188 



EARTH AND HEAVENLY BODIES 



(2) The first part of a steamship that a person, stand- 

 ing at a harbor, sees is the funnels. The body of the 

 ship comes into view last. This indicates that the 

 earth's surface is curved, for if it were flat we should 

 see the large body of the ship first. (3) The shadow 

 of the earth on the moon when the earth comes di- 

 rectly between the sun and the moon is round. (4) 

 Scientists have 1 accurately measured the dimensions 

 of the earth, and their measurements show that the 

 earth is a sphere, slightly flattened at the poles. (5) 

 Photographs taken of the earth on balloon flights to 

 the stratosphere also indicate that the earth is a 

 sphere. 



Courtesy BaiiscK and Lomb Optical Company 



FIG. 309. PHOTOGRAPH OF THE MOON 



The weight of the earth has also been determined. 

 Its weight was first determined by Cavendish, a cele- 

 brated English physicist, in 1798. In 1930 Dr. Paul 

 R. Heyl of the United States Bureau of Standards 

 repeated Cavendish's experiment. The earth weighs 

 about 6,000 million million million tons. 



The surface of the earth with its mountains and 

 valleys looks very uneven. The highest mountain is 

 Mount Everest, 29,141 feet. Canyons more than a mile 

 deep roughen its surface, and the Pacific Ocean has 



a depth at one place of 35,430 feet. Yet on a twelve- 

 inch model of the earth we should scarcely be able to 

 notice these irregularities of the surface, and if we 

 were able to view the earth from the sun it would ap- 

 pear only as a tiny speck in space. 



Why does the moon appear to change its shape? 

 The moon is about 240,000 miles from the earth. It is 

 the nearest of the heavenly bodies. Its diameter is 

 2,160 miles, a little more than one-fourth that of the 

 earth. It looks about the same size as the sun, but in 

 reality the sun is 400 times larger. The sun is 400 

 times farther from the earth than the moon ; hence it 

 looks about the same size. 



The naked eye sees many light and dark areas on 

 the moon. Through a telescope the dark areas are 

 smooth and the light areas are rough. Photographs 

 of the moon taken with our largest telescopes show 

 the surface of the moon to be very uneven and moun- 

 tainous (Fig. 309). The dark areas are great plains, 

 surrounded by great mountains. Some of the moun- 

 tains are in chains that have been named after simi- 

 lar formations on the earth, such as the Alps and 

 Caucasus. Others rise majestically as isolated peaks. 

 Many of the mountains are from 1,000 to 2,000 feet in 



FIG. 310. PHASES OF THE MOON 



height, and some have elevations over 25,000 feet. 



There are thousands of formations on the moon 

 that resemble the volcanic craters on the earth. They 

 are called lunar or moon craters. It is not known, how- 

 ever, whether they are of volcanic origin, and there 

 seems to be no volcanic action on the moon at the 

 present time. The lunar craters vary in size from mere 

 pits less than one eighth of a mile across to great de- 

 pressions more than a hundred miles wide. Some of 

 the larger craters have surrounding rims with eleva- 



