A circular harmonic orbit contains one center or focus or an equalized compound center. Forces of expansion and contraction are balanced or equated. The energy level is stable, neither increasing or decreasing.
See Also
9.24 - Elliptical Enharmonic Orbit
9.26 - Orbital Phases
12.38 - Orbital revolution
Cyclotronic Motion
Figure 9.11 - Compression Wave with expanded and contracted Orbits
Gyrokinetics
Orbital Plane