In addition to the geographical anomalies of gravity there exist, in effect, periodic anomalies of gravity with time at given geographic positions on the earth. The latter phenonenon is associated with the attractive forces of the sun and moon. The pure attractive forces of these bodies is offset, so far as the earth as a whole is concerned, by the centrifugal forces associated with the motion of the earth in its curved orbit through space. However, slight positive and negative forces exist at the earth's surface due to the combination of these two phenomena. The magnitude of these forces are extremely small relative to gravity, and cannot be detected in ordinary pendulum experiments. The forces, however, are important in connection with their effect on the oceans due to the fact that their period is in close accord with the natural period of oscillation of the ocean basins. Because of this quasi-resonance effect we refer to these net astronomical forces at the earth's surface as the tide producing or simply tidal forces.