Besselian Year</firstterm> 1950). 0089 </para> 0090 </sect2> 0091 0092 <sect2 id="horizontal"> 0093 <title>The Horizontal Coordinate System</title> 0094 0095 <indexterm><primary>Celestial Coordinate Systems</primary> 0096 <secondary>Horizontal Coordinates</secondary> 0097 <seealso>Horizon</seealso> 0098 <seealso>Zenith</seealso> 0099 </indexterm> 0100 <indexterm><primary>Azimuth</primary><see>Horizontal Coordinates</see></indexterm> 0101 <indexterm><primary>Altitude</primary><see>Horizontal Coordinates</see></indexterm> 0102 <para> 0103 The Horizontal coordinate system uses the observer's local <link 0104 linkend="ai-horizon">horizon</link> as the Fundamental Plane. This conveniently 0105 divides the sky into the upper hemisphere that you can see, and the lower 0106 hemisphere that you can't (because the Earth is in the way). The pole of the 0107 upper hemisphere is called the <link linkend="ai-zenith">Zenith</link>. The 0108 pole of the lower hemisphere is called the <firstterm>nadir</firstterm>. The 0109 angle of an object above or below the horizon is called the 0110 <firstterm>Altitude</firstterm> (Alt for short). The angle of an object around 0111 the horizon (measured from the North point, toward the East) is called the 0112 <firstterm>Azimuth</firstterm>. The Horizontal Coordinate System is sometimes 0113 also called the Alt/Az Coordinate System. 0114 </para><para> 0115 The Horizontal Coordinate System is fixed to the Earth, not the Stars. 0116 Therefore, the Altitude and Azimuth of an object changes with time, as the 0117 object appears to drift across the sky. In addition, because the Horizontal 0118 system is defined by your local horizon, the same object viewed from different 0119 locations on Earth at the same time will have different values of Altitude and 0120 Azimuth. 0121 </para><para> 0122 Horizontal coordinates are very useful for determining the Rise and Set times of 0123 an object in the sky. When an object has Altitude=0 degrees, it is either 0124 Rising (if its Azimuth is < 180 degrees) or Setting (if its Azimuth is > 0125 180 degrees). 0126 </para> 0127 </sect2> 0128 0129 <sect2 id="ecliptic"> 0130 <title>The Ecliptic Coordinate System</title> 0131 0132 <indexterm><primary>Celestial Coordinate Systems</primary> 0133 <secondary>Ecliptic Coordinates</secondary> 0134 <seealso>Ecliptic</seealso> 0135 </indexterm> 0136 <para> 0137 The Ecliptic coordinate system uses the <link 0138 linkend="ai-ecliptic">Ecliptic</link> for its Fundamental Plane. The 0139 Ecliptic is the path that the Sun appears to follow across the sky over 0140 the course of a year. It is also the projection of the Earth's 0141 orbital plane onto the Celestial Sphere. The latitudinal angle is 0142 called the <firstterm>Ecliptic Latitude</firstterm>, and the longitudinal angle 0143 is called the <firstterm>Ecliptic Longitude</firstterm>. Like Right Ascension 0144 in the Equatorial system, the zeropoint of the Ecliptic Longitude is the <link 0145 linkend="ai-equinox">Vernal Equinox</link>. 0146 </para><para> 0147 What do you think such a coordinate system would be useful for? If you 0148 guessed charting solar system objects, you are right! Each of the 0149 planets (except Pluto) orbits the Sun in roughly the same plane, so they always 0150 appear to be somewhere near the Ecliptic (&ie;, they always have small ecliptic 0151 latitudes). 0152 </para> 0153 </sect2> 0154 0155 <sect2 id="galactic"> 0156 <title>The Galactic Coordinate System</title> 0157 0158 <indexterm><primary>Celestial Coordinate Systems</primary> 0159 <secondary>Galactic Coordinates</secondary> 0160 </indexterm> 0161 <para> 0162 <indexterm><primary>Milky Way</primary></indexterm> 0163 The Galactic coordinate system uses the <firstterm>Milky Way</firstterm> as its 0164 Fundamental Plane. The latitudinal angle is called the <firstterm>Galactic 0165 Latitude</firstterm>, and the longitudinal angle is called the 0166 <firstterm>Galactic Longitude</firstterm>. This coordinate system is useful for 0167 studying the Galaxy itself. For example, you might want to know how the density 0168 of stars changes as a function of Galactic Latitude, to how much the disk of the 0169 Milky Way is flattened. 0170 </para> 0171 </sect2> 0172 </sect1>