Positions of the planet which are closest to, and farthest from, sun are called its perihelion and aphelion. Polar equation of the ellipse with directrix x= -d can be written in form r= a(1-e^2) / (1-e cos theta). Illustrate that perihelion distance from the planet to sun is a(1 - e) and aphelion distance is a(1 + e).