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Apparent Retrograde Motion

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Apparent Retrograde Motion - © Attention Deficit Disorder Prosthetic Memory Program

Apparent 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.

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 actual 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 it 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. This phenomenon also occurs on Mars, which has two natural satellites, Phobos and Deimos. Both moons orbit Mars 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 Mars due to their orbital periods in relation to the rotational period of the planet.

Apparent path of Mars in 2009–2010 relative to the constellation Cancer, showing its “opposition loop” or “retrograde loop”

The asteroid 514107 Kaʻepaokaʻawela has a retrograde orbit. Its apparent retrograde motion occurs at superior conjunction with the sun, as shown in this example in 2018.

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 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.[4] Inner planets Venus 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 center of the retrograde motion occurs when the body is exactly opposite the sun, and therefore high in the ecliptic at local midnight. 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 ellipse.

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

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