Orbital Motion

Motion is a change in which an object rotates around an axis or revolves around another object. In physics, two types are distinguished: rotation, in which an object spins on its own axis, and revolution (or orbit), in which an object moves around another object. The bodies of the solar system exhibit both: each rotates on its own axis and also revolves around the Sun.

The term 'orbital motion' is commonly used to describe the motion of an orbiting body, but the technical term 'orbital inclination' is also employed, referring to the angle between the plane of an object's orbit and a reference plane.

Rotation

Every object has an axis around which it rotates. The time required to complete one full rotation is called the rotation period. On Earth, this period produces the alternation of night and day. The angle between the rotation axis and the plane of Earth's orbit around the Sun causes the change of seasons.

Orbital Revolution

Orbital revolution depends on gravitational force. As Kepler's laws describe, orbital paths are not circles but ellipses, with the Sun occupying one of the two foci. An orbiting body moves faster when it approaches the Sun and slower when it moves away.

Orbital Inclination

Orbital inclination is measured in degrees. In the solar system, the standard reference is the plane of Earth's orbit around the Sun (the ecliptic). Zero inclination means the orbit lies in the reference plane; inclination greater than 90 degrees indicates retrograde motion. For natural satellites orbiting Earth, inclination is measured relative to the equator.