English: Illustration of the impact factor b for a blue particle moving in a straight line. The impact parameter is the distance of closest approach to a given center, here shown in yellow. The radius r obeys the equation b = r cos (θ₁ - θ₀), where θ₀ is the angle of the impact parameter direction (shown in red) relative to the x-axis (shown in thin black). This image is intended for the article on Newton's theorem of revolving orbits, but may have other applications.