Man's physical universe

xanabras

136 THE EARTH AS MAN'S ABODE

Inasmuch as warm water is less dense than cold water, the ocean

depths are nearly ice-cold (4° C.) even in the tropics.

The Earth Is Round.

The earth is a typical planet, and other planets are seen to be spherical

by observations with a telescope. The earth's shadow on the moon

is circular during an eclipse. Aviators and ships have circumnavigated

the earth. Photographs of the earth taken from great heights reveal a

curvature in all directions.

The circumference of the earth as measured checks with the circumference

calculated from the curvature obtained by measuring-instruments.

The circumference of the earth is about 24,800 miles. It is not

strictly spherical but is slightly bulged (to the extent of 27 miles in

diameter) at the equator.

You will recall that Newton suggested that

this bulging is due to centrifugal force produced by the rotation of the

earth.

The mass of the earth is about 6 X 10^^ tons.

The surface area is 197 X 10^ square miles.

The Lithosphere Is Thought to Consist of a Dense Core of Metal Surrounded

by a Thick Layer of Rock and a Relatively Thin Outside

Crust.

The average density of the earth is 5.5 times that of water, although

the average density of its crust, which is estimated to be about 60 miles

thick, is only 2.7 times that of water.

It thus appears that the center

of the earth is composed of denser material than the crust.

Earthquake waves showing abrupt changes in speed at a depth of

about 1800 miles suggest that the center of the earth, called the centrosphere,

is a sphere of iron having a diameter of about 4400 miles. There

is another abrupt change in the speed of earthquake waves at a depth

of about 300 miles. One hypothesis is that the earth's crust consists

of a layer of rock about 300 miles in depth and that there is a layer

of sulfides of nickel and iron about 1500 miles in depth between the

rocky crust and the metallic core. Tidal stresses of the sun and moon

deform the earth as they would deform a sphere of steel rather than

one of rock.

Examination of thousands of meteorites, as previously mentioned,

shows that they consist of rocks, nickel, and iron, the iron and nickel

being found in much larger quantities relative to their masses than

are found in the earth's crust. According to the planetesimal theory,

the earth and meteorites had a common origin. Where, then, is the

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