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59 lines
2.9 KiB
59 lines
2.9 KiB
15 years ago
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<sect1 id="ai-ecliptic">
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<sect1info>
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<author>
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<firstname>John</firstname>
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<surname>Cirillo</surname>
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</author>
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</sect1info>
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<title>The Ecliptic</title>
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<indexterm><primary>Ecliptic</primary>
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<seealso>Ecliptic Coordinates</seealso>
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</indexterm>
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<para>
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The ecliptic is an imaginary <link linkend="ai-greatcircle">Great Circle</link>
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on the <link linkend="ai-csphere">Celestial Sphere</link> along which the Sun
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appears to move over the course of a year. Of course, it is really the
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Earth's orbit around the Sun causing the change in the Sun's apparent
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direction. The ecliptic is inclined from the <link linkend="ai-cequator">Celestial
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Equator</link> by 23.5 degrees. The two points where the Ecliptic crosses
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the Celestial Equator are known as the <link
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linkend="ai-equinox">Equinoxes</link>.
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</para><para>
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Since our solar system is relatively flat, the orbits of the planets are
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also close to the plane of the ecliptic. In addition, the constellations of the
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zodiac are located along the ecliptic. This makes the ecliptic a very useful
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line of reference to anyone attempting to locate the planets or the
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constellations of the zodiac, since they all literally <quote>follow the
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Sun</quote>.
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</para><para>
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Because of the 23.5-degree tilt of the Ecliptic, the
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<firstterm>Altitude</firstterm> of the Sun at noon changes over the course of the
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year, as it follows the path of the Ecliptic across the sky. This causes the
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seasons. In the Summer, the Sun is high in the sky at noon,
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and it remains above the <link linkend="ai-horizon">Horizon</link> for more than
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twelve hours. Whereas, in the winter, the Sun is low in the sky at noon, and remains
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above the Horizon for less than twelve hours. In addition, sunlight is received at
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the Earth's surface at a more direct angle in the Summer, which means that a given
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area at the surface receives more energy per second in the Summer than in Winter.
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The differences in day duration and in energy received per unit area lead to the
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differences in temperature we experience in Summer and Winter.
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</para>
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<tip>
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<para>Exercises:</para>
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<para>
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Make sure your location is set to somewhere that is not very near the equator
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for these experiments. Open the <guilabel>Configure &kstars;</guilabel> window, and
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switch to Horizontal coordinates, with the Opaque Ground shown. Open the
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<guilabel>Set Time</guilabel> window
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(<keycombo action="simul">&Ctrl;<keycap>S</keycap></keycombo>),and change the
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Date to sometime in the middle of Summer, and the Time to 12:00 Noon. Back in
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the Main Window, point toward the Southern Horizon (press <keycap>S</keycap>).
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Note the height of the Sun above the Horizon at Noon in the Summer. Now, change
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the Date to something in the middle of Winter (but keep the Time at 12:00 Noon).
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The Sun is now much lower in the Sky. You will also notice that the day durations
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are different if you open the <guilabel>What's Up Tonight?</guilabel> tool for
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each date.
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</para>
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</tip>
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</sect1>
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