This study addresses the problem of orbit formation in planetary (including planet−moon), binary, and hierarchical triple systems by proposing an eccentricity excitation equation and investigating its underlying mechanism. The study introduces a synchronous orbit radius ratio and investigates the relationship between the semi-major axes and synchronous orbit radius ratios of known eccentric systems with similar mass ratio orders of magnitude and spin–orbit angles to discover the eccentricity excitation equation. In an eccentric system with a mass ratio order of magnitude higher than −7.5217, the semi-major axis exhibits a power-law distribution relationship with the product of mass ratio and square of rotation period ratio. By assuming that a gravitational field rotates along with the host celestial body, this study extends Einstein’s equivalence principle to analyze the mechanism underlying the eccentricity excitation equation. A frame exists in local physical space of a rotating gravitational field, which replaces gravity and exhibits a free-fall acceleration toward the host and an acceleration from an initial zero velocity to current velocity of the gravitational field. This frame can be divided into two subframes, one of which can be replaced by the drag force. The orbital eccentricity results from the combined effect of two drag forces caused by the gravitational field rotation of the primary and secondary bodies in a system. Moreover, a power-law relationship exists between the orbital velocity forming the semi-major axis caused by the drag force due to the gravitational field rotation of the secondary body and the ratio of the two drag forces. Orbital precession is driven by the drag force acting on the secondary body only in the aphelion region and is positively correlated with the eccentricity.