Editing Reaction (physics) (section)

===Gravitational forces===
[[File:orbit4.gif|thumb|right|Two bodies similar to the [[Sun]] and [[Earth]], i.e. with an extreme difference in mass – the red + marks the barycenter ]]
The [[Earth]], among other [[planet]]s, orbits the [[Sun]] because the Sun exerts a gravitational pull that acts as a [[centripetal force]], holding the Earth to it, which would otherwise go shooting off into space. If the Sun's pull is considered an action, then Earth simultaneously exerts a reaction as a gravitational pull on the Sun. Earth's pull has the same amplitude as the Sun but in the opposite direction. Since the Sun's [[mass]] is so much larger than Earth's, the Sun does not generally appear to react to the pull of Earth, but in fact it does, as demonstrated in the animation (not to precise scale). A correct way of describing the combined motion of both objects (ignoring all other celestial bodies for the moment) is to say that they both orbit around the [[center of mass]], referred to in astronomy as the [[barycenter]], of the combined system.