I looked at these 3 sites and I find that they give conflicting definitions even in their own lines.
They talk about the 2 focus points but we never know them.
If you know the definition of semimajoraxis that goes well with Celestia, I'm interested.
Hi Gironde... problem only that I do not know English well to explain this... but I will try to explain... See...
a^3=μ*T^2/4*Pi^2 a - semi-major axis.., μ - standard gravitational parameter (for the Earth - 398 600,44158 км^3c^2).., T - orbital period of satellite.., Pi - 3,14159265358979323846
In celestial mechanics, the standard gravitational parameter μ of a celestial body is the product of the gravitational constant G and the mass M of the body.
μ=G*M
For several objects in the Solar System, the value of μ is known to greater accuracy than either G or M... The SI units of the standard gravitational parameter are m^3 s^−2. However, units of km^3 s^−2 are frequently used in the scientific literature and in spacecraft navigation...
So if you know orbital period of satellite, you can find semi major axis easy... T- period in seconds... then a - in kilometers