Question

I am curious as to what causes the Earth's spin, and why does the Moon not spin?

Asked by: Peter Granka

 Answer

Stars and planets form as a result of the gravitational collapse of accreting material. Any net translational motion of that initial material is accelerated as its radius of rotation decreases. This is due to the same conservation of angular momentum principle that makes skaters and divers spin more rapidly when they bring their arms closer to their bodies. In its extreme, this effect can make collapsed neutron starts spin many times each second, creating the regular beat of pulsars as observed from Earth.

Once an initial spin was established for the Earth, the same conservation principle says that its angular momentum will continue unless it can be transferred to another object. With no significant frictional forces to allow that to happen, the Earth can continue to spin indefinitely without any further prodding. (More on that later.)

It is incorrect to say that the Moon does not spin. It rotates on its axis in synch with its revolution around the Earth, keeping the same face always pointed toward us. If it did not rotate at all, we would see all of its surface over the course of one month's orbit. This condition was created by tidal forces between the Earth and Moon, gradually slowing its initial rotation until the current more stable situation evolved. This same 'tidal lock' condition has been observed for satellite closely orbiting other planets. Because of its elliptical orbit, a slight 'wobble' allows us to actually see slightly more than half of the Moon's surface over the course of a month.

Tidal forces, by the way, continue to affect the Earth/Moon system. They exert enough frictional force on the Earth so that some of its angular momentum IS being transferred slowly to the Moon. The net effect of that is to gradually slow down Earth's rotation and lengthen the day. Where is that energy and momentum going? The moon is gradually receding from the Earth. Both of these effects are small. The Moon is receding at a rate of less than 2 inches per year and the day is getting longer by one second every 67,000 years or so.

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

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