quartic egg curve

The curve is formed by a set of two attached sticks, as follows:
A stick OP is fixed in O, and attached to a second stick in point P. The second stick AQ is free to move (in point A) along the x-axis.
Then the path of Q forms the quartic egg curve.

An Italian animation shows the construction very clearly.

The equation above follows from the expressions for the x- and y-coordinate of the point Q ¹⁾.

notes

1) As follows (let c=1):
x_Q = b cos a₁ - cos a₂
y_Q = b sin a₁ + sin a₂
with
sin a₂ = a sin a₁

Remark: the parameters a and b have been transposed.