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