 # When using Equinoctial Orbits, what does the position angle argument specifiy?

In the documentation for Equinoctial Orbits, the elements specified are A, Ex,Ey, Hx, Hy and Lv.
All of this makes sense. When mapping the orbit to an array, or getting an orbit’s jacobian one of the arguments to pass in is the PositionAngle. The options are true, eccentric, and mean. This makes sense for Keplarian elements but in the case of equinoctial elements, does this just chance the calculation of true longitude to mean or eccentric?

Yes, you can choose between mean longitude argument defined as λₘ= ω + Ω + M, eccentric longitude argument defined as λₑ = ω + Ω + E or true longitude argument defined as λₜ = ω + Ω + υ. In these definitions all the elements on the right hand sides correspond to classical Keplerian elements. ω is the argument of perigee, Ω is right ascension of ascending node, M is mean anomaly, E is eccentric anomaly and υ is true anomaly.

Beware, that true longitude argument does not really mean the exact longitude of the spacecraft, it is only a (bad) name. It is defined with respect to an inertial frame and not an Earth frame, and it is just a mathematical sum of angles with not exact geometric representation as the angles summed up are not all in the same plane. If the orbit were close to equatorial, then λₜ would be close to the right ascension of the spacecraft.

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Thanks that makes sense!