Ancient astronomers devised names to identify particular positions of the planets relative to the Earth and Sun on the celestial sphere. This early system forms the basis for current definitions of planetary configurations. Between the Earth and Sun revolve Mercury and Venus; because of their smaller orbits they are called the inferior planets. As seen from the Earth and measured relative to the Earth-Sun line, Mercury and Venus appear to move counterclockwise around the Sun while swinging from one side of the Sun to the other. The angular distance (in degrees) any planet appears east or west of the Sun is called its elongation.
From a position closest to the Earth called inferior conjunction, when it is in line with the Sun, either of the inferior planets appears to move rapidly westward from the Sun, which causes its phase to change from new to crescent. When the inferior planet reaches its greatest angular distance west of the Sun, known as maximum western elongation, it is conspicuous in our skies as a "morning star" and its phase is quarter. Thereafter, the planet appears to reverse its course and move eastward back toward the Sun until its elongation is a minimum, a configuration known as superior conjunction. At this point, the planet is on the opposite side of the Sun from the Earth, and its phase is full. Leaving superior conjunction, an inferior planet continues to move eastward on its way toward its greatest angular distance east of the Sun, known as maximum eastern elongation. Here it is seen in the heavens as an "evening star" and its phase is also quarter. Next, the inferior planet moves back toward the Sun and inferior conjunction, completing its cycle of planetary configurations and moonlike phases.
Planets with orbits outside Earth's are called superior planets. To the ancients, the superior planets were the three naked-eye planets Mars, Jupiter, and Saturn. When a superior planet is nearest to us and also brightest, its configuration is known as opposition--opposite the Sun in the sky and visible throughout the night. Although the superior planet moves eastward relative to the stars, it lags behind the swifter Earth's motion and so appears to drift westward from the Earth-Sun line until it is 90o east of the Sun, at which point it is in eastern quadrature. Here it rises at noon and is an "evening star." Although north and south are absolute directions in space defined by the Earth's axis of rotation, east and west are a sense of rotation defined by the Earth's rotation from west to east. Continuing to drift eastward relative to the Earth-Sun line, the superior planet's elongation decreases; it is thus approaching the Sun. It arrives at a configuration known as conjunction, where the superior planet is on the opposite side of the Sun from Earth and will rise and set with the Sun. From here the superior planet passes through western quadrature, 90o west of the Sun. At this point the superior planet rises at midnight and is a "morning star." Finally, the superior planet returns to opposition, its cycle of configurations complete. As seen from the Earth, superior planets do not exhibit a cycle of moonlike phases as do the inferior planets.
The length of time for one orbit of a planet around the Sun is known as its sidereal period. It is the time taken to complete a 360o circuit around the sky relative to the stars. Unfortunately, there is no marker along a planet's orbit to indicate when it has completed a 360o revolution. From the Earth we actually observe what is called the synodic period--the time it takes a planet to return to a particular configuration with respect to the Earth-Sun line (such as from opposition to opposition). The synodic and sidereal periods differ because the Earth is advancing in its own orbit as a given planet revolves around the Sun. A simple mathematical relation, known since the time of the Greeks, allows us to calculate the sidereal period after measuring the synodic period.
Since Earth's orbital period is shorter than that of a superior planet, the Earth overtakes a superior planet and passes it. This occurs while the planet's configuration changes from western quadrature through opposition to eastern quadrature. During this period of passing, the planet appears to temporarily interrupt its normal eastward motion relative to the stars and move westward. This countermotion is known as retrograde motion, in which the superior planet executes a closed or open loop and then continues its usual path eastward relative to the stars. Relative to the Earth-Sun line, however, it is moving toward an area of the sky east of the Sun.
Now having surveyed the common Earth-Sky relationships and the regularity of the heavens, and remembering that the Greeks and other peoples of that period were well aware of these regularities, we can discuss the historical development of astronomy, leading up to the modern conception of the dynamics of planetary motion.