Which term describes the surface of the oceans that would exist under gravity and rotation alone?

• A. Geoid
• B. Ellipsoid
• C. Spheroid
• D. Datum

Answer: (A)

The surface described is the geoid. Imagine the Earth’s gravity field together with the rotation of the planet; the ocean’s surface would settle into an equipotential surface of that combined field. This means every point on that surface has the same gravitational potential when rotation is included, so the sea level would align with this surface if all currents, winds, and tides were removed. That undulating, irregular surface is what we call the geoid, and it corresponds to mean sea level extended through the continents. In contrast, an ellipsoid (or spheroid) is a smooth, mathematical approximation of Earth’s shape used as a reference surface for maps and surveys, not the true physical surface defined by gravity. A datum is a reference framework that uses a chosen surface (often an ellipsoid) plus a coordinate system; it’s a setup for measurements, not the actual sea surface.