212 Trees, Stars, and Birds 



answers to all of them without understanding the reason. 

 The explanations for many of the things you can observe 

 about the stars may be found in some of the following 

 facts. This knowledge is of little value unless you 

 understand how these facts account for what you ob- 

 serve. 



You see stars and other objects only by means of 

 light that comes from them to your eyes. Light travels 

 in straight lines. It goes through empty space, through 

 certain substances as glass and air, but not through 

 everything. Dust, mist, cloud, water, and solid earth 

 either dim the light or shut out all of it. Air surrounds 

 the earth, and the atmospheric ocean at the bottom of 

 which we live is more than 100 miles deep ; but the 

 clouds, mist, and dust are most abundant in the lower 

 10 miles of the atmosphere. Light coming from a 

 body near the horizon, in order to reach our eyes, 

 must penetrate farther through the lower layers of the 

 atmosphere than light coming from overhead. 



Our eyes are affected by strong lights, so that they do 

 not perceive dim ones at the same time. Twilight, 

 moonlight, and artificial lights shining in our eyes make 

 them less sensitive to very faint lights. 



The earth rotates on its axis, carrying us with it. 

 While it rotates it is also moving around the sun at the 

 rate of about iS-J miles a second. The time of a rotation 

 is called a day, and the time it requires the earth to 

 make a complete journey around the sun is called a 

 year. These movements of the earth may be under- 

 stood by certain experiments that any pupil can make 

 for himself. 



