Editing Perturbation theory (section)

==Prototypical example==
The earliest use of what would now be called ''perturbation theory'' was to deal with the otherwise unsolvable mathematical problems of [[celestial mechanics]]: for example the [[orbit of the Moon]], which moves noticeably differently from a simple [[Kepler's laws of planetary motion|Keplerian ellipse]] because of the competing gravitation of the Earth and the [[Sun]].<ref>[https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.70.589 Martin C. Gutzwiller, "Moon-Earth-Sun: The oldest three-body problem", Rev. Mod. Phys. 70, 589 – Published 1 April 1998]</ref>

Perturbation methods start with a simplified form of the original problem, which is ''simple enough'' to be solved exactly. In [[celestial mechanics]], this is usually a [[Kepler's laws of planetary motion|Keplerian ellipse]]. Under [[Newton's law of universal gravitation|Newtonian gravity]], an ellipse is exactly correct when there are only two gravitating bodies (say, the Earth and the [[Moon]]) but not quite correct when there are [[three-body problem|three or more objects]] (say, the Earth, [[Moon]], [[Sun]], and the rest of the [[Solar System]]) and not quite correct when the gravitational interaction is stated using formulations from [[general relativity]].