Fundamentals of Geophysics

The shape of the Earth is determined primarily by two forces. Gravitational attraction, directed toward the center of the planet, results in an almost spherical shape. The Earth’s rotation produces a centrifugal force away from the rotation axis that flattens the sphere to a rotational ellipsoid. Differences in internal mass distribution produce bumps and hollows in the ellipsoid, forming a smooth but uneven surface called the geoid. Gravity at any place acts in the vertical direction, which is everywhere perpendicular to the local geoid. We explain how the gravitational attractions of the Moon and Sun deform the Earth’s free surface, creating tides in the oceans and in the solid planet. The Earth’s ellipsoidal shape allows the gravitational attractions of other planets to modulate both its rotation and its orbit cyclically with periodicities of 21,000 to 405,000 years, which are correlated to long-term climatic changes.