kstars/doc/equinox.docbook

0001 <sect1 id="ai-equinox">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>The Equinoxes</title>
0009 <indexterm><primary>Equinoxes</primary>
0010 <seealso>Celestial Equator</seealso>
0011 <seealso>Ecliptic</seealso>
0012 </indexterm>
0013 <para>
0014 Most people know the Vernal and Autumnal Equinoxes as
0015 calendar dates, signifying the beginning of the Northern hemisphere's Spring
0016 and Autumn, respectively.  Did you know that the equinoxes are also positions in
0017 the sky?
0018 </para><para>
0019 The <link linkend="ai-cequator">Celestial Equator</link> and the
0020 <link linkend="ai-ecliptic">Ecliptic</link> are two
0021 <link linkend="ai-greatcircle">Great Circles</link> on the
0022 <link linkend="ai-csphere">Celestial Sphere</link>, set at an angle of 23.5
0023 degrees.  The two points where they intersect are called the
0024 <firstterm>Equinoxes</firstterm>.  The <firstterm>Vernal Equinox</firstterm>
0025 has coordinates RA=0.0 hours, Dec=0.0 degrees.  The <firstterm>Autumnal
0026 Equinox</firstterm> has coordinates RA=12.0 hours, Dec=0.0 degrees.
0027 </para><para>
0028 The Equinoxes are important for marking the seasons.  Because they are on
0029 the <link linkend="ai-ecliptic">Ecliptic</link>, the Sun passes through each
0030 equinox every year.  When the Sun passes through the Vernal Equinox (usually on
0031 March 21st), it crosses the <link linkend="ai-cequator">Celestial Equator</link>
0032 from South to North, signifying the end of Winter for the Northern hemisphere.
0033 Similarly, when the Sun passes through the Autumnal Equinox (usually on September
0034 21st), it crosses the Celestial Equator from North to South, signifying the
0035 end of Winter for the Southern hemisphere.
0036 </para>
0037 </sect1>