The energy of the solar system is in part the kinetic energy of those parts of it which are in motion--planets, planetesimals,[9] and satellites. This quantity of energy is enormously great. In the case of our earth it is 1/2(_mv_^2), _m_ being the mass of the earth, and _v_ its velocity. Translated into numerical symbols we find this quantity almost inconceivable. The greater part of this energy is _unavailable_, that is, it can undergo no transformations. But because the earth is in rotation at the same time as it revolves round the sun, and because the moon revolves round the earth, there are tides in the watery and atmospheric envelopes of the earth. The energy of the tides is the kinetic energy of water or air in motion, and we can employ this energy in the production of transformations, and it is therefore _available_. But well-known investigations have shown that the tides produce friction, and that the period of rotation of the earth is slowly becoming greater. Ultimately the earth will rotate on its own axis in the same time that it revolves round the sun--then a year and day will be of the same length. When that occurs, the sun, earth, and moon will be in equilibrium, and tidal phenomena due to the sun will cease. The kinetic energy of the earth, rotating once in 24 hours is obviously greater than its kinetic energy when rotating in the period which will then be its year. What has become of the balance? It has been transformed into the mechanical friction of the tides against the surface of the earth,[10] and this friction has been transformed into low-temperature heat, and this heat has been radiated off into space. [9] Meteorites, cosmic dust, and other small particles moving in the solar system within influence of the sun’s gravity. [10] Not entirely, of course, but whatever be the transformation it ends in heat production.