The motion of planets in elliptical orbits is governed by the gravitational force exerted by the central body they orbit, such as the Sun in the case of the planets in our solar system. When a planet is in an elliptical orbit, its motion is influenced by two factors: the gravitational force pulling it toward the central body and its own inertia.
As a planet moves along its elliptical orbit, it experiences a varying gravitational force due to the changing distance from the central body. At the closest point of the orbit (the perihelion), the gravitational force is stronger, causing the planet to accelerate. This acceleration changes the planet's direction, causing it to turn around and start moving away from the central body.
As the planet moves away from the central body, the gravitational force weakens, gradually slowing down the planet's motion. Eventually, at the farthest point of the orbit (the aphelion), the gravitational force is at its weakest. However, the planet still possesses its inertia, which carries it forward.
Due to the interplay between gravitational attraction and inertia, the planet continues its motion along the elliptical orbit. The gravitational force from the central body acts as a centripetal force, continuously pulling the planet inward and preventing it from flying off into space. The planet keeps moving in its elliptical path, with the gravitational force and inertia balancing each other.
This combination of gravitational force and inertia ensures that the planet repeatedly turns around at the apex of its elliptical orbit and continues its orbit, tracing out a closed path around the central body.