# How is Earth's gravity modelled?

The Earth does not have a defined geometric shape. Then how do space scientists who want to put a satellite in orbit model the gravitational field of Earth? Do they assume a single object with minute variations from sphericity, or as a combination of multiple objects of defined shapes? Or, are there different ways of doing it right?

It's expressed as a bunch of coefficients of spherical harmonics. The monopole term is $$GM$$ the standard gravitational parameter, there is no dipole moment since the center is defined as the Earth's center of mass, and the quadrupole moment is expressed using $$J_2$$ and $$J_{22}$$, the first of which represents Earth's oblateness which is roughly a 0.1 % effect in low Earth orbit and is what makes Sun-synchronous and Molniya orbits possible.