doc/kstars/equinox.docbook

   1 <sect1 id="ai-equinox">
   2 <sect1info>
   3 <author>
   4 <firstname>Jason</firstname>
   5 <surname>Harris</surname>
   6 </author>
   7 </sect1info>
   8 <title>The Equinoxes</title>
   9 <indexterm><primary>Equinoxes</primary>
  10 <seealso>Celestial Equator</seealso>
  11 <seealso>Ecliptic</seealso>
  12 </indexterm>
  13 <para>
  14 Most people know the Vernal and Autumnal Equinoxes as
  15 calendar dates, signifying the beginning of the Northern hemisphere's Spring
  16 and Autumn, respectively.  Did you know that the equinoxes are also positions in
  17 the sky?
  18 </para><para>
  19 The <link linkend="ai-cequator">Celestial Equator</link> and the
  20 <link linkend="ai-ecliptic">Ecliptic</link> are two
  21 <link linkend="ai-greatcircle">Great Circles</link> on the
  22 <link linkend="ai-csphere">Celestial Sphere</link>, set at an angle of 23.5
  23 degrees.  The two points where they intersect are called the
  24 <firstterm>Equinoxes</firstterm>.  The <firstterm>Vernal Equinox</firstterm>
  25 has coordinates RA=0.0 hours, Dec=0.0 degrees.  The <firstterm>Autumnal
  26 Equinox</firstterm> has coordinates RA=12.0 hours, Dec=0.0 degrees.
  27 </para><para>
  28 The Equinoxes are important for marking the seasons.  Because they are on
  29 the <link linkend="ai-ecliptic">Ecliptic</link>, the Sun passes through each
  30 equinox every year.  When the Sun passes through the Vernal Equinox (usually on
  31 March 21st), it crosses the <link linkend="ai-cequator">Celestial Equator</link>
  32 from South to North, signifying the end of Winter for the Northern hemisphere.
  33 Similarly, whenthe Sun passes through the Autumnal Equinox (usually on September
  34 21st), it crosses the Celestial Equator from North to South, signifying the
  35 end of Winter for the Southern hemisphere.
  36 </para>
  37 </sect1>