at the Earth-Sun system provides important insight into current environmental
phenomena. Since the beginning of the Earth's evolution four and a half
billion years ago, the atmosphere and landscape have changed significantly.
Today, global atmospheric problems such as global warming and ozone depletion
arise from changes in the delicate balance of the Earth's atmosphere that
alter the amount of the sun's radiation reaching humans and other living
creatures. The Earth's immediate physical environment is patterned by
three primary influences:
nearness of the sun.
- The Earth's
atmosphere, a mixture of gases held in a layer surrounding the Earth
by the Earth's gravitational force. This layer has reached the current
and somewhat steady composition of approximately 80% nitrogen and 20%
oxygen over the last four and a half billion years.
composition of the solid mass of the Earth, which gives rise to the
materials in and on the Earth (including water, an essential to our
type of life). Also resulting from the composition and manner of the
origin of the Earth is the temperature of the Earth's core.
will focus predominantly on the first two influences: the Earth's atmosphere,
and the Earth's relationship to the sun.
From a physical
point of view alone, the Earth-Sun system can be represented simply as
in Figure 1. The two main forces and effects of the sun on the Earth are
the gravitational force (maintaining the Earth in an orbit around the
sun), and the electromagnetic radiation from the sun (keeping the Earth's
atmosphere at a particular temperature).
1: Earth-Sun system with primary influences.
components of the sun's (solar) spectrum interact with the atmosphere.
Over billions of years, this interaction has produced both the ozone layer
and current climatic conditions with feedback from life that evolved.
These phenomena form the major topics of this unit. We begin with a description
of solar radiation.