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

Apparent retrograde motion is the motion of a planetary body 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. Prograde was first seen in an abstract of an astronomy-related professional article in 1963.^[1]

[edit] Etymology

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^[2].

"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 (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 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.^[3]

[edit] Apparent 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 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^[4]. 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^[4]. It was not understood to be an illusion until the time of Copernicus. The accompanying animated diagram shows the retrograde motion of Mars for the year 2003, which occurs against the background of the constellation Aquarius.

[edit] See also

• Actual Retrograde motion
• Hipparchus
• Ptolemy
• Shen Kuo
• Spherical astronomy
• Wei Pu

[edit] References

[edit] External links

• Animated explanation of the mechanics of a retrograde orbit of a planet, University of Glamorgan
• NASA: Mars retrograde motion