What is the cause of day and night?

When the sunlight strikes the surface of earth, the opposite side of the surface is dark. Everyone on earth konws that the cause of day and night is a result of earth's rotation.

But, no one exactly knows the cause of planet's rotation.

Galileo(?), Newton, Huygens analyzed the uniform circular motion, but their understandings of circular motion have had a limits.

I think that physics has to be able to explain theoretically the planet's rotation. If we speculate about the formular of circular motion, it has only analyzed planet's revolution. The planets have a motion of rotation and revolution at the same time. Their analysis are clearly wrong.

Using the length theory, this diffcult problem is easily solved.

If the circumference of circle (A) is 2*pi*r

During the time t, if the dispalcement of the planet moving in a circle is l

then 2*pi*r is a absolute length (spacelength),

l is a length traveled by the planet.

therefore,

From the above equation, we can know that the centripetal force is influenced not only by its radius, but also by its small angle.