This change in the size of the orbit of a satellite or planet is known as the eccentricity of the orbit, which eccentricity is constantly changing, being sometimes greater and sometimes less. We will look at this truth in its relation to the moon first, and then consider the same principle in its relation to the earth and other planets later on. For the purpose of illustration, we will consider the earth as being circled round the sun by the electro-magnetic Aether currents in a closed orbit, _A_ _B_ _C_ _D_, which forms a perfect ellipse, the sun occupying one of the foci _S_ (Fig. 28), the earth occupying a position in the orbit represented by point _C_, with the moon being circled round the earth by that planet's aetherial currents. As we have already seen in Art. 103, according to Kepler's Second Law, at this point the earth is furthest from the sun, being now at a distance of 94-1/2 millions of miles, and therefore its orbital velocity will be slowest at that part of its orbit. If it were absolutely at rest in space, and simply revolving on its own axis, then the result would be that the moon would be circled round the earth in an orbit _M_ _C_ _F_ which is perfectly circular in form; but, as the earth is being carried along slowly through space by the circulating Aether currents, this onward movement changes the circular orbit into an orbit of elliptic form. [Illustration: Fig: 28.] The eccentricity of the moon's orbit when the earth is at its aphelion, or furthest from the sun, is now at a minimum, for the simple reason that the earth is proceeding slowly through space, owing to the decreased kinetic energy of the aetherial currents at the increased distance. So that, at this point of the earth's orbit, the difference between the two axes of the moon's orbit will be the least, and its orbit at that point will be the nearest approach to that of a circle. But, as we have already seen, as soon as the earth leaves this part of its orbit, and begins to get nearer to the sun, it passes into a part of the aetherial medium possessing greater kinetic energy, with the result that its own velocity is accelerated. Now what is the effect of this increased acceleration of the earth on the eccentricity of the orbit of the moon?