Orbital Parameters of the Earth
The amount of radiation received from the sun and its distribution on the Earth’s surface varies with the relative position between the sun and the Earth. These natural variations in the Earth's orbit impact the planet’s climate and appear to set the conditions for the cooler and warmer periods of glacial and interglacial (non-glacial) stages. Figure 2 depicts the three parameters that describe the Earth’s orbit around the sun: (1) eccentricity, (2) axial tilt (or obliquity), and (3) time of perihelion (or precession). In 1920, Milutin Milankovitch proposed a theory that changes in climate cycles of glacial-interglacial periods were initiated by both the amount and the distribution of radiation received from the sun. Every 100,000 years or so, these orbital parameters vary in such a way to reduce the amount of sunlight energy received at midlatitudes in the northern hemisphere. This reduction in sunlight energy and is thought to lead to the onset of an ice age due to reduced warming of the Earth's surface.
Figure 2. Earth's orbital paramenters: (a) eccentricity, (b) tilt (c) time of perihelion, and (d) amount of solar radiation received in the Northern Hemisphere between 60 and 70 degrees as a function of these three parameters.
The Milankovitch theory of climate change during the last 1.6 million years theorizes that the onset of ice ages is due to variations in three orbital parameters of Earth. In Figure 2, the eccentricity (a) is the degree to which Earth’s orbit departs from a circle. Times of maximum eccentricity are separated by roughly 100,000 years. The tilt angle (b) is the angle between Earth’s axis and a line perpendicular to the plane of the orbit of the planet. The time of perihelion (c) involves the tilt of Earth’s axis at its closest approach to the sun. The cycles of tilt and time of perihelion are roughly 40,000 and 20,000 years, respectively. The calculated amount of sunlight (d) received at 60o to 70o north latitude during the summer (summer insolation, July) is based on the cycles of variation of these three orbital parameters.