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


  • $\begingroup$ Related question on defining argument of perigee when inclination is zero $\endgroup$
    – Jack
    Apr 21, 2019 at 19:18

1 Answer 1


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.

  • $\begingroup$ Is it means that longiture of ascending node is -90 degrees? $\endgroup$
    – Robotex
    Nov 9, 2021 at 9:36
  • $\begingroup$ @Robotex The longitude of the ascending node of a circular orbit can still be anywhere from 0 to 360 degrees. For orbits with zero inclination, the RAAN would be undefined and is set to 0 by definition. $\endgroup$ Nov 10, 2021 at 10:11
  • $\begingroup$ I assumed that 0 degrees is my argument of pericenter $\endgroup$
    – Robotex
    Nov 10, 2021 at 13:14

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