The acceleration due to Earth Gravity, lowercase, g is commonly stated to be equal to 9.8 meters/second2. When dealing with space-going objects with ballistic trajectories (such as ballistic missiles), using this assumption will often result in the prediction of incorrect trajectories.

This is because the Earth is appreciably oblate, the equatorial diameter being 42.77 km greater than the polar diameter. Changes in the gravity field caused by this oblateness creates an asymmetric potential in earth's gravitational field. The cyclical characterization of this perturbation requires a rather high level of degree and order in the spherical harmonic expansion representation of the gravity field in order to predict precise effects on ballistic trajectories or satellite orbits.

As a result, there exist a host of gravitation models for Earth, the complexity of which is dependent on the required navitational precision. These can range from treating the Earth as a perfectly smooth sphere, a perfectly smooth oblate spheroid, or a much more complex topographically generated empirical model of very high order.