Ellipsoidal movement easing equation

I want to simulate a elipsoidal movement of an object wit开发者_运维知识库h easing equations, more specificly like a spinning top, but i cannot get an easing equation that helps me to do that, can anyone knows one?

i am using html/js, but no matter if is in another language, or maybe could made one here, i'll keep searching for a method, while i'll wait your answers.

An ellipse is parameterized by the equation:

var x = Math.cos(t) * smja;
var y = Math.sin(t) * smna;

Where smja and smna are the semi-major and semi-minor axes respectively. Run t between 0 and 2π and (x, y) will describe an ellipse.