<sect1 id="ai-ecliptic">
<sect1info>
<author>
<firstname>John</firstname>
<surname>Cirillo</surname>
</author>
</sect1info>
<title>The Ecliptic</title>
<para>
The ecliptic is an imaginary <link linkend="ai-greatcircle">Great Circle</link>
on the <link linkend="ai-csphere">Celestial Sphere</link> along which the Sun
appears to move over the course of a year.  Of course, it is really the
Earth's orbit around the Sun causing the change in the Sun's apparent
direction.  The ecliptic is inclined from the <firstterm>Celestial
Equator</firstterm> by 23.5 degrees.   The two points where the ecliptic crosses
the celestial equator are known as the <link
linkend="ai-equinox">Equinoxes</link>.
</para><para>
Since our solar system is relatively flat, the orbits of the planets are
also close to the plane of the ecliptic.  In addition, the constellations of the
zodiac are located along the ecliptic. This makes the ecliptic a very useful
line of reference to anyone attempting to locate the planets or the
constellations of the zodiac, since they all literally <quote>follow the
Sun</quote>.
</para><para>
The <firstterm>Altitude</firstterm> of the ecliptic above the Horizon changes
over the course of the year, because of the 23.5 degree tilt of the Earth's spin
axis.  This causes the seasons.  When the ecliptic (and therefore the Sun) is
high above the horizon, the days are longer, and you have Summer. When the
ecliptic is low in the sky, you have Winter.
</para>
<tip>
<para>Exercises:</para>
<para>
Open the <guilabel>View Options</guilabel> window, and switch to Horizontal
coordinates, with the Opaque Ground shown.  Open the <guilabel>Set
Time</guilabel> window
(<keycombo><keycap>Ctrl</keycap><keycap>s</keycap></keycombo>),and change the
Date to sometime in the middle of Summer, and the Time to 12:00 Noon.  Back in
the Main Window, point toward the Southern Horizon (press <keycap>S</keycap>).
Note the height of the Sun above the Horizon at Noon in the Summer.  Now, change
the Date to something in the middle of Winter (but keep the Time at 12:00 Noon).
The Sun is now much lower in the Sky.
</para>
</tip>
</sect1>
