150 Our Surroundings 



The study of the skies, which deals with all the heavenly 

 bodies, we call astronomy. Every bit of knowledge that astron- 

 omy has gathered and classified concerning the heavens is highly 

 interesting, but it is necessary for us to limit our study to the sun, 

 the earth and its moon, and to a few of the stars and constella- 

 tions, or groups of stars. 



The Solar System. The solar system consists of the sun 

 and the bodies which revolve around it. The largest of these 

 are called the major planets. They are Mercury, Venus, Earth, 

 Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Most of 

 these, in their turn, have satellites, or moons, which revolve around 

 them. In addition to the major planets, there are over 250 minor 

 or lesser planets, numerous meteors, or shooting stars, and a num- 

 ber of comets, consisting of bright heads and long luminous tails. 



The Sun. The sun is the center of the solar system and is 

 its largest member. Its diameter is estimated to be 866,000 miles, 

 which is over 100 times the diameter of the earth. Its volume is 

 1,300,000 times that of the earth, and its weight is so great that 

 it can be expressed in tons only by a number made up of twenty- 

 eight figures. Its power of attraction is so strong that it over- 

 comes the centrifugal force, or tendency of the revolving planets 

 and other bodies of its system to fly off on straight lines, and 

 compels them to revolve about it. The path of the earth around 

 the sun is not a perfect circle but an ellipse an oval-shaped curve 

 having greater length than width. Because of this, the distance 

 from the earth to the sun is not always the same. 



Composition. There are various theories in regard to the 

 composition of the sun. One theory, Wilson's, assumes that the 

 sun consists of a solid dark globe surrounded by three atmos- 

 pheres. The atmosphere nearest the dark center is a dense, 

 cloudy substance having great reflecting power. The middle atmos- 

 phere is made up of a vast quantity of glowing gas from which 

 light and heat radiate. It is called the photosphere and is the 

 part that is visible to us. The outer atmosphere is quite similar 

 to our atmosphere and is transparent. 



The Kirchoff theory, the one now generally accepted, supposes 

 there are four different parts of the sun, arranged as follows: 



