The pole of a line L in a circle C is a point P that is the inversion in C of the point Q on L that is closest to the center of the circle. Conversely, the polar line of a point P in a circle C is the line L such that its closest point Q to the center of the circle is the inversion of P in C. The relationship between poles and polars is reciprocal. Thus, if a point A lies on the polar line q of a point Q, then the point Q must lie on the polar line a of the point A. The two polar lines a and q need not be parallel. There is another description of the polar line of a point P in the case that it lies outside the circle C. In this case, there are two lines through P which are tangent to the circle, and the polar of P is the line joining the two points of tangency. This shows that pole and polar line are concepts in the projective geometry of the plane and generalize with any nonsingular conic in the place of the circle C.
The concepts of a pole and its polar line were advanced in projective geometry. For instance, the polar line can be viewed as the set of projective harmonic conjugates of a given point, the pole, with respect to a conic. The operation of replacing every point by its polar and vice versa is known as a polarity. A polarity is a correlation that is also an involution.
The concepts of pole, polar and reciprocation can be generalized from circles to other conic sections which are the ellipse, hyperbola and parabola. This generalization is possible because conic sections result from a reciprocation of a circle in another circle, and the properties involved, such as incidence and the cross-ratio, are preserved under all projective transformations.
Calculating the polar of a point
A general conic section may be written as a second-degree equation in the Cartesian coordinates of the plane where Axx, Axy, Ayy, Bx, By, and C are the constants defining the equation. For such a conic section, the polar line to a given pole point is defined by the equation where D, E and F are likewise constants that depend on the pole coordinates
Calculating the pole of a line
The pole of the line , relative to the non-degenerated conic section can be calculated in two steps. First, calculate the numbers x, y and z from Now, the pole is the point with coordinates
Given four points forming a complete quadrangle, the lines connecting the points cross in an additional three diagonal points. Given a point Z not on conic C, draw two secants from Z through C crossing at points A, B, D, and E. Then these four points form a complete quadrangle with Z at one of the diagonal points. The line joining the other two diagonal points is the polar Z, and Z is the pole of this line.
Applications
Poles and polars were defined by Joseph Diaz Gergonne and play an important role in his solution of the problem of Apollonius. In planar dynamics a pole is a center of rotation, the polar is the forceline of action and the conic is the mass–inertia matrix. The pole–polar relationship is used to define the center of percussion of a planar rigid body. If the pole is the hinge point, then the polar is the percussion line of action as described in planar screw theory.
