Revolution with and without rotation

Scroll down for a detailed explanation.

The animations show two spheres revolving on a circular path around a centre. To illustrate the orientation of the spheres in space they are divided into a green hemisphere and a red hemisphere separated by a magenta line that can be considered the "axis" of the spheres.

The left animation shows revolution with rotation. While the sphere revolves around the circle centre it rotates around it: The axis of the sphere always points towards the centre of revolution and rotation. Different points on the surface of the sphere experience different centrifugal forces: Consider the two points on the axis of the sphere. The point closest to the centre of revolution and rotation moves on a smaller circle than the point on the opposite side of the sphere and thus experiences a smaller centrifugal force.

The right animation shows revolution without rotation. While the sphere revolves around the circle centre it does not rotate around it: The axis of the sphere always maintains the same orientation in space. As a result, all points on the surface of the sphere move on circles of identical radius: Consider again the two points on the axis of the sphere. The point with the green hemisphere on its left moves on a circle that is placed slightly below the yellow circle, while the point on the opposite side moves on a circle that is placed slightly above the yellow circle; but their radius is the same. As a consequence, in revolution without rotation all points on the surface of the sphere experience an identical centrifugal force, both in direction and in magnitude.

Both forms of movement are observed in the universe. An example for revolution with rotation (the left animation) is given by the moon, which revolves around the earth, always showing the earth the same side. An example of revolution without rotation (the right animation) is given by the earth, which revolves around the sun without changing the orientation of its axis.

(For simplicity the animations show situations where the axis of the sphere lies in the plane of revolution. The earth's axis is inclined with respect to the plane of revolution, but the principle remains unchanged. Note also that in addition to revolving around the sun without rotation, the earth spins around its axis. This rotation around its own axis is an entirely different issue and does not invalidate the findings about centrifugal force with respect to the earth's revolution around the sun.)

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