Let a planet be moving in an elliptical orbit around the sun. Since the mass of the sun is very large than that of the planet, the sun will exert a gravitational force (attractive) on the planet.
Since the eccentricities are small and sun is more massive than the planets, the path can be assumed as approximately a circle. The centripetal force, exerted by the sun on the planet, is given by
Thus the areal velocity in circular orbit is constant.
