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Retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Direct motion or prograde motion is motion in the same direction as other bodies. While the terms direct and prograde are equivalent in this context, the former is the traditional term in astronomy. The earliest recorded use of prograde was in the early 18th century, although the term is now less common. == Etymology == The term ''retrograde'' is from the Latin word ''retrogradus'' – "backward-step", the affix ''retro-'' meaning "backwards" and ''gradus'' "step". ''Retrograde'' is most commonly an adjective used to describe the path of a planet as it travels through the night sky, with respect to the zodiac, stars, and other bodies of the celestial canopy. In this context, the term refers to planets, as they appear from Earth, to stop briefly and reverse direction at certain times though in reality, of course, we now understand that they perpetually orbit in the same uniform direction.〔Carrol, Bradley and Ostlie, Dale, ''An Introduction to Modern Astrophysics'', Second Edition, Addison-Wesley, San Francisco, 2007. pp. 3〕 "Mercury in retrograde" is an example of the term used as a noun for retrograde ''motion''. ''Retrograde'' is also sometimes used as an intransitive verb meaning to become, to appear, to behave—or appear to move—in a retrograde fashion. Although planets can sometimes be mistaken for stars as one observes the night sky, the planets actually change position from night to night in relation to the stars. Retrograde (backward) and prograde (forward) are observed as though the stars revolve around the Earth. Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the solar system but still used the terms ''retrograde'' and ''prograde'' to describe the movement of the planets in relation to the stars. Although it is known today that the planets revolve around the sun, the same terms continue to be used in order to describe the movement of the planets in relation to the stars as they are observed from Earth. Like the sun, the planets appear to rise in the East and set in the West. When a planet travels eastward in relation to the stars, it is called ''prograde''. When the planet travels westward in relation to the stars (opposite path) it is called ''retrograde''. ==Apparent motion== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「apparent retrograde motion」の詳細全文を読む スポンサード リンク
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