In
astrodynamics the
argument of periapsis ω can be calculated as follows: :\omega = \arccos{{\mathbf{n} \cdot \mathbf{e}} \over {\mathbf{\left| n \right|} \mathbf{\left| e \right|}}} ::If
ez \omega = \mathrm{atan2}\left(e_y, e_x\right) ::If the orbit is clockwise (i.e. (
r ×
v)
z x
and ey
are the x
- and y''-components of the eccentricity vector
e. In the case of circular orbits it is often assumed that the periapsis is placed at the ascending node and therefore
ω = 0. However, in the professional exoplanet community,
ω = 90° is more often assumed for circular orbits, which has the advantage that the time of a planet's inferior conjunction (which would be the time the planet would transit if the geometry were favorable) is equal to the time of its periastron. ==See also==