A
rotation is a movement of an object in a
circular motion. A two-
dimensional object rotates around a
center (or
point)
of rotation. A
three-dimensional object rotates around a
line called an
axis. If the axis of rotation is within the body, the body is said to rotate upon itself, or
spin—which implies relative
speed and perhaps free-movement with
angular momentum. A circular motion about an external point, e.g. the
Earth about the
Sun, is called an
orbit or more properly an
orbital revolution.All rigid body movements are rotations, translations, or combinations of the two.
A Rotation is simply a progressive radial orientation to a common point. That common point lies within the axis of that motion. The axis is 90 degrees perpendicular to the plane of the motion. If the axis of the rotation lay external of the body in question then the body is said to Orbit. There is no fundamental difference between a “rotation” and a “orbit” and or "spin". The key distinction is simply where the axis of the rotation lay, either within or without a body in question. This distinction is and can be demonstrated in and for both “ridged” and “non ridged” bodies.
If a rotation around a point or axis is followed by a second rotation around the same point/axis, a third rotation results. The reverse (inverse) of a rotation is also a rotation. Thus, the rotations around a point/axis form a group. However, a rotation around a point or axis and a rotation around a different point/axis may result in something other than a rotation, e.g. a translation.
