deflection of the vertical

The deflection of the vertical refers to the angle between the local direction of gravity and the normal to a reference ellipsoid. This phenomenon is a key concept in geodesy, the science of measuring and understanding Earth's shape, orientation in space, and gravity field.1

The geoid is a model of Earth's figure that aligns with mean sea level. It's an equipotential surface, meaning gravity's potential is constant across it. However, due to uneven mass distribution within Earth, the geoid is irregular. The deflection of the vertical arises because the local gravity direction is perpendicular to the geoid, not necessarily to the simpler, smoother reference ellipsoid used for geometric calculations. This deflection is crucial for accurate astronomical determinations of latitude and longitude.12