Curves63 of 113
Nephroid
Huygens and Tschirnhaus, 1679; named by Richard Proctor, 1878
An epicycloid with two cusps, the track of a point on a wheel running round the outside of a circle twice its radius. Parallel light reflected inside a circular mirror gathers along it, which is why half a nephroid shows in a cup of coffee. Proctor named it in 1878, from the Greek for kidney.
x = 3a cos φ - a cos 3φ, y = 3a sin φ - a sin 3φ
On mathcurve.com as Néphroïde