An object’s ORBIT is the path it takes around another, more massive object in space. Each of the nine planets in the Solar System is held in orbit by the Sun’s gravitational pull. However, the planets do not orbit the Sun in circular paths but in elliptical (oval) ones. Orbit lengths, and the orbital period (the time it takes a planet to complete one orbit) increase with successively distant planets.
Orbit is the path of a body as it moves under the influence of a second body. An example is the path of a planet or comet as it moves around the Sun. Planets and satellites that orbit other bodies trace out a path called an ellipse. An ellipse is a closed curve of oval shape wherein the sum of the distances from any point on the curve to two internal focal points is constant. In everyday life you probably just call this an oval. As shown in the picture below, an ellipse has a major axis and a minor axis.
The major axis is always at least as long as or longer than the minor axis. When both the major and minor axes are the same length, this is a special case of an ellipse we commonly call a circle. Therefore, orbiting bodies can also trace out a circular path. Although a circle is a special type of ellipse, people commonly refer to satellite and planetary orbits as either circular or elliptical. The orbital period is the time to complete one full orbit.
After ten years of work, Kepler discovered the relationship between the time it takes a planet to orbit the Sun and its distance from the Sun. Kepler’s third law says that the square of the orbital period of a planet is directly proportional to the cube of the average distance of the planet from the Sun. Mathematically, this is given by the ratio T^2/r^3 and applies to all planets. The practical application of Kepler’s third law is to calculate the radius of a planet’s orbit by observation of that planet’s orbital period.