Abstract <p>The intensity of irradiation of the Earth and hemispheres in the Late Pleistocene has been calculated with high spatial and temporal resolution. The effect of division of seasonal irradiation of the hemispheres by phases of increasing and decreasing eccentricity of the Earth’s orbit has been discovered. The effect is manifested in the fact that the intensity of summer irradiation in the Northern Hemisphere in the phase of increasing eccentricity exceeds the intensity of its irradiation in the phase of decreasing eccentricity. The effect in the Northern Hemisphere in the winter half-year is opposite. The irradiation intensity in the phase of increasing eccentricity is inferior to the irradiation intensity in the phase of decreasing eccentricity. In the Southern Hemisphere, the effect of seasonal division is direct in the winter half-year and is opposite in the summer half-year. The effect of seasonal division is associated with the fact that the intensity of annual irradiation of the Earth and hemispheres, as well as the amplitude of the intensity of seasonal irradiation are determined by eccentricity. The effect of division of seasonal irradiation explains the mechanism of formation and manifestation of the 100 000-year cycle in the change of the natural environment in the Late Pleistocene.</p>