Apparent retrograde motion is the apparent motion of a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

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 Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

word – "backward-step", the affix meaning "backwards" and "step". ''Retrograde'' is most commonly an

adjective In linguistics, an adjective ( abbreviated ) is a word that generally modifies a noun or noun phrase or describes its referent. Its semantic role is to change information given by the noun. Traditionally, adjectives were considered one of the ...

used to describe the path of a planet as it travels through the night sky, with respect to the

zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The pa ...

, stars, and other bodies of the celestial 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. 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 The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

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''.

Apparent motion

From Earth

When standing on the Earth looking up at the sky, it would appear that

the Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

travels from east to

west West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth. Etymology The word "west" is a Germanic word passed into some ...

, 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

to east, as do the vast majority of manmade satellites such as the

International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...

. 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

is able to complete one orbit. As a result, it looks like

is travelling in the opposite direction, otherwise known as apparent retrograde motion. This phenomenon also occurs on

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

, which has two natural satellites, Phobos and

Deimos Deimos, a Greek word for ''dread'', may refer to: * Deimos (deity), one of the sons of Ares and Aphrodite in Greek mythology * Deimos (moon), the smaller and outermost of Mars' two natural satellites * Elecnor Deimos, a Spanish aerospace company * ...

. Both moons orbit

in an eastward ( prograde) direction; however,

has an orbital period of 1.23 Martian sidereal days, making it supersynchronous, whereas Phobos has an orbital period of 0.31 Martian sidereal days, making it 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

due to their orbital periods in relation to the rotational period of the planet. Retrogradation1

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 An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

and Kuiper Belt objects (including

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

) exhibit apparent retrogradation. 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.Carrol, Bradley and Ostlie, Dale, ''An Introduction to Modern Astrophysics'', Second Edition, Addison-Wesley, San Francisco, 2007. pp. 4 Inner planets

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

and Mercury appear to move in retrograde in a similar mechanism, but as they can never be in 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 retrogradation 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 Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...

ellipse. The center of the retrograde motion occurs at the planet's opposition which is when it is exactly opposite the Sun. This 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; if it occurs around the Winter Solstice when the Sun passes lowest in the sky, the retrograde will pass high in the sky at midnight, and conversely it will pass low in the sky when retrograde occurs around the Summer 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. Apparent retrograde motion of Mars in 2003

Apparent retrograde motion of Mars in 2003

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 model In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under most geocentric models, the Sun, Moon, stars, an ...

of the Solar System proposed by Apollonius in the third century BCE, retrograde motion was explained by having the planets travel in deferents and epicycles. It was not understood to be an illusion until the time of

Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulat ...

, although the Greek astronomer Aristarchus in 240 BCE proposed a heliocentric model for the Solar System.

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

's drawings show that he first observed

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...

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 Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

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 A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...

From Mercury

From any point on the daytime surface of 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 In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an obje ...

.Strom, Robert G.; Sprague, Ann L. (2003). ''Exploring Mercury: the iron planet''. Springer. . 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 Mercurian day.

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