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59 lines
2.7 KiB
59 lines
2.7 KiB
15 years ago
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<sect1 id="ai-cpoles">
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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>The Celestial Poles</title>
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<indexterm><primary>Celestial Poles</primary>
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<seealso>Equatorial Coordinates</seealso>
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</indexterm>
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<para>
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The sky appears to drift overhead from east to west, completing a full circuit
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around the sky in 24 (<link linkend="ai-sidereal">Sidereal</link>) hours. This
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phenomenon is due to the spinning of the Earth on its axis. The Earth's
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spin axis intersects the <link linkend="ai-csphere">Celestial Sphere</link> at
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two points. These points are the <firstterm>Celestial Poles</firstterm>. As the
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Earth spins; they remain fixed in the sky, and all other points seem to rotate
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around them. The celestial poles are also the poles of the <link
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linkend="equatorial">Equatorial Coordinate System</link>, meaning
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they have <firstterm>Declinations</firstterm> of +90 degrees and -90 degrees
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(for the North and South celestial poles, respectively).
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</para><para>
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The North Celestial Pole currently has nearly the same coordinates as
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the bright star <firstterm>Polaris</firstterm> (which is Latin for <quote>Pole Star</quote>).
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This makes Polaris useful for navigation: not only is it always above the North
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point of the horizon, but its <link
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linkend="horizontal">Altitude</link> angle is always (nearly)
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equal to the observer's <link linkend="ai-geocoords">Geographic Latitude</link>
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(however, Polaris can only be seen from locations in the Northern hemisphere).
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</para><para>
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The fact that Polaris is near the pole is purely a coincidence. In fact,
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because of <link linkend="ai-precession">Precession</link>, Polaris is only near
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the pole for a small fraction of the time.
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</para>
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<tip>
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<para>Exercises:</para>
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<para>
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Use the <guilabel>Find Object</guilabel> window
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(<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to locate
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Polaris. Notice that its Declination is almost (but not exactly) +90 degrees.
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Compare the Altitude reading when focused on Polaris to your location's
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geographic latitude. They are always within one degree of each other.
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They are not exactly the same because Polaris isn't exactly at the Pole.
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(you can point exactly at the pole by switching to Equatorial
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coordinates, and pressing the up-arrow key until the sky stops scrolling).
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</para><para>
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Use the <guilabel>Time Step</guilabel> spinbox in the toolbar to accelerate time
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to a
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step of 100 seconds. You can see the entire sky appears to rotate around
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Polaris, while Polaris itself remains nearly stationary.
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</para><para>
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We said that the celestial pole is the pole of the Equatorial coordinate
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system. What do you think is the pole of the horizontal (Altitude/Azimuth)
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coordinate system? (The <link linkend="ai-zenith">Zenith</link>).
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</para>
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</tip>
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</sect1>
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