<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>
