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Planetary Motions
Mercury and Venus, the inner planets, sometimes called inferior planets, speed around the sun more quickly than Earth while the outer, or superior, planets take longer to orbit. Planets appear to move west to east across the sky, relative to fixed, distant stars. Planetary motion in this forward direction is called prograde. When Earth passes a superior planet in orbit, the motion of the planet appears to change direction. This backward motion is called retrograde and also occurs when the swifter inferior planets pass Earth in its orbit. Retrograde motion is the movement of planets from east to west.
Opposition
Opposition describes the position of a planet relative to Earth and the sun. A planet in opposition is aligned with Earth and the sun, with Earth in the middle and the other planet and the sun on opposite sides of Earth. Planets in opposition appear high in the sky around midnight, when the viewer's position on Earth is turned away from the sun. A planet appears in opposition at a particular time during its orbit around the sun.
Sidereal Period
A planet's sidereal period is the amount of time it takes for the planet to orbit the sun. Earth's sidereal period is 365.25 days. Mars takes 1.88 years to complete its orbit. Sixteenth-century astronomer Copernicus determined that sidereal periods correlate to the distance from the sun; the length of sidereal periods increase in length based on the sequence of the planets. Mercury, the closest planet to the sun, has the shortest sidereal period of 0.241 years. Neptune, the farthest planet in our solar system, has a sidereal period of nearly 166 years. However, there is no way to determine when another planet has completed its journey around the sun. We can only see when the planet passes in front of us because Earth and fixed stars are our only reference points. To viewers on Earth, a planet's sidereal period is the time it takes for a planet to return to a particular point in the night sky.
Synodic Period
The synodic period of planets describes the time it takes for a planet to resume a particular position in the sky, relative to Earth and the sun. A synodic period is the time it takes for a planet in opposition to return to this position of alignment. For example, when Mars is lined up on the opposite side of Earth from the sun, its synodic period is the amount of time elapsed until it resumes opposition and once again lines up with Earth between it and the sun. For Mercury and Venus, planets that are closer to the sun than Earth, the synodic period is longer than the sidereal period. The synodic period for Jupiter, Saturn, Uranus and Neptune is shorter than their sidereal period.