<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>
