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55 lines
2.5 KiB
55 lines
2.5 KiB
<sect1 id="ai-precession">
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<sect1info>
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<author>
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<firstname>Jason</firstname>
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<surname>Harris</surname>
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</author>
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</sect1info>
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<title>Precession</title>
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<indexterm><primary>Precession</primary>
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</indexterm>
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<para>
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<firstterm>Precession</firstterm> is the gradual change in the direction of the
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Earth's spin axis. The spin axis traces a cone, completing a full circuit in
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26,000 years. If you have ever spun a top or a dreidel, the
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<quote>wobbling</quote> rotation of the top as it spins is precession.
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</para><para>
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Because the direction of the Earth's spin axis changes, so does the location of
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the <link linkend="ai-cpoles">Celestial Poles</link>.
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</para><para>
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The reason for the Earth's precession is complicated. The Earth is not a
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perfect sphere, it is a bit flattened, meaning the
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<link linkend="ai-greatcircle">Great Circle</link> of the equator is longer
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than a <quote>meridonal</quote> great circle that
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passes through the poles. Also, the Moon and Sun lie outside the Earth's
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Equatorial plane. As a result, the gravitational pull of the Moon and Sun on
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the oblate Earth induces a slight <emphasis>torque</emphasis> in addition to a
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linear force. This torque on the spinning body of the Earth leads to the
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precessional motion.
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</para>
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<tip>
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<para>Exercise:</para>
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<para>
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Precession is easiest to see by observing the <link
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linkend="ai-cpoles">Celestial Pole</link>. To find the pole, first switch to
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Equatorial Coordinates in the <guilabel>Configure &kstars;</guilabel> window, and
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then hold down the <keycap>Up arrow</keycap> key until the display stops
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scrolling. The declination displayed in the center of the
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<guilabel>Info Panel</guilabel> should be +90 degrees, and the bright star
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Polaris should be nearly at the center of the screen. Try slewing with the left
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and right arrow keys. Notice that the sky appears to rotate around the Pole.
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</para><para>
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We will now demonstrate Precession by changing the Date to a very remote year,
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and observing that the location of the Celestial Pole is no longer near Polaris.
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Open the <guilabel>Set Time</guilabel> window
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(<keycombo action="simul">&Ctrl;<keycap>S</keycap></keycombo>), and set the date
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to the year 8000 (currently, &kstars; cannot handle dates much more remote than
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this, but this date is sufficient for our purposes). Notice that the sky
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display is now centered at a point between the constellations Cygnus and
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Cepheus. Verify that this is actually the pole by slewing left and right: the
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sky rotates about this point; in the year 8000, the North celestial pole will no
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longer be near Polaris.
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</para>
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</tip>
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</sect1>
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