I learn that any work needs a source of energy, the work of gravity force (like keeping the Moon in the orbit by the Earth gravity force) comes from where? Can we find any mass reduction (transform to energy source) during the time?
But work is the integral f ds
If the orbit is not decaying then no work is performed
If the orbit were to degrade then the moon would have less potential energy
Think of putting a satellite in orbit. The potential energy is the work against gravity to get it to that altitude and the work to get it up to speed for orbit.