THE SUN AND HIS FAMILY 



181 



stood would, aid in the mat- 

 ter of weather prediction. 



The chemical composition 

 of the sun has also been de- 

 termined, an achievement 

 which less than a hundred 

 years ago was thought im- 

 possible. Every chemical 

 element in the gaseous 

 state gives off its own dis- 

 tinctive kind of light. By 

 means of the spectroscope, a 

 marvelous instrument that 

 separates light from any 

 source into its parts, scien- 

 tists determine what chem- 

 ical elements are in the sun 

 or any other star. It is cer- 

 tain from an analysis of 

 sunlight that the sun is 

 composed of enormous 

 quantities of nearly all the 

 elements, such as iron, cop- 

 per, zinc, carbon, nickel, 

 oxygen, hydrogen, etc., 

 that are known on the 

 earth. 



We are dependent upon 

 the sun for our existence. 

 The sun is the source of 

 practically all our energy. 

 Without sunlight plants 

 could not grow and manu- 

 facture food. Since animals 

 depend upon plants for 

 their food, no animals could 

 exist on the earth without 

 sunlight. Even the wood, 

 coal, gas, and oil we use for 

 fuel possess energy ob- 

 tained from the sun at some 

 time in the past. When coal 

 burns it releases energy 

 that plants obtained from 

 the sun millions of years 

 ago. Nearly all the energy 

 and forces man directs on 

 the earth have their origin 

 in the sun. Because the heat 

 of the sun evaporates large 

 quantities of water into 

 the atmosphere, we have 

 streams and waterfalls. 

 These man harnesses to 

 run dynamos and generate 



Venus 

 Earth 

 Mars 



Asteroids - J 



Saturn/ 



Uranus 



Pluto- 



F1G. 298. COMPARISON OF 

 ORBITS OF THE PLANETS 



electricity which furnishes us with light and heat in 

 our homes. When we work and play we are directing 

 energy that was received by^plants from the sun. Can 

 you think of any form of energy, on the earth, that 

 does not originate in the sun? 



The sun is a huge, globular, gaseous mass, and it 

 is inconceivably hot. Its surface temperature is 

 10,000F., and at the center the temperature rises 

 enormously to 30,000,000 F. The sun has radiated 

 heat and light upon the earth hundreds of millions of 

 years, and scientists think that it will continue to 

 radiate heat and light for millions of years in the fu- 

 ture. The source of the vast amount of energy the sun 

 gives out is still a mystery. 



What has science learned of the planets? Revolving 

 around the sun are nine major planets. Look at Fig- 

 ure 298. Notice that there are four planets with orbits 

 quite close to the sun. These small planets are Mer- 

 cury, Venus, Earth, and Mars, which taken together 

 are known as the earth-like planets. The orbits of Jupi- 

 ter, Saturn, Uranus, and Neptune are farther away 

 from the sun ; these large planets are called the great 

 planets. The most recently discovered planet, Pluto, is 

 farthest from the sun. Notice also that the orbits of all 

 the planets lie in the same plane and that they all 

 move in the same direction in their orbits. Some de- 

 tails about each planet will be given in the following 

 paragraphs. 



Mercury. Mercury, the smallest of the nine major 

 planets, is the nearest to the sun. Its average distance 

 from the sun is about thirty-six million miles. Like all 

 the other planets it is named for a heathen god. Be- 

 cause this planet moves so swiftly, it was named for 

 the messenger of the Roman gods. It travels between 

 twenty-three and thirty-five miles a second and re- 

 quires only eighty-eight days to complete one revolu- 

 tion about the sun. It is not known whether Mercury 

 rotates on an axis. If it does not, the side toward the 

 sun must be a burning desert and the other side a 

 frozen waste. Mercury has little or no atmosphere. 

 Because of its small size, its gravitational attraction is 

 not sufficient to hold an atmosphere to it. 



Venus. Venus is more like the earth than any other 

 planet. It is almost as large as the earth and it has an 

 atmosphere nearly as dense as ours. Because the at- 

 mosphere of Venus is always cloudy, no definite mark- 

 ings on the surface can be seen. This makes it diffi- 

 cult to determine how much time is required for it to 

 make one rotation on its axis. At present it is thought 

 that the period of rotation of the planet Venus is long 

 in comparison with that of the earth. Therefore one 

 side of the planet is very hot and the other side very 

 cool. Since Venus has an atmosphere and is farther 

 from the sun, the difference in temperature of the two 

 sides would not be so great as on Mercury. 



