2 a

x(t) = a cos(t) y(t) = b sin(t)

x^2/a^2 + y^2/b^2 = 1

r(θ) = (a b)/sqrt((b^2 - a^2) cos^2(θ) + a^2)

(for an ellipse with center at the origin, semimajor axis a parallel to the x-axis, and semiminor axis b parallel to the y-axis)