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
Frozen orbit propellant. For most spacecraft, changes to orbits are caused by the oblateness of the Earth, gravitational attraction from the sun and moon, solar radiation pressure, and air drag. These are called "perturbing forces". They must be counteracted by maneuvers to keep the spacecraft in the desired orbit. For a geostationary spacecraft, correction maneuvers on the order of 40–50 m/s per year are required to counteract the gravitational forces from the sun and moon which move the orbital plane away from the equatorial plane of the Earth. For sun-synchronous spacecraft, intentional shifting of the orbit plane (called "precession") can be used