# Primary direction in planet-centered equatorial reference frame

I am given the classical orbital elements of the orbit of a spacecraft around a planet which is not the Earth, say Venus. I assume those are referred to a reference frame whose fundamental plane is the equator of the planet, but which is its primary direction?

I am hesitating between two options: either it is the intersection of the plane of the orbit with the ecliptic, or the intersection of the plane of the orbit with the J2000 Earth equator. While the ecliptic options seems more reasonable to me, I've been reading lots of documentation on the Internet but either they are all Earth-minded (i.e. Wikipedia) or there is some implicit convention I am missing.

The thing is that this is not important if for example I am just analyzing the motion of my spacecraft around my main attractor (Venus), but when I want to look at it from an interplanetary point of view, I think I need this information.

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Wait, wait. Are you talking about rotating frame or non-rotating frame? For the rotating frame, you have 0 longitude in the Ariadne crater, and the other direction is given by the need for a right-hand-sided system (90E). – Deer Hunter Jul 2 '13 at 18:42
Sorry, I am talking about the non-rotating frame. – astrojuanlu Jul 2 '13 at 19:14
What about Venus mean equator and J2000 vernal equinox? Suggest reading SPICE's doc on frames: naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/req/frames.html and the IAU doc: Seidelmann, P.K., Abalakin, V.K., Bursa, M., Davies, M.E., Bergh, C. de, Lieske, J.H., Oberst, J., Simon, J.L., Standish, E.M., Stooke, P., and Thomas, P.C. (2002). "Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000," Celestial Mechanics and Dynamical Astronomy, v.82, Issue 1, pp. 83-111. – Deer Hunter Jul 2 '13 at 19:29

From Seidelmann, P.K., Abalakin, V.K., Bursa, M., Davies, M.E., Bergh, C. de, Lieske, J.H., Oberst, J., Simon, J.L., Standish, E.M., Stooke, P., and Thomas, P.C. (2002). "Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000," Celestial Mechanics and Dynamical Astronomy, v.82, Issue 1, pp. 83-111:

(Table I)

Recommended values for the north pole of rotation and the prime meridian at standard epoch (J2000 = JD2451545.0 barycentric coordinate time).

Venus:

$\alpha_0 = 272.16$

$\delta_0 = 67.16$

$W = 160.20$ (at J2000, for other values, the rotation term given is $-1.4813688d$ where $d$ is days since J2000)

The values of $\alpha_0$ and $\delta_0$ are given in standard equatorial coordinates. The prime meridian is at the center peak of Ariadne crater.

Thank you for your answer, but I think I would need further explanation on it. On the one hand, I don't know what $W$ is, and on the other hand, though I understood how the direction of the pole is defined, I fail to see how does it help to choose a primary direction in the fundamental plane: does it mean that there's not really a canonical choice between the intersection of the plane of the orbit with the ecliptic, or the intersection of the plane of the orbit with the J2000 Earth equator? – astrojuanlu Jul 3 '13 at 15:10