Caption: "An animation showing an oblique view (more exactly a view at high inclination) of the orbits of Pluto and Charon. Note Pluto and Charon are mutually tidally locked to each other, and so always turn the same side to the other. Charon is massive enough that the center of mass (AKA barycenter) of the Pluto system (which includes 4 other small moons which have little effect on the location of the center of mass) lies outside of Pluto, and thus Pluto and Charon are sometimes considered to be a binary system." (Somewhat edited.)
Orbit Definition and What Orbits What
An orbit in general is just a trajectory relative to an inertial frame (usually a true free-fall frame) of an astronomical object, spacecraft, etc. under the gravitational force aside from small astrophysical perturbations NOT due to gravity.
However, when we say something orbits something, we usually mean that the 1st something goes around a center of mass (AKA barycenter in celestial mechanics jargon) defining an inertial frame closely located to the 2nd something with the angular motion measured relative to the observable universe.
This is a "physical" orbit as opposed to what is called a "geometrical orbit" which is just a rotation around any point taken as an observation point.
For example, the Earth orbits the Sun, but from the Earth's perspective the Sun "geometrically orbits" the Earth. True the geometrical "orbit" of the Sun is relative to the observable universe, but the Earth is NOT near the center of mass of the Earth-Sun system: the Sun is.
For another example, the Pluto-Charon system which is illustrated in the animation. We can say that Pluto and Charon orbit their mutual center of mass or less precisely that Charon orbits Pluto. However, clearly Pluto only geometrically "orbits" Charon since Charon is relatively far from their center of mass.
Credit/Permission: ©
Stephanie Hoover (AKA User:StephHoover),
2013 /
CC BY-SA 1.0.
Image link: Wikimedia Commons:
File:Pluto-Charon System.gif.
Local file: local link: orbit_defined.html.
File: Orbit file:
orbit_defined.html.