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Size of the Earth - The diameter of the earth through the poles is 7,899.6 miles; through the equator 7,926.6 miles. Compute the circumference, the area, the volume and the weight of the earth in tons from the following data:
  • The circumference equais the diameter multiplied by 3.14159.
  • The area equals the product of the diameter by the circumference of a great circle.
  • The volume equals the area of the surface multiplied by one-third of the radius.
  • The mass equals the volume multiplied by the density. The mean density of the earth is 5.6. A cubic foot of water weighs 62.5 lbs.
Problem of Eratosthenes - The diameter of the earth was not the dimension first determined, as there is no way of measuring it directly. The part that was actually measured was an arc of the circumference. This problem was first solved by Eratosthenes two centuries before the Christian era. He determined that Syene in Egypt was close to the same meridian, hence to the same great circle as Alexandria. 

He had observed that at noon on the longest day in midsummer the sun's rays shone on the bottom of a deep well at Syene in Egypt. ' What inference could he draw from this? He measured the angular distance of the sun from the zenith at Alexandria on the same day at noon and found it equaled 7 degrees and 12 minutes, or exactly one-fiftieth of a circle, which is the same as the angle at the center of the earth formed by the radii from these cities. 

Prove this. The distance between the two cities had been measured and found to be 5,000 stadia, hence by multiplying this distance by fifty he obtained the total distance around the earth as 250,000 stadia. Unfortunately we have no means of knowing at the present time the length of a stadium in any of our units of measurement, so that we have no certain means of comparing the accuracy of the result obtained by Eratosthenes with those obtained by similar means in more receot times.

Structure of the Earth - The earth is frequently divided for convenience of study into three parts or spheres :
  • The outer gaseous envelope, the atmosphere; 
  • the liquid envelope, the water or hydrosphere which nearly surrounds 
  • the solid rocky part, the lithosphere, the inner portion of which is sometimes called the centrosphere.
  • A fourth is sometimes added called the biosphere, or life sphere. These are not true mathematical spheres, nor are they very sharply separated at times. 
Both the hydrosphere and the atmosphere penetrate the lithosphere and large quantities of the atmosphere are dissolved in the hydrosphere as well as large quantities of water as invisible vapor in the atmosphere. The life sphere is confined chiefly to the water and the contact of the atmosphere with the lithosphere. 

It is scattered through the water sphere to a greater depth, probably, than in either the gaseous or the rock portions, yet the greater portion of it lies close to the lower portions of the atmosphere. The air, water and land portions of the earth, which form the greater part of the subject of physical geography or physiography, are discussed in the following chapters.

Related article : Overview of Earth Science
Physical geography


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