Orbit, in astronomy, is the path of an object whose motion is influenced by the gravitational attraction of another object. Often, orbit refers to the nearly circular path maintained by an object under the gravitational pull of a much larger object. The larger object is often called the primary, and the smaller one is the secondary. For example, Earth is a secondary orbiting the sun, its primary.
No secondary known to scientists orbits in a perfect circle. Instead, most orbits form a closed curve called an ellipse. In an elliptical orbit, the primary is not in the exact center. As a result, the distance from primary to secondary varies along the orbit. A secondary of Earth is nearest to Earth at a point called perigee. It is farthest at apogee. For a secondary of the sun, the nearest point is called perihelion, and the farthest point is aphelion.
A secondary is often described as if orbiting a fixed (motionless) primary. In actuality, both objects orbit a common center of mass, the point where their masses would balance. If the difference in mass is great, the center of mass lies close to the primary, so the primary appears not to move. If the masses are similar, however, the objects appear to orbit a point between them.
A secondary may gain enough speed to escape the pull of its primary, called escape velocity. Its orbit then becomes an open curve called a parabola. If a secondary moves faster than escape velocity, its orbit is a broader curve, called a hyperbola. Spacecraft leaving Earth orbit travel a hyperbolic path.
See also Ellipse; Planet (Orbits); Satellite, Artificial (Satellite orbits).
