Which tide theory assumes Earth is covered by an ocean of great and uniform depth capable of instantaneous response to the gravitational and inertial forces of the sun and moon?

• A. Dynamic Theory of Tides
• B. Mean Sea Level
• C. Ebb Current
• D. Equilibrium Theory of Tides

Answer: (D)

The concept being tested is how tides can be modeled when you treat the ocean as an idealized, endless sheet that responds instantly to the Sun’s and Moon’s gravitational forces. In the equilibrium theory, the ocean depth is considered very large and uniform, so water height adjusts immediately to the tidal potential. This means bulges form directly under and opposite the celestial bodies, because at each point the water height is determined by the instantaneous gravitational pull, without accounting for motion, inertia, or bottom geometry.

This is why it’s the best description for this scenario: it strips away dynamics like wave propagation, friction, and Coriolis effects to focus on the immediate gravitational forcing, giving a simple, static picture of tides. In contrast, the dynamic theory includes how tides propagate as waves and interact with the coastlines and seafloor, mean sea level is just a reference height, and ebb current is a phase of the tidal cycle, not a theory of the tide’s cause.