Retrograde and direct motion

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As Earth passes a superior planet, such as Mars, the superior planet will temporarily appear to reverse its motion across the sky.

Direct motion is the motion of planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion. Retrograde motion is motion in the opposite direction. In the case of celestial bodies, such motion may be real, defined by the inherent rotation or orbit of the body, or apparent, as seen in the skies from Earth.

While the terms direct and prograde are equivalent in this context, the former is the traditional term in astronomy. Prograde was first seen in an abstract of an astronomy-related professional article in 1963 (J. Geophys. Res. 68, 4979).

[edit] Retrograde

The term Retrograde is from the Latin word Retrogradus--"backward-step". The affix Retro- meaning backwards and gradi step or to go. 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 canopy. In this context, the term refers to planets as they appear from Earth, to briefly stop and reverse direction at certain times, though in reality they perpetually orbit in the same uniform direction.

"Mercury in Retrograde" is a good example of the term used as a noun for retrograde motion. Retrograde is also sometimes used as an intransitive verb meaning to become, to appear, to behave --or appear to move-- in a retrograde fashion.

Although planets can sometimes be mistaken for stars as we observe the night sky, the planets actually change position from night to night in relation to the stars. Retrograde (to go backward) and Prograde (to go 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 but still used the terms Retrograde and Prograde to describe the movement of the planets in relation to the stars. Although we know today that the planets revolve around the sun, we continue to use the same terms in order to describe the movement of the planets in relation to the stars as we observe them 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. ^[1]

[edit] Apparent retrograde motion

T1, T2, ..., T5 - positions of Terra
P1, P2, ..., P5 - positions of a planet
A1, A2, ..., A5 - projection to celestial sphere

When we observe the sky, the Sun, Moon, and stars appear to move from east to west because of the rotation of Earth (so-called diurnal motion). However, orbiters such as the Space Shuttle and many artificial satellites appear to move from west to east. These are direct satellites (they actually orbit Earth in the same direction as the Moon), but they orbit Earth faster than Earth itself rotates, and so appear to move in the opposite direction. Mars has a natural satellite Phobos, with a similar orbit. From the surface of Mars it appears to move in the opposite direction to Earth's moon (Luna), even though both Phobos and Luna have direct orbits, because its orbital period is less than a Martian day, whereas Luna's orbital period (one month) is longer than a Terrestrial day. There are also smaller numbers of truly retrograde artificial satellites orbiting Earth which counter-intuitively appear to move westward, in the same direction as the Moon.

Apparent path of Mars in 2009-2010 relative to the constellation Cancer.

As seen from Earth, all the true planets appear to periodically switch direction as they cross the 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. 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, we periodically overtake them, like a faster car on a multi-lane highway. When this occurs, the planet we are passing 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. Inner planets Venus and Mercury appear to move in retrograde in a similar mechanism, though their retrograde cycles are also tied to their conjunctions with the Sun. The apparent retrograde motion is explained by the same mechanism as the outer planets. Asteroids and Kuiper Belt Objects (including Pluto) also exhibit apparent retrogradation.

Interestingly, 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. This apparent backward motion is created when the orbit of the Earth takes it past an outer planet. 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.

The more distant planets retrograde more frequently:

Mars retrogrades for 72 days every 25.6 months.
Jupiter for 121 days every 13.1 months.
Saturn for 138 days every 12.4 months.
Uranus for 151 days every 12.15 months and
Neptune for 158 days every 12.07 months.

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

Apparent retrograde motion of Mars in 2003 as seen from Earth

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 of the solar system, 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. The accompanying map shows the retrograde motion of Mars for the year 2003, which occurs against the background of the constellation Aquarius.

[edit] Examples

Some significant examples of retrograde motion in the solar system:

Venus rotates slowly in the retrograde direction.
The moons Ananke, Carme, Pasiphaë and Sinope all orbit Jupiter in a retrograde direction. Many other minor moons of Jupiter orbit retrograde.
The moon Phoebe orbits Saturn in a retrograde direction, and is thought to be a captured Kuiper belt object.
The moon Triton orbits Neptune in a retrograde direction, and is also thought to be a captured Kuiper belt object.
The planet Uranus has an axial tilt of 98°, which is near 90°, and can be considered to be rotating in a retrograde direction depending on one's interpretation.

[edit] Retrograde motion in astrology

In astrology, the retrograde motion of the planets () was traditionally thought to be unlucky or inauspicious, as it went against the 'natural' order of movement, and a planet which was retrograde at the time of birth was considered a weak spot in the natal chart.^[2]

Retrogradeness is accompanied by inversion ("passivation") of the planet's quality.

Example 1:
Mercury is a symbol of communication.^[3]

The direct Mercury (the ruler of Gemini) is active and symbolises speaking.

The retrograde Mercury (the ruler of Virgo) is passive and symbolises listening.

Example 2:
Mars is a symbol of fighting; the sigil of Mars ♂ is a spear and a shield.^[4]

The direct Mars is active and symbolises offence (the spear).

The retrograde Mars is passive and symbolises defence (the shield).

In classical astrology, the influence of a planet is a function of its geocentric motion, which can be proper or improper. Only the inferior planets—Mercury and Venus—can become properly retrograde. The retrogradeness of the superior planets is improper (that is why the retrograde Pluto does not qualitatively equal Mars and does not co-rule Aries):

The actual reason of the apparent retrogradation is, indeed, different in the case of the inferior and of the superior planets. As regards the former, when they are traversing the inferior portion of their orbits, or are nearly between the sun and the earth, their heliocentric eastward motion becomes, of course, as seen from the earth, westward, or retrograde; by the parallax of the earth's motion in the same direction this apparent retrogradation is diminished, both in rate and in continuance, but is not prevented, because the motion of the inferior planets is more rapid than that of the earth. The retrogradation of the superior planets, on the other hand, is due to the parallax of the earth's motion in the same direction when between them and the sun, and is lessened by their own motion in their orbits, although not done away with altogether, because their motion is less rapid than that of the earth.^[5]