The Solar System and the Planet Earth 307
our galaxy and in the universe as a whole. Most of the exoplanets
observed are large gas giants like Jupiter because they are the easiest to
observe. Rocky planets like the Earth have been observed and are
believed to exist in numbers equal to or greater than that of gas giants.
Many astronomers and other scientists believe that many of these planets
support life and that there must exist numerous intelligent civilizations
out there somewhere in our universe, but because of their vast distances
from us we are unaware of them and they of us.
We now return to our Solar System and study in detail the third planet
from the Sun, our very own planet Earth.
The Earth
The third planet from the Sun the Earth completes a revolution once
every 365 ¼ days and turns upon its axis once every 24 hours. The Earth
has a mass of 6 × 10^27 grams and an average density of 5.4 grams per
cubic centimeter. The Earth is almost a perfect sphere with an average
diameter of 12735 kilometers. Because the Earth rotates about the axis of
its North and South Pole its shape is slightly distorted from that of a
perfect sphere. The planet is slightly flat at the poles and bulges at the
equator so that the equatorial diameter is 42.8 kilometers greater than the
diameter from the North to South pole. This discrepancy of 1/3 of 1% of
the Earth’s diameter is due to the centrifugal forces generated by the
Earth’s rotation and indicates that while the Earth is solid it also
possesses plastic qualities.
The planet Earth consists of three basic zones corresponding for
the most part to the three states of matter, solid, liquid and gas. The
principal part of the Earth is the solid globe, which as we will shortly
discover consists of the core, the mantle and the crust. There is new
evidence that the inner core of the Earth rotates slightly faster than the
rocky mantle and crust that cover it by a very small amount which is
somewhere between 0.3 to 0.5 degrees per year. The core, which is made
of iron, consists of a solid inner core and a liquid outer core. Above the
solid Earth lie the oceans, which cover 71% of the Earth’s surface. These
vast bodies of water have depths up to 10 km. Finally, floating above
both the oceans and dry land is the gaseous component of the planet
Earth, our atmosphere, which consists basically of nitrogen, oxygen,
carbon dioxide, water and traces of other gases.