Thanks @ConnorGarcia for your Answer. Avoiding 'Tidal Locking' for now - the rotational angular momentum Lrot is 10,000,000 times smaller, so negligible, but could it still be a factor in eccentricity? I realise this is a controversial question, but, does the angular momentum between two rotating planets 'literally' take Relativity into account? i.e. if Planet-Earth with 'clockwise' rotation: Se, is measured relative to 'clockwise' rotation of Planet Mars: Sm, then their nearest equatorial 'purely-relative' rotation speed will be: (Se + Sm) (for Mars and Earth, this is approximately twice their individual rotation speed relative to a 'Rest' frame). If two fictitious planets had the same rotation speed S and anti-parallel rotation (one clockwise and one anticlockwise), they would - by similar logic - have 'relatively' Zero rotation speed (along the shortest distance between them). General Relativity influences the precession of the perihelion of Mercury, but I'm wondering about Special Relativity) My question is: if these 'Relative' speeds were used in the Lorentz transformation, then the masses would change - by a tiny amount. Could this deflect the orbits by a minute amount over a long period?