Earth's Shape and Reference Ellipsoid

Earth's True Shape and Its Consequences

Earth's rotation causes it to bulge at the equator and flatten at the poles, creating an oblate spheroid, also known as an ellipsoid. The equatorial diameter is about 12,756 km, while the polar diameter is slightly less, around 12,714 km. This subtle difference is crucial for precise navigation.

The Reference Ellipsoid in Navigation

To simplify calculations, geodesists use mathematically defined ellipsoids to approximate Earth's shape. These reference ellipsoids allow for consistent latitude, longitude, and elevation measurements. The World Geodetic System 1984 (WGS-84) is the standard reference ellipsoid for global navigation, including all aviation charts and GPS systems. Other ellipsoids exist, but WGS-84 is required for aviation coordinates.

Geoid vs Ellipsoid

The geoid represents the shape the oceans would form under only gravity and Earth's rotation—an irregular, undulating surface reflecting Earth's mass distribution. In contrast, the reference ellipsoid is smooth and regular, making it practical for mapping and navigation. The difference between the geoid and ellipsoid can be several tens of meters, which is significant for precise altitude and position work.

Practical Navigation Implications

• At the equator, 1 minute of longitude equals roughly 1 nautical mile (NM) on the WGS-84 ellipsoid.
• 1 NM is defined as 1.852 km, which is also the average distance of 1 minute of latitude.
• The actual ground distance represented by a given chart length varies with latitude due to Earth's curvature.

Earth's Rotation and Navigation

Earth rotates around its own axis, which is tilted about 23.5° relative to the plane of its orbit (the ecliptic). This tilt affects celestial navigation and the apparent movement of the Sun and stars, which is foundational for traditional navigation techniques.