The power spectral density (frequency dependence of the variance) of the annual Stuiver radiocarbon record (Δ14C) contains a major line with a 17.5¿0.5(2&sgr;) year period which is proposed to result from a hitherto unexpected forcing of the cosmic ray (CR) flux at the top of the atmosphere. The only known natural period with a corresponding value is the synodic Saros cycle, the retrograde period of rotation of the Moon's nodal plane; 18.5 years is the sidereal nodal regression period. It is proposed that the intersection of the Moon's downstream diamagnetic solar wind cavity with the Earth at the Saros period is the source of the apparent cosmic ray anomaly leading to this periodic change of Δ14C; arguably, eclipsing and/or a low energy acceleration of the GCR could take place, though the evidence from the Fourier spectrum appears to favor acceleration. ¿ 1999 American Geophysical Union