* The conservation of angular momentum. As a planet orbits the Sun, its angular momentum remains constant. This means that the product of its mass, velocity, and distance from the Sun must be the same at all points in its orbit. As the planet moves closer to the Sun, its velocity must increase in order to maintain the same angular momentum. This causes the planet to move in an elliptical orbit, with the Sun at one focus of the ellipse.

* The gravitational force between the Sun and the planet. The gravitational force between the Sun and the planet is always pulling the planet towards the Sun. This force is inversely proportional to the square of the distance between the Sun and the planet. As the planet moves closer to the Sun, the gravitational force becomes stronger and pulls the planet in more quickly. This causes the planet to accelerate as it moves closer to the Sun.