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authortoma <toma@283d02a7-25f6-0310-bc7c-ecb5cbfe19da>2009-11-25 17:56:58 +0000
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+<sect1 id="ai-equinox">
+<sect1info>
+<author>
+<firstname>Jason</firstname>
+<surname>Harris</surname>
+</author>
+</sect1info>
+<title>The Equinoxes</title>
+<indexterm><primary>Equinoxes</primary>
+<seealso>Celestial Equator</seealso>
+<seealso>Ecliptic</seealso>
+</indexterm>
+<para>
+Most people know the Vernal and Autumnal Equinoxes as
+calendar dates, signifying the beginning of the Northern hemisphere's Spring
+and Autumn, respectively. Did you know that the equinoxes are also positions in
+the sky?
+</para><para>
+The <link linkend="ai-cequator">Celestial Equator</link> and the
+<link linkend="ai-ecliptic">Ecliptic</link> are two
+<link linkend="ai-greatcircle">Great Circles</link> on the
+<link linkend="ai-csphere">Celestial Sphere</link>, set at an angle of 23.5
+degrees. The two points where they intersect are called the
+<firstterm>Equinoxes</firstterm>. The <firstterm>Vernal Equinox</firstterm>
+has coordinates RA=0.0 hours, Dec=0.0 degrees. The <firstterm>Autumnal
+Equinox</firstterm> has coordinates RA=12.0 hours, Dec=0.0 degrees.
+</para><para>
+The Equinoxes are important for marking the seasons. Because they are on
+the <link linkend="ai-ecliptic">Ecliptic</link>, the Sun passes through each
+equinox every year. When the Sun passes through the Vernal Equinox (usually on
+March 21st), it crosses the <link linkend="ai-cequator">Celestial Equator</link>
+from South to North, signifying the end of Winter for the Northern hemisphere.
+Similarly, whenthe Sun passes through the Autumnal Equinox (usually on September
+21st), it crosses the Celestial Equator from North to South, signifying the
+end of Winter for the Southern hemisphere.
+</para>
+</sect1>