Editing Apparent retrograde motion (section)

== Etymology and history ==
[[File:Apparent retrograde motion of Mars in 2003.gif|thumb|250px|Apparent retrograde motion of Mars in 2003 as seen from Earth]]
The term ''retrograde'' is from the [[Latin]] word {{lang|la|retrogradus}} – "backward-step", the affix {{lang|la|retro-}} meaning "backwards" and {{lang|la|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 [[wikt:canopy|canopy]]. In this context, the term refers to planets, as they appear from Earth, stopping briefly and reversing direction at certain times, though in reality, of course, we now understand that they perpetually orbit in the same uniform direction.<ref>Carrol, Bradley and Ostlie, Dale, ''An Introduction to Modern Astrophysics'', Second Edition, Addison-Wesley, San Francisco, 2007. pp. 3</ref>

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&nbsp;AD believed that the Earth was the center of the [[Solar System]] and therefore 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''.<ref>{{cite web|url=http://dictionary.reference.com/browse/retrograde |title=Retrograde &#124; Define Retrograde at Dictionary.com |publisher=[[Dictionary.reference.com]] |access-date=2012-08-17}}</ref>

This apparent retrogradation puzzled ancient astronomers, and was one reason they named these bodies 'planets' in the first place: 'Planet' comes from the Greek word for 'wanderer'. In the [[geocentric universe|geocentric model]] of the Solar System proposed by [[Apollonius of Perga|Apollonius]] in the third century BCE, retrograde motion was explained by having the planets travel in [[deferent and epicycle|deferents and epicycles]].<ref name="Carrol 2007. pp. 4" /> It was not understood to be an illusion until the time of [[Nicolaus Copernicus|Copernicus]], although the Greek astronomer [[Aristarchus of Samos|Aristarchus]] in 240 BCE proposed a [[heliocentrism|heliocentric model]] for the Solar System.

[[Galileo]]'s drawings show that he first observed [[Neptune]] on December 28, 1612, and again on January 27, 1613. On both occasions, Galileo mistook Neptune for a fixed star when it appeared very close—in conjunction—to [[Jupiter]] in the night sky, hence, he is not credited with Neptune's discovery. During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that very day. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small [[telescope]].