What is Orbital resonance?
[[File:Mercury's orbital resonance.svg|thumb|Diagram showing how [[Mercury (planet)|Mercury]]'s orbital period and [[Rotational energy|rotational]] period are locked in a 3:2 resonance.]] '''Orbital resonance''' is a [[resonance]] of two [[orbit]]ing bodies exerting a regular, periodic [[gravitational]] effect on each other. Their [[orbital period]]s may be related by a ratio of two small [[integers]]. It is caused by the changing [[gravitational force]]s of bodies which go round each other. The [[stability]] of the [[Solar System]] was first investigated by [[Laplace]], and there is still much that is not known about it.<ref>Murray C.D. & Dermott S.F. 1999. ''Solar System dynamics''. Cambridge University Press. {{ISBN|0-521-57597-4}}.</ref> As a [[satellite (natural)|satellite]] goes round a [[planet]] or two [[stars]] go round each other, the gravitational forces can change, sometimes hugely. This is partly because orbits are usually [[ellipse]]s, not circular, and so the forces change accordingly. Also, the planets and stars are usually not [[spherical]]. They spin, and vary in their degree of [[oblateness]]. This also changes the forces on an orbiting body. In particular, the forces may be [[stability|unstable]], so the smaller partner may change until the forces are stable (do not change with time).<ref>Malhotra, Renu 1998. Orbital resonances and chaos in the Solar System. In ''Solar System formation and evolution'', ASP Conference Series. '''149'''. [http://www.lpl.arizona.edu/people/faculty/malhotra_preprints/rio97.pdf preprint] {{Webarchive|url=https://web.archive.org/web/20131211165545/http://www.lpl.arizona.edu/people/faculty/malhotra_preprints/rio97.pdf |date=2013-12-11 }}.</ref> Satellites often end up with one face towards their planet, because that is the most stable position ([[tidal locking]]). There are other stability effects. Gaps in [[Saturn's rings]] are caused by the particles shifting into more stable positions. In the rings of [[Saturn]], the [[Cassini Division]] is a gap between the in