The true shape of the Earth is best described as an oblate spheroid, which is a slightly flattened sphere. The Earth is not a perfect sphere but is bulged at the equator and flattened at the poles. This shape is often approximated as an ellipsoid.
The Earth's shape is influenced by its rotation, which causes a centrifugal force that pushes material away from the axis of rotation. As a result, the equatorial region bulges outward, and the polar regions are flattened. This shape is known as the geoid, which represents the shape the Earth would take if it were entirely covered by an ocean at rest, influenced only by gravity.
To simplify calculations and measurements, the Earth's shape is often approximated as an ellipsoid, which is a mathematical representation of a slightly flattened sphere. This ellipsoidal model, known as the reference ellipsoid, provides a good approximation of the Earth's shape over large areas. Different reference ellipsoids have been defined to best fit different regions or continents, such as the WGS84 ellipsoid commonly used in global navigation systems.
However, it's important to note that the Earth's surface is not perfectly smooth due to irregularities in topography, such as mountains and valleys, and variations in the distribution of mass within the Earth. To account for these irregularities and create a more accurate representation of the Earth's shape, advanced measurements and modeling techniques are used to create a geoid model that best fits the actual shape of the Earth's surface.
In summary, the Earth's true shape is an oblate spheroid, but it is often approximated as an ellipsoid for practical purposes, while the geoid represents a more detailed model accounting for irregularities in the Earth's surface.