This article investigates the Sun-Earth Harmonic Orbit, which synchronizes the argument of periapsis change per lunar gravity assist and the Earth's argument of latitude change during an orbital period of the Earth-Moon resonance orbit. Resonance orbits in the Earth-Moon rotating frame appear repetitive; however, the argument of periapsis varies due to the third body effect from the lunar flyby. The magnitude of the argument of periapsis change depends on the Jacobi integral of the resonance orbit. We map the argument of periapsis change versus the Jacobi integral for each prospective resonance orbit to identify the Sun-Earth Harmonic Orbit. This synchronization enables the spacecraft to travel within a confined region in space when viewed from the Sun-Earth rotation frame, which benefits space missions, such as Earth magnetosphere science and space telescope missions. The applications of the Sun-Earth harmonic orbit, such as orbit-orientation reconfiguration (station keeping) and stabilities, are presented.