Apparent retrograde motion is the apparent motion of a

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

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 and history

Apparent retrograde motion of Mars in 2003

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 spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

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

adjective An adjective (abbreviations, abbreviated ) is a word that describes or defines a noun or noun phrase. Its semantic role is to change information given by the noun. Traditionally, adjectives are considered one of the main part of speech, parts of ...

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 and south celestial latitude of the ecliptic – the apparent path of the Sun across the celestial sphere over the course of the year. Within this zodiac ...

, 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 Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...

in 150 AD believed that the Earth was the center of the

Solar System The Solar SystemCapitalization 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 "Sola ...

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''. 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 scientific theories, superseded description of the Universe with Earth at the center. Under most geocentric m ...

of the Solar System proposed by

Apollonius Apollonius () is a masculine given name which may refer to: People Ancient world Artists * Apollonius of Athens (sculptor) (fl. 1st century BC) * Apollonius of Tralles (fl. 2nd century BC), sculptor * Apollonius (satyr sculptor) * Apo ...

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 (19 February 1473 – 24 May 1543) was a Renaissance polymath who formulated a mathematical model, model of Celestial spheres#Renaissance, the universe that placed heliocentrism, the Sun rather than Earth at its cen ...

, 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), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...

's drawings show that he first observed

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

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 List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

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 (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

Apparent motion

From Earth

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

east East is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth. Etymology As in other languages, the word is formed from the fact that ea ...

west West 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 Romance langu ...

, 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

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

International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...

. 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 Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, which has two natural satellites, Phobos and Deimos. Both moons orbit

in an eastward ( prograde) direction; however, Deimos 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. 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—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

and

Kuiper Belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...

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 Su ...

) 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.Carrol, Bradley and Ostlie, Dale, ''An Introduction to Modern Astrophysics'', Second Edition, Addison-Wesley, San Francisco, 2007. pp. 4 File:Retrograde Motion.bjb.svg, As

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

(blue) passes a superior planet such as

(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 Inner planets

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

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 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 Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

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.

From Mercury

From any point on the daytime surface of Mercury when the planet is near

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

(closest approach to the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

), the Sun undergoes apparent retrograde motion. This occurs because, from approximately four Earth days before

until approximately four Earth days after it, Mercury's angular

orbital speed In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or ...

exceeds its angular rotational velocity.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.

References

External links