Basics of Environmental Science

Earth Sciences / 77

we are at perihelion in January. In AD 15000, one half-cycle from now, we will reach perihelion
in June.

The third cycle, called the ‘obliquity of the ecliptic’, relates to the angle of the Earth’s rotational axis
to the ecliptic, the plane of the Earth’s orbit. Imagine the rotational axis as a straight rod projecting
at both ends and forming an angle with the ecliptic. At present, that angle is 66.5° and, therefore, the
axis is 23.5° from the vertical (90° to the ecliptic). Over a cycle of about 41000 years this angle
varies by about 1.5° about a mean of 23.1° (see Figure 2.28).

Figure 2.26 Orbital stretch

Period: about 95000 years

Figure 2.27 Wobble of the Earth’s axis