# Argument of Perigee for a circular orbit

For a perfectly circular orbit (e=0), how is the argument of perigee defined??

Thanks.

• Related question on defining argument of perigee when inclination is zero – Jack Apr 21 '19 at 19:18

$$\omega = \text{arccos}\!\left(\frac{n \cdot e}{|n|\cdot |e|}\right)$$
For circular orbits ($$e=0$$) this is division by zero. This is why it is by definition set to zero.