Principles of Geography
Contents
Chapter 1 the earth in the Universe
Chapter 2 location time and maps
Chapter 3 the Continents
Chapter 4 Mountains
Chapter 5 Plateaus and Uplands
Chapter 6 Plains
Chapter 7 the Oceans
Chapter 8 Rivers and Canals
Chapter 9 lakes and Underground Water
Chapter 10 the Atmosphere and its Weather
Chapter 11 Elements of Climate
Chapter 12 Natural Vegetation and the Climatic Regions
Chapter 13 Agriculture
Chapter 14 Grazing and Forestry
Chapter 15 Fishing
Chapter 16 Mining
Chapter 17 Manufacturing
Chapter 1
The earth in the universe
We cannot understand the earth without knowing something about its place in the universe the limitless space that surrounds us. Our world is only one of a group of planets that circle around the sun. The sun is only one of an immense number of stars in the group called the Milky Way galaxy. Uncounted other galaxies probably exist even beyond the many that can be seen with the best telescopes.
The sizes and distances of all these inhabitants of space are so great that we can hardly understand them. The surface of the planet earth includes about 200 million square miles. The sun is about 110 times bigger than the earth. As a star the sun is not very large and the Milky Way contains about 40 billion stars. Distances between stars and galaxies must be measured in light years the distance covered by light in one year. Since light travels at a speed of 186000 miles each second a light year is an almost unimaginable distance.
The solar system of which our earth is a part was probably formed at least four and a half billion years ago. No one knows exactly how it came about. New discoveries in science tell us much about the nature of the earth but each attempt to solve the mystery of the earth’s origin leads to now questions. In the study of geography however we are most interested in the earth as it is today. We need to remember nevertheless that present-day conditions are the result of a long history.
WHAT ARE THE EARTHS MOTIONS?
Many natural conditions which affect human life are the result of the earth’s motions. These motions are of several kinds. The causes for them are too complicated to discuss fully here but the motions can be described.
Revolution
The earth as a member of the solar system revolves around the sun. the sister planets of the earth are Mercury , Venus , Mars , Jupiter , Saturn , Uranus , Neptune , and Pluto.
The earth is neither the nearest nor the farthest from the center. Two planets, Mercury and Venus, are nearer, and the earth is about 93 million miles from the sun.
The revolution of all the planets depends on the force of gravity. Every object, whatever its size, has gravity. Any two particles or bodies, if free to move, are attracted to each other. Only under certain conditions, however, does the attraction produce effects that can be noticed or measured without special scientific equipment. The force of gravity depends on the mass of the objects and their distance from each other. The sun, because of its great mass compared to the planets, has a very strong gravitational attraction. It causes the planets to move in a path that is a circle.
This path is called on orbit. Some of the planets have satellites, smaller bodies that revolve around them. The moon is a satellite of the earth. The earth’s gravitational attraction keeps the moon moving in its orbit. In turn, the moon’s attraction for the earth causes the movement of ocean water called the tides. (The tides are described in Chapter7.)
The planets in our solar system revolve at different speeds. Those closest to the sun revolve fastest, because the gravitational force is greatest. The earth takes 365 days to travel around the sun. Pluto, the planet farthest from the sun, takes 249 earth years to complete one revolution.
The orbit of the earth is not quite a circle. The earth draws somewhat nearer to the sun in January and is slightly farther. away in July.
Rotation
The earth rotates, or turns on its axis. Once in 24 hours. The axis of the earth is an imaginary line running through the north and south poles. The rotation of the earth causes day and night; it also helps determine the pattern of the seasons. Day and night and the progress of the seasons result in changes of temperature in the various regions of the earth. Seasons change because of the angle of the earth’s axis. If we think of the earth as moving along a track on a level surface, the axis is not exactly straight up and down, but is tipped slightly. This angle does not change. Therefore, as the earth moves around the sun, first one pole and then the other is nearest the sun. During part of year, the northern half of the earth is turned toward the sun. During the rest of the year, the southern half is turned toward the sun.
As the earth rotates, one half of the earth is in sunlight and the other half is in shadow. At the equator, days and nights are always of equal length. Because of the angle of the earth’s axis, however, days and night are of different lengths in all other parts of the world. There are two exceptions. On March 21 and September 23, which are known as the spring and fall equinoxes, the circle that divides light from darkness passes through the Poles. Then day and night are equal everywhere. From March to September, the northern half of the earth has spring and summer, and the southern half has autumn and winter. From September to march, the opposite is true. During summer, the days are longer and the nights are shorter. On June 22, sun’s direct rays fall farthest north of the equator. On December 22 they fall farthest south. The circles which are the outer limits of the sun’s direct rays are called the tropic of cancer (north) and the Tropic of Capricorn (south).
In north temperate regions, summers are warmer than winters. Changes in temperature are directly related to the amount of sunlight received. Such changes will be described in chapter 11.
THE SUN AND ITS ENERGY
The sun, the center of the solar system, is a huge ball of intensely hot gases. It is about 864000 miles in diameter. The sun is the most important single influence on the earth. It is the source of all energy, through its light and heat. Without this energy, the earth would be cold and lifeless, the sun’s light and heat are changed into countless forms of energy. All the fuels we burn and all the water power we change in to electricity owe their energy to the sun.
The source of the sun’s energy has long been a puzzle to mankind. Early attempts to explain it assumed that the sun is burning as fire burns. Later thinkers, however, could not understand how the sun could burn so fiercely day after day without eventually burning out. Calculations made of the sun’s size and energy output made clear that it could not possibly be burning in any ordinary way. To produce the energy given off in one second, more than its entire substance would have to be burned up.
Only in recent years, since the discoveries made by atomic scientists, has there been any clear theory of the source of the sun’s energy. Studies show that the sun’s interior must have a temperature of about 18 to 36 million degrees F. the elements that make up matter, show that at very high temperatures, certain changes may occur in the nature of the atom. The sun’s energy seems to be the result of processes that change the element hydrogen into helium, releasing tremendous amounts of energy. This energy radiates from the sun as light and heat. Only a very small part of the sun’s total energy reaches the earth.
In recent years, inventors have tried to find ways to use the sun’s energy directly. Sunshine can be used to heat water in copper pipes, and if it can be stored properly, it can heat houses or other small areas. It can also be used to evaporate sea water, providing fresh water for drinking or other purposes. However, the equipment needed to make use of the sun’s energy in this way is expensive and usually takes up a great deal of space. Further study is needed before we can use solar energy.
Review Questions
1. What is geography?
2. What is a light year?
3. Why do the planets revolve around the sun?
4. What causes the seasonal changes in the length of a day at a given point on earth?
5. In what forms does the sun’s energy reach the earth?
6. How does the sun maintain its constant output of energy?
