Figure 13.5: The evolution of the amplitude (top) and phase (bottom) of the first
harmonic in the spin-synchronous component of the OBC drift. The amplitude
variations reflect the varying distance between Earth and Sun (smooth reference
curve), but show on top of that strong modulations. The same modulations are
observed in the phase. The smooth curve in the lower graph shows the beat pattern
produced by periods of 57 and 584 days (see text).