<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, when the 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>
