Gravity Variations

changes in gravity at a given point on the earth in the course of time. There are both periodic and secular gravity variations.

Periodic gravity variations are caused primarily by the attractions of the sun and moon, which change the gravity on the earth. They arise because of the earth’s rotation, as a result of which the relative positions of the observation point and the heavenly body change. Tidal deformations of the earth lead to a redistribution of its mass and a change in the distance between the observation point and the earth’s center, which further increases the amplitude of periodic gravity variations by approximately 1.2 times. The total value of gravity variations caused by the moon reaches 0.2 milligal and that caused by the sun reaches 0.1 milligal (1 gal = 1 cm/sec²). The basic cycles of periodic variations are semidiurnal and diurnal. As a result of the motions of the heavenly bodies and the earth’s poles, long-term changes in the angular velocity of the earth’s diurnal rotation, and other factors, small long-term gravity variations arise.

Secular gravity variations are caused by physical processes within the earth, such as changes in rock density and shifts of masses within the earth, and retardation of the earth’s rotation. It has not yet been possible to directly measure secular gravity variations. Only theoretical estimates, which show that the size of secular gravity variations is barely measurable, have been made. Lunar and solar gravity variations are accounted for in gravimetric field measurements, and tables and nomograms have been created for this. Continuous long-term observations of periodic gravity variations (extending over many months) aid the study of the earth’s internal structure and elasticity. Gravity variations are measured by means of highly accurate stationary gravimeters.