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45 lines
2.0 KiB
45 lines
2.0 KiB
<sect1 id="ai-retrograde">
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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>Retrograde Motion</title>
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<indexterm><primary>Retrograde Motion</primary>
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</indexterm>
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<para>
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<firstterm>Retrograde Motion</firstterm> is the orbital motion of a body in a
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direction opposite that which is normal to spatial bodies within a given system.
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</para><para>
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When we observe the sky, we expect most objects to appear to move in a
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particular direction with the passing of time. The apparent motion of
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most bodies in the sky is from east to west. However it is possible to
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observe a body moving west to east, such as an artificial satellite or
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space shuttle that is orbiting eastward. This orbit is
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considered Retrograde Motion.
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</para><para>
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Retrograde Motion is most often used in reference to the
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motion of the outer planets (Mars, Jupiter, Saturn, and so forth).
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Though these planets appear to move from east to west on a nightly
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basis in response to the spin of the Earth, they are actually drifting
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slowly eastward with respect to the stationary stars, which can be
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observed by noting the position of these planets for several nights in a
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row. This motion is normal for these planets, however, and not
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considered Retrograde Motion. However, since the Earth completes its
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orbit in a shorter period of time than these outer planets, we
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occasionally overtake an outer planet, like a faster car on a
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multiple-lane highway. When this occurs, the planet we are passing will
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first appear to stop its eastward drift, and it will then
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appear to drift back toward the west. This is Retrograde Motion, since
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it is in a direction opposite that which is typical for planets. Finally,
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as the Earth swings past the the planet in its orbit, they appear to
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resume their normal west-to-east drift on successive nights.
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</para><para>
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This Retrograde Motion of the planets puzzled ancient Greek
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astronomers, and was one reason why they named these bodies <quote>planets</quote>
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which in Greek means <quote>wanderers</quote>.
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</para>
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</sect1>
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