This work develops a unified theory based on the Harmonic Field Equation, a wave operator defined on a five-dimensional scalar field whose dynamics generate, as a macroscopic limit, Einstein's equations. We show that gravity is not a fundamental curvature of spacetime, but rather the elastic response of the vacuum to the vibrational pressure of the scalar field. General Relativity is recovered as a hydrodynamic approximation obtained by averaging the phase oscillations of the field, while the coherence terms discarded in the classical framework are revealed as crucial for describing the fine structure of the Solar System. Starting from vortex solutions of the field equation, we model celestial bodies as harmonic vortices whose interference explains phenomena traditionally considered anomalous: the heliocentric wobble of the Moon, its secular recession, the stability of multiple resonances such as the Laplace resonance, and the fractal distribution of Saturn's rings. We show that planetary orbits are not continuous geodesic trajectories, but rather stationary states of quantized phase determined by the uniqueness conditions of the global wavefunction. This orbital quantization allows us to derive the existence of gaps such as the Asteroid Belt and reproduces the structure of the Jupiter and Saturn systems without resorting to empirical fitting. Finally, we demonstrate that tidal forces and the quantum tunneling effect are isomorphic manifestations of the same wave equation under different scales and boundary conditions. Both describe the response of a confined wavefunction to the periodic modulation of its potential barrier, thus establishing the universality of Simple Harmonic Motion from the atomic to the celestial scale. The Harmonic Model thus offers a scalar unification in which gravity, orbital quantization, planetary resonances, and interference phenomena emerge as expressions of a single dynamic principle: the stationary resonance of the vacuum. The work concludes by proposing specific observational predictions-including the detection of higher harmonic modes in the tidal spectrum and resonant patterns in extrasolar systems-that allow the model to be subjected to empirical verification.