Seiffert's spherical spiral is a curve on a sphere
made by moving on the sphere with constant speed and angular velocity with respect to a fixed diameter. If the selected diameter is the line from the north pole to the south pole, then the requirement of constant angular velocity means that the longitude of the moving point changes at a constant rate.
The cylindrical coordinates of the varying point on this curve are given by the Jacobian elliptic function
* Rhumb line