THE EFFECT OF THE EARTH'S FORM AND MOTIONS The Earth as a Globe.—In the diagram on page 3, location stands first because upon it depend a great many other geographical condi tions such as distances and climate. The only possible way of stating the location of a place is by giving its position in reference to some thing else. The fact that the earth is a rotating globe is highly impor tant in this respect, for it means that the earth has two fixed points, the poles, whose position can be determined with absolute precision and in reference to which all other positions can be fixed. The equator, for example, is merely an imaginary line half way between the poles.
The evidence that the earth is a globe is abundant. The hull of a distant ship disappears before the sails or smoke stacks. Hence the intervening surface must be curved. The altitude of the stars changes by a practically uniform number of degrees for each hundred miles that one travels northward or southward. This is possible only on a globular earth. Moreover, thousands of people have actually gone around the globe in many different directions since Magellan's first circumnavigation.
The evidence that the earth rotates on an axis is not so clear as the evidence that it is a globe. The sun, moon, and stars, to be sure, rise and set as if the earth rotated, but this might be because each heavenly body revolves around the earth, as was supposed by the ancients. So firmly was this idea established that when Galileo announced that the earth's rotation accounts for day and night and the rising and setting of the stars, he risked violent persecution and even death. One of the most convincing proofs that the earth rotates is the course of a ball dropped from a great height. Barring a slight deflection due to the varying density of different parts of the earth, a plumb line suspended from a lofty structure such as the Eiffel Tower in Paris points straight toward the earth's center. If a ball be dropped from the point of suspension, it will not strike the earth at the point toward which the plumb line was directed, but an inch or more to the east. During the few seconds while the ball is falling both ball and
earth move eastward by rotation. The ball falls perfectly straight, but it has an eastward motion greater than that of the point below it.
The Meaning of Latitude and Longitude.—The relation of lati tude and longitude to the globular form and rotation of the earth may be illustrated by an umbrella. The handle represents the earth's axis upon which rotation takes place. It passes through the cloth at the pole, while the lower edge of the cloth, if it were straight instead of scalloped, would represent the equator. The ribs represent meridians by which longitude, or angular distance east or west, is measured. Circles parallel to the umbrella's lower edge would cor respond to the parallels by which latitude, or angular distance north or south is measured. If a marked rib serves as the prime meridian of London, the position of any point on the umbrella may be indicated as so many degrees of longitide east or west of the prime meridian, and so many degrees of latitude from the equa tor. All these relationships, as they appear upon a globe, are shown in Fig. 4.
Although latitude and longitude are angular distances they can readily be converted into distances in miles pro vided the size of the globe is known. The length of a degree of latitude is everywhere about 69 miles, while a degree of longitude has the same length at the equator, but decreases steadily until it becomes nothing at the poles where the meridians con verge.
The distinction between longitude and latitude can easily be remembered by bearing in mind that longitude not only runs to 180°, but is the long dimension of the Mediterranean Sea where the terms were first used. Latitude runs only to 90°. The word comes from the Latin for " wide," meaning the width of the Medi terranean.