All planets, comets and asteroids in our solar system have approximately elliptical (a kind of non-circular) orbits. Thus, they all have a closest and a farthest point from the sun: a perihelion and an aphelion. Orbital eccentricity measures the flatness of the orbit. Any single revolution of a body around the sun is only approximately elliptical, because the precession of the perihelion prevents the orbit from being a simple closed curve such as an ellipse. This causes Milankovich cycles. 