Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 243

9.2 VARIABILITY OF THE EARTH'S ORBIT
9.2.1 Variability within the orbital plane
The orbit of the Earth about the Sun is illustrated in Figure 9.1 .
The Earth's orbit has three characteristic parameters that are relevant to the
variability of solar energy input to higher latitudes.
The Earth spins on its axis, and that axis is
(a) fixed in space during the course of a year; and
(b) tilted with respect to the plane of the orbit at an angle designated the obliquity.
The obliquity is presently 23.45 but has slowly varied over many thousands
of years typically between about 22 and 24.5 with a 41,000-year period.
The Earth's orbit is not quite circular and possesses a small eccentricity, which
is dependent on the distance between the center of an ellipse and the position of a
focus. In simplistic terms, it is a measure of how elongated one axis is compared
with the other. Eccentricity varies with a period of roughly 100,000 years but the
amplitude of these variations is highly variable.
The summer solstice (June 21) occurs in the Northern Hemisphere when the
spin axis of the Earth points exactly toward the Sun, and winter solstice
(December 21) occurs in the Northern Hemisphere when the spin axis points
directly away from the Sun. This is illustrated for the present case in Figure 9.1 .
Figure 9.1. Motion of Earth about the Sun. Seasons are for the Northern Hemisphere. The
current obliquity is 23.45 .