A planet revolves round the Sun by the action of the gravitational force between the sun and the planet. Depending up on the distance from the sun in the elliptic orbit, the speed of revolution of the planet changes.
The formula to calculate the linear speed of a planet in an elliptical orbit can be written as
v=GM(r2−a1)
From the above expression, it can be concluded that the linear speed of the planet is not constant during its revolution.