Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 245

Figure 9.4. Variation of the longitude of perihelion over the past 400,000 years.
uniformly. When the longitude of perihelion occurs near 90 the Earth is closest to
the Sun in northern summer, and when the longitude of perihelion occurs near
270 the Earth is closest to the Sun in northern winter. Since ice age formation or
interglacial period formation are influenced by peak solar input to higher northern
latitudes in summer, one might expect that a longitude of perihelion near 90 is
conducive to interglacial conditions while a longitude of perihelion near 270 may
contribute to glacial conditions. However, if peak solar input to higher southern
latitudes were more important, the conditions for glacial and interglacial con-
ditions would be just the opposite. At present, the longitude of perihelion is about
250 , so that the perihelion of the Earth's orbit occurs near the winter solstice in
the north. This is illustrated in the upper figure in Figure 9.4 . The angle measured
counterclockwise from where the Earth is on March 20 to the position of the