All planets, comets and asteroids orbiting a central sun have elliptical orbits. Thus, they all have a closest (aphelion) and a farthest (perihelion) point from their sun. The planetary oscillation is the difference between these two points.
See Also
8.18 - Oscillation
8.19 - Law of Corporeal Oscillations
8.2 - Oscillation versus Vibration
8.20 - Law of Sympathetic Oscillation
8.27 - Law of Sympathetic Oscillation
Gravitation
Gravity
Law of Corporeal Oscillations
Oscillation
Part 13 - Rotation from Vibration and Oscillation
Part 22 - Solar Rings and Planetary Formation
Planetary Vibration
Sympathetic Oscillation
Sympathetic Planetary Oscillation