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Earth's insolation variations are confined largely to the seasonal and diurnal frequencies (Rubincam, 1994; Huybers & Curry, 2006). The Earth's orbital variations participate mainly as long-period modulators of these high frequencies. The orbital frequencies are propagated into the climate record by "rectification" of the seasonal and/or diurnal frequencies by the climate and climate recorder, as demonstrated below. Rectification was originally envisaged by Milankovitch (1941), who showed that over the past 600 millennia the European glacial periods coincided with deep minima in northern summer half-year insolation.
A--RED curve: Seasonally resolved insolation spectrum for 65 ° North, 0-800 Ka. GREEN curve: Rectification of the seasonal insolation, e.g., clipping the lower halves of the seasonal cycles diverts ca. 20% of the seasonal power into the orbital band. B--Seasonal band of the insolation shows multiple lines in the neighborhood of the annual frequency caused by the orbital modulator. C--Orbital band of the rectified insolation shows clear expression of obliquity and precession frequencies. Note: p1=1/(23ky), p2=1/(19ky), o1=1/(41ky), o2=1/54ky) and o3=1/(29ky). Based on Huybers & Wunsch (2003).