2l6 



Trees, Stars, and Birds 



FIG. 133. Measuring the size of a distant 

 object. By this same method astronomers, 

 knowing the distance to the sun, moon, and 

 planets, can calculate their size. 



By this same method 

 the astronomers can 

 calculate the actual size 

 of the sun, moon, and 

 planets. They find the 

 angle made by two lines 

 running from opposite 

 points on the edges of 

 one of these bodies. 

 Then, having already de- 

 termined the distance of 

 the body, they are able 

 to calculate its size. 

 This method cannot be 

 used to determine the 



size of the stars, for stars appear as mere points of light 

 even when viewed with large telescopes. 



Material of the sun. Astronomers are sure that iron, 

 nickel, carbon, and many other substances that we are 

 familiar with on the earth, exist also in the sun. The 

 sun is so hot that even the iron in it is not solid or 

 liquid but is in the condition of vapor. Substances in 

 this condition occupy more space than they do when 

 condensed to a liquid or solid; hence vapors are 

 light. The sun in proportion to its size is only about 

 one fourth as heavy as the earth. Ask a teacher 

 of physics how we know what materials are in the 

 sun. 



Granular appearance and spots on the sun. The 

 appearance of the sun is always changing; nothing 

 seems to be fixed or solid. The surface appears granular, 



