There's a celestial coordinate system used to measure the orbital elements.  Zero longitude points in the direction of a point in space that is fixed relative to the stars.  In real life, zero longitude is along the line formed by the intersection of Earth's equatorial plane and the ecliptic plane, pointing in the direction of the vernal equinox.  In KSP it is not entirely clear what defines the direction of zero longitude. One thing I've noticed, and this is totally unofficial, is that whe