182 



EARTH AND HEAVENLY BODIES 



With the exception of our sun and moon, Venus 

 far outshines any other heavenly body. Sometimes it 

 comes up in the east before the sun, and it is then 

 called the morning "star." For several months it ap- 

 pears after sunset in the west, and it is then called 

 the evening "star." Of course it is not a star because 

 it shines by reflecting light from the sun. Stars are 

 self-luminous bodies. 



Earth. The earth is the largest of the earth-like 

 planets. It is about 8,000 miles in diameter and 25,000 

 miles around at the equator. It rotates on its axis once 

 in twenty-four hours and completes its revolution 

 around the sun in 365 l /$ days. More information about 

 the earth will be given in the next topic and also in the 

 next unit. 



Mars. Mars perhaps has excited more interest than 

 any other planet, and many persons wonder whether 

 there are living things upon it. Before considering the 

 possibility of life on this planet, however, it is neces- 

 sary to know some facts about it. The diameter of 

 Mars is about half that of the earth and its mass is 

 only one-ninth as great. Because of its small mass, the 

 gravitational force of Mars is much less than that of 

 the earth. Hence it must have a thin atmosphere. Life 

 as we know it also requires water and the proper tem- 



Courtcsy Mount Wilson Observatory 



FIG. 299. TWO VIEWS OF MARS 



Notice the "ice caps." 



peratures. Astronomers have noticed the appearance 

 and disappearance of white caps at the polar regions 

 of Mars, which are thought by some to be ice caps, 

 similar to the ice caps in the polar regions of the earth. 

 It has never been conclusively proved, however, that 

 the polar white caps are formed from water. Concern- 

 ing the temperature of Mars it has been estimated that 

 the sun's heat on Mars has about two-thirds the in- 

 tensity of the heat which warms the earth. This 

 would be sufficient heat for life, but since Mars has 

 a very limited atmosphere, at night its surface must 

 cool very rapidly and the temperature probably falls 

 below the freezing point every night. 



In 1877 the Italian astronomer G. V. Schiaparelli 



announced the discovery of strange markings on the 

 surface of Mars. He described them as long, narrow, 

 straight dark streaks which he called "canali" (chan- 

 nels). Since Schiaparelli's time many other observers 

 have claimed to have verified his observations. Some 

 people with vivid imaginations have brought forward 

 the claim that these markings on Mars are great canals 

 constructed by engineers for running the melting 

 snow water of the polar caps down toward the central 

 regions of the planet for irrigation purposes. This 

 view has been defended by men of great ability, but 

 many other competent observers say they are unable 

 to find evidences of such intelligent and extraordinary 

 feats of engineering. 



The only scientific conclusion we can come to at 

 the present time concerning life on Mars is that there 

 is a possibility that Mars supports life, but so far we 

 have no direct evidence of life of any sort there. In 

 fact, in many ways Venus is more like the earth than 

 Mars, and it may be better able to support life. 



Jupiter. Jupiter is sometimes called the giant planet 

 because it is the largest. It is 86,000 miles in diameter 

 and 317 times as massive as the earth. Jupiter is 483 

 million miles from the sun, 

 so that the sun's rays which 

 strike it have only one 

 thirtieth of their intensity 

 on the earth. Although 

 Jupiter travels at the rate 

 of eight miles a second, 

 twelve of our .years are re- 

 quired for it to make one 

 revolution around the sun. 



Jupiter is an interesting 

 sight through a small tele- 

 scope. In addition to the 

 planet four little moons that revolve about it in a row 

 can be seen. They were the first objects in the sky 

 discovered by Galileo in 1610 with the telescope he 

 invented. Jupiter has five more moons, but they are 

 so small that they are visible only through a large 

 telescope. 



Saturn. To the unaided eye Saturn appears like a 

 yellowish star, but when viewed through a telescope 

 it is one of the wonderful sights of the heavens. Saturn 

 possesses a beautiful system of rings that encircle it 

 (Fig. 301), and it is also attended by ten moons that 

 revolve about it. Saturn has three rings. The diameter 

 of Saturn itself is about 70,000 miles, but the diameter 

 of its outer ring is 167,000 miles. The rings are very 

 thin, the outer one having a thickness of about ten 

 miles. The exact nature of these rings is not known. 

 Moulton, a noted American astronomer, thinks they 

 are swarms of bodies individually small and probably 

 dust-like in character. 



FIG. 300. JUPITER WITH 

 ITS MOONS 



