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Ivan,
We know the gravitational interaction between two bodies is given by F=GMm/r^2. From this we can determine the orbital period of one body about another. Or for that matter, the orbits of two bodies about another. However, it tells us nothing about the speed of propagation of gravity.
As both the Earth and the satellite have an effective angular momentum about the Sun, you not are actually measuring the speed of gravity but their relative speeds of rotation as a result of the inverse square law used for gravity. With the satellite at the Lagrangian point, this speed is the same.
The evidence for gravity propagating at the speed of light is primarily theoretical and, as far as I know, not proven conclusively by any observational evidence.
Regards, Rob.
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