# 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 at 19:18

## 1 Answer

In case of a circular orbit, the argument of periapsis is defined as 0. This means the periapse is located at the ascending node.

If you would try to calculate the argument of periapse it doesn't work:

$$\omega = \text{arccos}\!\left(\frac{n \cdot e}{|n|\cdot |e|}\right)$$

For circular orbits ($$e=0$$) this is division by zero. This is why it is by definition set to zero.