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 NOT rotating with respect to the observable universe) 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 of the first something 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 in an unqualified (i.e., physical) sense, but from the Earth's perspective the Sun "geometrically orbits" the Earth. Now 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, consider the Pluto system (overwhelmingly dominated in mass by Pluto and its major moon Charon) which is illustrated in the animation (which does NOT show the 4 minor moons of Pluto). Exactly speaking, Pluto and Charon orbit the Pluto system center of mass (which is very close to Pluto and is the center of the circular orbit in the animation) or, much less exactly speaking, Charon orbits Pluto. However, clearly Pluto only geometrically "orbits" Charon since Charon is relatively far from the Pluto system 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.
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