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Astronomical coordinate systems
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<p>In <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>, <a href="/facts/Coordinate_systems/Mtd0q4SP">coordinate systems</a> are used for specifying <a href="/facts/Position_(geometry)/Tpk5d9f2">positions</a> of <a href="/facts/Astronomical_object/BCHLa0Q2">celestial objects</a> (<a href="/facts/Natural_satellite/AE4KQKbq">satellites</a>, <a href="/facts/Planet/rbPjE6Bx">planets</a>, <a href="/facts/Star/PFbd6ICx">stars</a>, <a href="/facts/Galaxy/zCyUvEf7">galaxies</a>, etc.) relative to a given <a href="/facts/Reference_frame/15yYfB9M">reference frame</a>, based on physical reference points available to a situated observer (e.g. the true <a href="/facts/Horizon/LxwxWfC2">horizon</a> and <a href="/facts/True_north/276EMRXX">north</a> to an observer on Earth's surface). Coordinate systems in astronomy can specify an object's relative position in <a href="/facts/Three-dimensional_space/KAqKWUbS">three-dimensional space</a> or <a href="/facts/Plot_(graphics)/dxuSqKN9">plot</a> merely by its <a href="/facts/Direction_(geometry)/Tj24mTJ8">direction</a> on a <a href="/facts/Celestial_sphere/TYAtXgor">celestial sphere</a>, if the object's distance is unknown or trivial. </p><p><a href="/facts/Spherical_coordinate_system/5cVnNof3">Spherical coordinates</a>, projected on the celestial sphere, are analogous to the <a href="/facts/Geographic_coordinate_system/RkjfcY6k">geographic coordinate system</a> used on the surface of <a href="/facts/Earth/HLLmDfj0">Earth</a>. These differ in their choice of <a href="/facts/Fundamental_plane_(spherical_coordinates)/n5cWNMq2">fundamental plane</a>, which divides the celestial sphere into two equal <a href="/facts/Sphere/jywYLNM2">hemispheres</a> along a <a href="/facts/Great_circle/x4fq6eNm">great circle</a>. <a href="/facts/Cartesian_coordinate_system/lkTqvMmB">Rectangular coordinates</a>, in appropriate <a href="/facts/Units_of_measurement/pvTpWbmC">units</a>, have the same fundamental (<i>x, y</i>) plane and <a href="/facts/Primary_direction/oEEQuut7">primary (<i>x</i>-axis) direction</a>, such as an <a href="/facts/Rotation_around_a_fixed_axis/PGcg3IY5">axis of rotation</a>. Each coordinate system is named after its choice of fundamental plane. </p>