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38 lines
1.6 KiB
38 lines
1.6 KiB
<sect1 id="ai-equinox">
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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 Equinoxes</title>
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<indexterm><primary>Equinoxes</primary>
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<seealso>Celestial Equator</seealso>
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<seealso>Ecliptic</seealso>
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</indexterm>
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<para>
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Most people know the Vernal and Autumnal Equinoxes as
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calendar dates, signifying the beginning of the Northern hemisphere's Spring
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and Autumn, respectively. Did you know that the equinoxes are also positions in
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the sky?
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</para><para>
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The <link linkend="ai-cequator">Celestial Equator</link> and the
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<link linkend="ai-ecliptic">Ecliptic</link> are two
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<link linkend="ai-greatcircle">Great Circles</link> on the
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<link linkend="ai-csphere">Celestial Sphere</link>, set at an angle of 23.5
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degrees. The two points where they intersect are called the
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<firstterm>Equinoxes</firstterm>. The <firstterm>Vernal Equinox</firstterm>
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has coordinates RA=0.0 hours, Dec=0.0 degrees. The <firstterm>Autumnal
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Equinox</firstterm> has coordinates RA=12.0 hours, Dec=0.0 degrees.
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</para><para>
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The Equinoxes are important for marking the seasons. Because they are on
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the <link linkend="ai-ecliptic">Ecliptic</link>, the Sun passes through each
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equinox every year. When the Sun passes through the Vernal Equinox (usually on
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March 21st), it crosses the <link linkend="ai-cequator">Celestial Equator</link>
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from South to North, signifying the end of Winter for the Northern hemisphere.
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Similarly, whenthe Sun passes through the Autumnal Equinox (usually on September
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21st), it crosses the Celestial Equator from North to South, signifying the
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end of Winter for the Southern hemisphere.
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</para>
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</sect1>
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