Editing Apparent retrograde motion (section)

==Apparent motion==<!-- This section is linked from [[Astrological aspect]] -->
=== From Earth ===
When standing on the Earth looking up at the sky, it would appear that  [[the Moon]] travels from [[east]] to [[west]], just as the Sun and the stars do. Day after day however, the Moon appears to move to the east with respect to the stars. In fact, the Moon orbits the Earth from [[west]] to [[east]], as do the vast majority of manmade satellites such as the [[International Space Station]]. The apparent westward motion of the Moon from the Earth's surface is actually an artifact of its being in a [[supersynchronous orbit]]. This means that the Earth completes one [[sidereal rotation]] before [[the Moon]] is able to complete one orbit. As a result, it looks like [[the Moon]] is travelling in the opposite direction, otherwise known as apparent retrograde motion. A person standing on Earth "catches up" to the Moon and passes it because the Earth completes one rotation before the Moon completes one orbit.

This phenomenon also occurs on [[Mars]], which has two natural satellites,  [[Phobos (moon)|Phobos]] and  [[Deimos (moon)|Deimos]]. Both moons orbit [[Mars]] in an eastward ([[Retrograde and prograde motion|prograde]]) direction; however, [[Deimos (moon)|Deimos]] has an orbital period of 1.23 Martian  [[Sidereal Time|sidereal days]], making it [[supersynchronous orbit|supersynchronous]], whereas [[Phobos (moon)|Phobos]] has an orbital period of 0.31 Martian  [[Sidereal Time|sidereal days]], making it [[subsynchronous orbit|subsynchronous]]. Consequently, although both moons are traveling in an eastward (prograde) direction, they appear to be traveling in opposite directions when viewed from the surface of [[Mars]] due to their orbital periods in relation to the rotational period of the planet.

All other planetary bodies in the Solar System also appear to periodically switch direction as they cross Earth's sky. Though all stars and planets appear to move from east to west on a nightly basis in response to the rotation of Earth, the outer planets generally drift slowly eastward relative to the stars. [[Asteroids]] and [[Kuiper Belt]] objects (including [[Pluto]]) exhibit apparent retrograde motion. This motion is normal for the planets, and so is considered direct motion. However, since Earth completes its orbit in a shorter period of time than the planets outside its orbit, it periodically overtakes them, like a faster car on a multi-lane highway. When this occurs, the planet being passed will first appear to stop its eastward drift, and then drift back toward the west. Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east.<ref name="Carrol 2007. pp. 4">Carrol, Bradley and Ostlie, Dale, ''An Introduction to Modern Astrophysics'', Second Edition, Addison-Wesley, San Francisco, 2007. pp. 4</ref><gallery mode="packed" heights="200">
File:Retrograde Motion.bjb.svg|As [[Earth]] (blue) passes a superior planet such as [[Mars]] (red), the superior planet will temporarily appear to reverse its motion across the sky.
File:An animation to explain the (apparent) retrograde motion of Mars, using actual 2020 planet positions.webm|An animation to explain the (apparent) retrograde motion of Mars, using actual 2020 planet positions
</gallery>Inner planets [[Venus]] and [[Mercury (planet)|Mercury]] appear to move in retrograde in a similar mechanism, but as they can never be in [[Opposition (astronomy)|opposition]] to the Sun as seen from Earth, their retrograde cycles are tied to their inferior conjunctions with the Sun. They are unobservable in the Sun's glare and in their "new" phase, with mostly their dark sides toward Earth; they occur in the transition from evening star to morning star.

The more distant planets retrograde more frequently, as they do not move as much in their orbits while Earth completes an orbit itself.  The retrograde motion of a hypothetical extremely distant (and nearly non-moving) planet would take place during a half-year, with the planet's apparent yearly motion being reduced to a [[parallax]] ellipse.

The center of the retrograde motion occurs at the planet's opposition which is when the planet is exactly opposite the Sun. This position is halfway, or 6 months, around the ecliptic from the Sun. The planet's height in the sky is opposite that of the Sun's height. The planet is at its highest at the winter solstice, and at its lowest at the summer solstice, on those (rare) occasions when it passes through the center of its retrograde motion near a solstice. Note particularly that the hemisphere the observer is in is critical to what they observe. The December Solstice will place the planet high in the northern hemisphere sky where it is winter and place it low in the southern hemisphere sky where it is summer. The opposite is true if this happens at the June Solstice. 

Since the planet's opposition retrograde motion is when the Earth passes closest, the planet appears at its brightest for the year.

The period between the center of such retrogradations is the [[synodic period]] of the planet.

{| class="wikitable"
 |+ Planetary retrograde constants
 |-
 ! Planet
 ! Synodic period (days)
 ! Synodic period (mean months)
 ! Days in retrogradation
 |-
 ! [[Mercury (planet)|Mercury]]
 | 116
 | 3.8
 | ≈ 21
 |-
 ! [[Venus]]
 | 584
 | 19.2
 | 41
 |-
 ! [[Mars]]
 | 780
 | 25.6
 | 72
 |-
 ! [[Jupiter]]
 | 399
 | 13.1
 | 121
 |-
 ! [[Saturn]]
 | 378
 | 12.4
 | 138
 |-
 ! [[Uranus]]
 | 370
 | 12.15
 | 151
 |-
 ! [[Neptune]]
 | 367
 | 12.07
 | 158
 |-
 ! Hypothetical [[planets beyond Neptune|far-out planet]]
 | 365.25
 | 12
 | 182.625
 |}

=== From Mercury ===

From any point on the daytime surface of [[Mercury (planet)|Mercury]] when the planet is near [[perihelion]] (closest approach to the [[Sun]]), the Sun undergoes apparent retrograde motion. This occurs because, from approximately four Earth days before [[perihelion]] until approximately four Earth days after it, Mercury's angular [[orbital speed]] exceeds its angular [[rotational velocity]].<ref>Strom, Robert G.; Sprague, Ann L. (2003). ''Exploring Mercury: the iron planet''. Springer. {{ISBN|1-85233-731-1}}.</ref> Mercury's elliptical orbit is farther from circular than that of any other planet in the Solar System, resulting in a substantially higher orbital speed near perihelion. As a result, at specific points on Mercury's surface an observer would be able to see the Sun rise part way, then reverse and set before rising again, all within the same [[Extraterrestrial skies#Mercury|Mercurian day]].