476 GENERAL SCIENCE 



largest is only about 500 miles, and some of them so small that a 

 person would be able to ride around one in a few minutes in an 

 automobile. A farmer would need the entire planet for his garden. 



The pull of gravity on some of these is so slight that a man 

 could leap 60 feet and descend upon the planet without sustaining 

 any injury. These planets collectively are called asteroids. 



Stars. All the stars are much like our own sun. Each twinkling 

 star may have a system of worlds or planets traveling around it. 

 It is possible that some of the worlds are inhabited and that our 

 sun is seen shining only as a small star in the far distant unknown. 



The stars seem to be too numerous to count but in reality only 

 about 3000 stars are visible to the naked eye. With the aid of a 

 powerful telescope the number of stars seen increases to many millions. 



How the Distance to the Stars is Measured. The distance to 

 the stars is measured in a very interesting way : Let us look at some 

 object a short distance from us and then move in a straight line to 

 some other spot and look at the same object again. Draw a line 

 from the first position to the object and another one from the second 

 position to the object, forming an angle. Return to the first posi- 

 tion and look at some object farther away than the first one. Move 

 to the second position and view the same object. We find the new 

 angle formed to be much smaller than the first angle. We learn 

 that the farther the object is from us the smaller the angle becomes. 



The basis for the calculation of the distance of stars is measured 

 much in the same way except that two positions of the earth are 

 chosen from which to observe a star. The distance is the mean 

 radius of the earth's orbit (the earth's distance from the sun). This 

 distance serves as a base line and can be used to view the angular 

 displacement of stars viewed just as a surveyor may use a base line 

 to measure the angle displacement of objects on the earth. In 

 the case of the stars this displacement is called the parallax. Stars 

 are not listed according to distance, but according to their parallax. 

 For example, Alpha Centaura, probably the nearest star except 

 our sun, is at a distance of about 25,000,000,000,000 miles. To 

 find the distance of any star from the earth we divide 19,000,000,- 

 000,000, the mile value of one second, by a fraction representing 

 the actual parallax. For example, the North Star has a parallax 



