The Earth/Moon System

The Earth/Moon System is an elegant example of the amazing consequences of the parts of a system interacting. This system is about as simple as one can imaging: two bodies isolated in space, with only one force acting between them, gravity. One would not expect anything interesting from such a simple system. Yet, the fact that we exist is strong evidence that this system is not that simple.

The forces imposed on the Earth/Moon system by the Sun are much weaker than the force acting between these two bodies. Therefore, for the sake of this discussion, we can neglect the effects of the Sun and all other external forces on this system and treat the Earth/Moon as a closed system. However, someday in the far, far future this will not be the case, as the following discussion will reveal.

The result of the gravitational force between the Earth and the Moon are the tides. The tides cause deformation of the solid part of both bodies and of course the oceans of the Earth. The Earth spins on its axis and the Moon orbits the Earth. Both the spinning and orbiting are expressions of angular momentum. Like a spinning wheel these bodies tend to spin at a constant rate unless some force acts on them. This is a basic characteristic of nature. A body in motion will remain in motion unless an outside force acts on it (Newton’s 2^nd Law). The energy of motion, in this case the energy of spinning is gradually being dissipated. The tides are gradually using the energy of spinning to break down rocks on the beach, to move sand, to make frictional heat, and to produce internal heat and friction inside the Earth and Moon. This loss of the spinning energy is measurable. The Earth’s spin is slowing down (0.002 seconds/century), the days are getting longer and the Moon is gradually moving away from the Earth (3ft/century), the months are getting longer!

Ages ago (3-4 billion years) the Moon was much closer to the Earth (19-30 thousand km compared to the current 384 thousand km) and the tides were stupendous. Imagine the Moon 4,000 times closer than it is today, and the days were only 18 hours long! Do you think that the full Moon is impressive today? Think of what it would have looked like then! Wow!

 Another result of the interaction of the Earth and Moon is that the Moon has lost all of its spin. The tides within the Moon have dissipated the rotational energy to the final state; the same side of the Moon always faces the Earth. In addition, one can predict the final state of the Earth/Moon system- the day and the month will be equal at 1320 hours, or 55 today days long. The Earth/Moon system is changing as a result of the two system components, the Earth and the Moon, interacting with each other, as is true of most systems. Amazing!

There are many amazing consequences resulting from the interaction of the components of this system. The Earth/Moon system is spectacular considering the relative sizes of these two bodies. The Earth/Moon system is really a binary planet. In fact, these two celestial bodies orbit each other (around a point about 4670 km from the Earth's center). As a result of the Moon’s huge size and the small distance between the Earth and Moon, the ocean tides on Earth are very large; these tides create a broad high energy, transition zone between the water-breathing world and the air-breathing world, that is, thousands of miles of beaches. These beaches present a very challenging environment for life on Earth. This zone tests new adaptations of life on Earth. An ocean creature being beached or isolated in a brackish tide pool faces a tremendous challenge to life. Only a few live, those gifted with the ability to survive exposure to air, drying heat and direct sunlight. There is little doubt that having this large, hugely challenging environment has sped up biological change on this planet. Thank goodness for a big Moon! A wise man never looses a childlike wonder of the the world.

(system principles)