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+<sect1 id="ai-ecliptic">
+<sect1info>
+<author>
+<firstname>John</firstname>
+<surname>Cirillo</surname>
+</author>
+</sect1info>
+<title>The Ecliptic</title>
+<indexterm><primary>Ecliptic</primary>
+<seealso>Ecliptic Coordinates</seealso>
+</indexterm>
+<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 <link linkend="ai-cequator">Celestial
+Equator</link> 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>
+Because of the 23.5-degree tilt of the Ecliptic, the
+<firstterm>Altitude</firstterm> of the Sun at noon changes over the course of the
+year, as it follows the path of the Ecliptic across the sky.  This causes the
+seasons.  In the Summer, the Sun is high in the sky at noon,
+and it remains above the <link linkend="ai-horizon">Horizon</link> for more than
+twelve hours.  Whereas, in the winter, the Sun is low in the sky at noon, and remains
+above the Horizon for less than twelve hours.  In addition, sunlight is received at
+the Earth's surface at a more direct angle in the Summer, which means that a given
+area at the surface receives more energy per second in the Summer than in Winter.
+The differences in day duration and in energy received per unit area lead to the
+differences in temperature we experience in Summer and Winter.
+</para>
+<tip>
+<para>Exercises:</para>
+<para>
+Make sure your location is set to somewhere that is not very near the equator
+for these experiments.  Open the <guilabel>Configure &kstars;</guilabel> window, and
+switch to Horizontal coordinates, with the Opaque Ground shown.  Open the
+<guilabel>Set Time</guilabel> window
+(<keycombo action="simul">&Ctrl;<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.  You will also notice that the day durations
+are different if you open the <guilabel>What's Up Tonight?</guilabel> tool for
+each date.
+</para>
+</tip>
+</sect1>