Retrograde motion is the apparent motion of a planet to move 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 word
retrogradus – "backward-step", the affix retro- meaning "backwards" and gradi to 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 stop briefly and reverse direction at certain times though in reality, of course,
we now understand that they perpetually orbit in the same uniform direction.
"Mercury in retrograde" is an 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 one observes the night sky, the planets actually change position from night
to night in relation to the stars. Retrograde and prograde 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 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 it is called retrograde.
Apparent motion
From Earth When we observe the sky, the Sun, Moon, and
stars appear to move from east to west because of the rotation of Earth. However, orbiters
such as the Space Shuttle and many artificial satellites appear to move from west to east.
These are direct satellites, but they orbit Earth faster than Earth itself rotates, and
so appear to move in the opposite direction of the Moon. Mars has a natural satellite
Phobos, with a similar orbit. From the surface of Mars it appears to move in the opposite
direction because its orbital period is less than a Martian 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.
As seen from Earth, all the other planets, asteroids and all objects in our solar system
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. Asteroids and Kuiper
Belt Objects 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. 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, and are thus unobservable in the
Sun's glare and because the planets are in their "new" phase, with mostly their dark
sides toward Earth; they occur in the transition from morning star to evening star.
The more distant planets retrograde more frequently, as they don't move as far 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 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.
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 proposed by Appolonius 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, although the Greek
astronomer Aristarchus in 240 BCE proposed a heliocentric model for the solar system.
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.
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.
From Mercury At certain 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. This apparent retrograde motion of the Sun
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. Mercury's elliptical orbit is farther from circular than that of any other planet
in our solar system, resulting in a substantially higher orbital speed near perihelion.
See also Retrograde and prograde motion
Hipparchus Ptolemy
Shen Kuo Spherical astronomy
Wei Pu References
External links Composite photograph of the 2009/2010 retrograde
motion of Mars Animated explanation of the mechanics of a
retrograde orbit of a planet, University of South Wales
NASA: Mars retrograde motion