PURPOSE: To provide measurability for the transmitting speed of the gravitational force from the present location by accurately measuring the direction of the tidal power due to the attracting force of the moon, and thereby knowing the measuring position for the moon through utilization of the gravitational force.

CONSTITUTION: A pendulum (with known period) 20 at a standstill is displaced by the tidal power. Optical wave interferometers 21, 22 are installed at a right angle with the pendulum 20 at the appex, and the angle to the position of the moon with the light velocity corrected has been known with high precision. The displacement of this pendulum 20 as the output of the interferometers 21, 22 in such a positioning varies in sine wave form substantially, and displacement related to the period of earth rotation, revolution, moon revolution, etc., can be observed. From these direct pieces of data, only the data about the period due to the moon revolution is extracted mathematically using Fourie transform. This is followed by corrections for the phase delay due to the period of the pendulum 20 and angle displacement of the rotation axis of the earth relative to the revolution plane of the moon. If two displacement data values are subjected to vector synthetization, the moon direction due to the tidal force is defined.