Pole and Polar of an Ellipse

In geometry, the concepts of pole and polar are powerful tools used to analyze curves in polar coordinates. The pole of a curve is a point with specific geometric properties related to that curve, and the polar is a curve associated with a given point, often the pole. Let's delve into how these concepts apply to an ellipse.

Pole of an Ellipse:

The pole of an ellipse in polar coordinates is a point that corresponds to the center of the ellipse in Cartesian coordinates. Denoted as P, the pole has coordinates (h,k), where (h,k) is the center of the ellipse in Cartesian coordinates.

The pole is a fundamental point because the polar coordinates of any point on the ellipse can be expressed in terms of its distance from the pole (r) and the angle formed with the positive x-axis (θ). The relationship is given by:


where a and b are the semi-major and semi-minor axes, respectively.

Polar of an Ellipse:

The polar of a point with respect to the ellipse is a curve that represents all possible points for which the given point is the pole. For the ellipse, the polar equation is derived from the relationship mentioned above:

r(θ)=aba2sin2(θ)+b2cos2(θ)This equation defines the locus of points in polar coordinates for a specific pole, in this case, the center of the ellipse.

Special Cases:

  1. Circle: If the ellipse is a circle (a=b, the polar equation simplifies to r(θ)=a2. The pole would be at the center of the circle.

  2. Horizontal Ellipse: When the major axis is along the x-axis (a>b), the polar equation becomes r(θ)=aba2sin2(θ)+b2cos2(θ)

  3. Vertical Ellipse: When the major axis is along the y-axis (b>a), the polar equation is r(θ)=aba2cos2(θ)+b2sin2(θ)