Orbit

In reality, an object orbiting another object actually orbits the center-of-mass location of the two objects. Typically, one of the objects is very much more massive than the other, so that the center of mass location very nearly coincides with that of the larger object. Examples include planets orbiting a more massive sun, or satellites orbiting a planet.

Actual orbits deviate from true ellipses owing to a number of factors:

• Other objects exerting gravitational forces in addition to the force of gravity that acts between the two objects
• Orbital precession due to general relativistic effects
• Deviation from spherical symmetry in the objects, for example the flattening of a star or planet due to its rotation, or deformations due to tidal effects

If the speed of the smaller-mass object equals or exceeds the escape speed, then it will follow a path that is approximately parabolic or hyperbolic, respectively, rather than orbiting the larger object.