Introduction

(Claudius Ptolemy's Geographia, Library of Congress, Geography and Maps, An Illustrated Guide. http://www.loc.gov/rr/geogmap/guide/gm004001.jpg)

Earlier in the course, it was mentioned that nearly 2000 years ago a man by the name of Ptolemy used mathematical calculations to prove that the earth is a sphere. He was born in Alexandria, Egypt, about 85 AD and he was a great scientist for his time. He made several interesting discoveries. You may have also heard about him in your math classes, because some of his mathematical theories are still used today. During Ptolemy's time, world travel was quite limited, so Ptolemy's view of the earth's features was also quite limited; however, he attempted to map the entire world as he knew it. He created enough maps to fill eight books, and this map collection is entitled Geography. The map above is included in one of his books and it shows the world as Ptolemy saw it. If you look towards the center of the picture, just left of the page seam, you will see the Arabian Peninsula; to the left of this is Africa.

These days even a child in elementary school can see that Ptolemy's view of the world was a bit off! For one thing, Ptolemy's map shows a larger portion of the world as landmasses, with a smaller portion as bodies of water. When you look at a modern globe it is clear that a much larger portion of the earth is covered with water than Ptolemy thought. Thanks to photos of the earth taken from space, combined with other modern technology, our picture of the world is becoming clearer and more exact. We know the exact size of the earth; we are familiar with all of its seven continents and four oceans. Most of the earth's rivers, lakes, mountains, and even ocean floors have been explored and mapped.

Scientists have also discovered that our world is constantly changing; this means that the earth today is not exactly the same as the earth during Ptolemy's time. Earthquakes occur and volcanoes erupt continually. In some areas of the world the crust is destroyed, and in other areas new crust is formed.

In this lesson you will take a closer look at some of the earth's major features and how they have been formed. You will learn why the earth is shaped like it is. This is also the lesson that was mentioned in the course introduction, in which you will identify many of the world's most prominent geographical features. Perhaps you have already been playing the game, Where in the World Is That? If not, now is the time to memorize the features that are listed at the end of this lesson.

• Objective 1:  Discuss features of the ocean floor.
• Objective 2:  Discuss significant details about the oceans.
• Objective 3:  Compare different types of lakes.
• Objective 4:  Compare different types of desertification.
• Objective 5:  Compare different types of mountains.
• Objective 6:  Identify some of the earth's major features.

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