ASTRONOMY 440 



But they early discovered the fact that five of 

 the stars were not fixed in the sky as the 

 others seemed to be, but moved in paths like 

 the moon's. These five the Greeks called 

 planets, a word which means wanderers. The 

 Chaldeans are said, too, to have been able to 

 predict eclipses with accuracy, and to have 

 known that the rotation of the four seasons 

 occupied 365% days. 



Ptolemy, an Egyptian of the second century 

 A. D., outlined the mechanism of the universe 

 as he believed it to be, and the world accepted 

 his views almost as late as our own time. 

 Ptolemy's earth was an immovable globe in 

 the center of the universe; around it revolved 

 a huge hollow sphere in whose walls were set 

 the stars, and inside of which, at different dis- 

 tances, traveled the sun, the moon and the 

 five planets then known, Mercury, Venus, 

 Mars, Jupiter and Saturn. As we watch the 

 Great Dipper slowly turn about the North Star 

 it is easy to understand how Ptolemy thought 

 as he did, though we know now that it is the 

 spinning of the earth on its axis that causes 

 this effect. 



Early in the sixteenth century Copernicus, 

 a Pole, learned that the doctrines of Ptolemy 

 were almost entirely wrong, for the sun is the 

 center about which the earth and the planets 

 revolve. But, as in the time of Pythagoras, 

 who had come to the same conclusion a little 

 more than 2,000 years earlier, people were not 

 ready to believe such teachings, and Coperni- 

 cus was excommunicated by the Pope. Within 

 the next two centuries lived four great astron- 

 omers, Tycho Brahe, Galileo, Kepler and New- 

 ton, who made many valuable observations 

 and discovered several of the fundamental laws, 

 the most vital of which is the one known as 

 Newton's Law of Gravitation (see NEWTON, 

 ISAAC). 



What the Heavens Contain. Because of the 

 faithful work of these men and those who 

 have followed them, astronomers now know 

 to a surprising extent how the material uni- 

 verse is arranged. We no longer think, with 

 Milton, that the Creator 



"made the stars, 



And set them In the firmament of heav'n, 

 T illuminate the earth," 



for we know that our own world is a very, 

 very small part of the universe which we can 

 see, and that beyond the reach of our eyes 

 in every direction there must be worlds upon 

 worlds and suns upon suns, to a truly infinite 

 extent. 



ASTRONOMY 



The earth is a planet like the five which the 

 ancients knew, like Uranus and Neptune, dis- 

 covered in modern times, and like the number- 

 less little earths called asteroids or planetoids, 

 many of which are yet undiscovered. All the 

 planets travel 

 around the sun, 

 whirling on their 

 axes as they go. 

 Together with 

 their moons, or 

 satellites, they 

 make up our 

 solar system. 

 Mercury, the in- 

 nermost planet, 

 has a year only 



as long as three HOW THE CAMERA DIS- 

 COVERS PLANETOIDS 



of our months, If a por tion of the heavens 

 while Neptune, is Photographed, the tele- 

 scope being moved by clock- 

 tne lartnest out, work to keep pace with the 



nassps around thp fixed stars ' a P lanet wil1 

 passes arouna tne show as a streak In thls 



sun only once in wa y hundreds of tiny worlds 

 -. f have been found. 



164.78 of our 



years. Each satellite revolves about its planet 

 much as the earth does about the sun, but sev- 

 eral of them circle in the opposite direction, and 

 the satellite of Uranus moves from south to 

 north. Saturn is known to have ten moons and 

 Jupiter nine, one of which takes two years and 

 two months to complete each journey. One of 

 the moons of Mars is only ten miles in diam- 

 eter, the other thirty-six. 



But the sun is not, as even Copernicus 

 thought, the center of all creation. It, too, 

 spins on its axis, once in twenty-five days, and 

 is carrying the solar system through space at 

 a rate of about sixteen miles a second, not 

 quite as fast as the earth moves in its annual 

 journey. This we know because the stars in 

 one direction are gradually spreading apart 

 (just as the two sides of a road open in front 

 of you as you speed along in an automobile), 

 while in the opposite direction they are mov- 

 ing together. Possibly our sun is revolving 

 about some other what one we do not know. 



All of the stars we see, the planets alone 

 excepted, are suns, some of them thousands 

 of times as big as our sun, which is itself 

 860,000 miles in diameter. Many of them, 

 perhaps all, have companions, some bright 

 suns, others dark globes which we cannot see, 

 with which they spin about as though they 

 were two stones chained together. The period 

 of revolution has in some cases been deter- 

 mined, and in others it can be estimated with 



