You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
59 lines
2.9 KiB
59 lines
2.9 KiB
<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>
|