E

Earth

Earth is the formal name for the planet on which we live. Although we know of

other planets, including some outside of our own solar system, there is not another

known planet with the combination of physical characteristics as ours. Earth is

large enough to support several billion human inhabitants and one-and-a-half

million species of plants and animals. It is small enough to present a finite set of

resources in terms of long-term sustainability of life as we know it. Moreover, it

is subject to all sorts of natural and human caused catastrophes, some of which

could possibly destroy the viability of life on the planet.

Seemingly large to humankind, Earth is incredibly small when considering the

size of the known universe. The third planet from its local star, the sun, Earth

orbits the sun at an average distance of 150,000,000 kilometers while rotating on

its axis (seeSeasons). Earth was created approximately 4.6 billion years ago out

of the conglomeration of materials from a disk-shaped concentration of gas and

dust widely surrounding the sun.

The planet seems massive by human standards. Earth contains about 5.97×

1024 kg of mass, although it is not nearly the most massive planet in our own solar

system. Mass is added on a daily basis as dust and meteorites enter the atmos-

phere. Conversely, some mass is lost by gases escaping the microgravity at the

top of theatmosphere. At present, gains and losses do not seem to have an appre-

ciable imbalance. Something of Earth’s size can be gleaned from watching a ship

disappear over the horizon at sea or observing the planet from a commercial jet

aircraft: it is clear the Earth has a rounded shape. The circumference of the planet

approximates 40,000 km at the equator.

The shape is roundish, but Earth is not a perfect sphere on two counts. First, the

radius—the distance from mean sea level to Earth’s center—is not consistent in all

latitudes. At the equator, the radius is 6,378 km while at the North and South

Poles, the radius is only 6,357 km. Earth belongs to a class of shapes called oblate

spheroids. Earth’s spheroid is “fat” in its tropical latitudes and this is caused by

shape deformation of Earth’s non-solid interior by virtue of outward forces caused

by rotation. Notice the polar and equatorial radii differ by only three-tenths of a

percent. For all but the most esoteric mapping functions and to the astronaut

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