Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 272

tioned previously. Conversely, the Earth is moving slowest through its aphelion
when the Earth is far from the Sun producing low peak solar intensity. Some
investigators have used the length of summer (defined as the number of days with
some minimal solar intensity) as a measure of the driving force due to changes in
the Earth's orbit (e.g., Huybers and Denton, 2008; Huybers, 2009a). However, the
length of summer in one hemisphere varies with time in a very similar manner to
peak intensity in the opposite hemisphere. Hence, it is dicult (if not impossible)
to distinguish between a forcing due to length of summer in one hemisphere and
peak solar intensity in the opposite hemisphere.