# post-Newtonian formalism

For situations in which gravity is very weak, general relativity and Newton's theory of gravity lead to very similar predictions for the motion of bodies and the propagation of light. Such situations can be described by starting out with the Newtonian description and then, step by step, adding correction terms that take into account the effects of general relativity. The post-Newtonian formalism is a method for performing those step-by-step corrections. As the correction terms are ordered in a systematic way (the largest effects are called "of first post-Newtonian order, 1pN", the next smallest ones of second order, and so on), the progression of ever smaller corrections is also called the post-Newtonian expansion.

The post-Newtonian formalism is used to describe planetary motion as well as the propagation of light within our solar system; it is also used to describe the relativistic effects within binary neutron star systems.

### Variants

- post-Newtonian expansions