# Libration vs. rotation for mass on rotating disc

Hello I have a question about the difference between rotation and libration. In some textbook it is stated that for libration $$$$q (t+\tau) = q(t) \\ p (t + \tau) = p(t)$$$$ with the coordinates $$q$$, the momentum $$p$$, time $$t$$ and period $$\tau$$. Rotation is given by $$$$p (q + q_0) = p(q)$$$$
Now I am wondering if a mass is fixed on a rotating disc, rotating with a constant angular velocity, then this would be a libration, but I would expect it to be a rotation, since I had the feeling that libration and oscillation should be similar, but this example is not an oscillation.

• I believe rotation is required for libration. For instance, the Moon rotates, librates, and nutates - and probable has other small oscillations. Commented Jul 14, 2019 at 0:05
• And I think of the libration, nutation, etc. as being a result of the mathematical description of the motion being imprecise. For instance, the case of the Moon, Cassin's Laws being imprecise. Commented Jul 14, 2019 at 0:08