• Sum of the distance from two special points inside the ellipse to any point is always the same
  • The two points are called its foci
  • Widest diameter is its major axis
  • The distance from the center to one end is the semi-major axis


  • Roundness is determined by how close the foci are
  • How "squashed" an ellipse is
    • A circle has an eccentricity of 0

Physics Astronomy