Low head hydro power with speed and direction as caused by the gravitational forces of the sun and the moon, and centrifugal and inertial forces on the Earth's waters. Due to its proximity to the earth, the moon exerts roughly twice the tide raising force of the sun. The gravitational forces of the sun and moon and the centrifugal/inertial forces caused by the rotation of the earth around the center of mass of the earth-moon system create two "bulges" in the Earth's oceans: one closest to the moon, and the other on the opposite side of the globe.(CNW Group, 2008). This kind of energy
Orbital station-keeping caused by the oblateness of the Earth is a desirable feature that is part of the mission design but the inclination change caused by the gravitational forces of Sun/Moon is undesirable. For geostationary spacecraft the inclination change caused by the gravitational forces of the Sun & Moon must be counteracted by a rather large expense of fuel, as the inclination should be kept sufficiently small for the spacecraft to be tracked by a non-steerable antenna. For spacecraft in low orbits the effects of atmospheric drag must often be compensated for. For some missions this is needed simply to avoid re-entry;