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0001 <sect1 id="ai-ecliptic"> 0002 <sect1info> 0003 <author> 0004 <firstname>John</firstname> 0005 <surname>Cirillo</surname> 0006 </author> 0007 </sect1info> 0008 <title>The Ecliptic</title> 0009 <indexterm><primary>Ecliptic</primary> 0010 <seealso>Ecliptic Coordinates</seealso> 0011 </indexterm> 0012 <para> 0013 The ecliptic is an imaginary <link linkend="ai-greatcircle">Great Circle</link> 0014 on the <link linkend="ai-csphere">Celestial Sphere</link> along which the Sun 0015 appears to move over the course of a year. Of course, it is really the 0016 Earth's orbit around the Sun causing the change in the Sun's apparent 0017 direction. The ecliptic is inclined from the <link linkend="ai-cequator">Celestial 0018 Equator</link> by 23.5 degrees. The two points where the Ecliptic crosses 0019 the Celestial Equator are known as the <link 0020 linkend="ai-equinox">Equinoxes</link>. 0021 </para><para> 0022 Since our solar system is relatively flat, the orbits of the planets are 0023 also close to the plane of the ecliptic. In addition, the constellations of the 0024 zodiac are located along the ecliptic. This makes the ecliptic a very useful 0025 line of reference to anyone attempting to locate the planets or the 0026 constellations of the zodiac, since they all literally <quote>follow the 0027 Sun</quote>. 0028 </para><para> 0029 Because of the 23.5-degree tilt of the Ecliptic, the 0030 <firstterm>Altitude</firstterm> of the Sun at noon changes over the course of the 0031 year, as it follows the path of the Ecliptic across the sky. This causes the 0032 seasons. In the Summer, the Sun is high in the sky at noon, 0033 and it remains above the <link linkend="ai-horizon">Horizon</link> for more than 0034 twelve hours. Whereas, in the winter, the Sun is low in the sky at noon, and remains 0035 above the Horizon for less than twelve hours. In addition, sunlight is received at 0036 the Earth's surface at a more direct angle in the Summer, which means that a given 0037 area at the surface receives more energy per second in the Summer than in Winter. 0038 The differences in day duration and in energy received per unit area lead to the 0039 differences in temperature we experience in Summer and Winter. 0040 </para> 0041 <tip> 0042 <para>Exercises:</para> 0043 <para> 0044 Make sure your location is set to somewhere that is not very near the equator 0045 for these experiments. Open the <guilabel>Configure - &kstars;</guilabel> window, and 0046 switch to Horizontal coordinates, with the Opaque Ground shown. Open the 0047 <guilabel>Set Time</guilabel> window 0048 (<keycombo action="simul">&Ctrl;<keycap>S</keycap></keycombo>),and change the 0049 Date to sometime in the middle of Summer, and the Time to 12:00 Noon. Back in 0050 the Main Window, point toward the Southern Horizon (press <keycap>S</keycap>). 0051 Note the height of the Sun above the Horizon at Noon in the Summer. Now, change 0052 the Date to something in the middle of Winter (but keep the Time at 12:00 Noon). 0053 The Sun is now much lower in the Sky. You will also notice that the day durations 0054 are different if you open the <guilabel>What's up Tonight?</guilabel> tool for 0055 each date. 0056 </para> 0057 </tip> 0058 </sect1>