The Starry Heavens 179 



earth's revolution about the sun. Each successive 

 night they rise nearly 4 minutes earlier, or, in the course 

 of a month, about 2 hours earlier. 



The stars do not appear to change their positions with 

 respect to other stars. Any triangle that you may 

 notice tonight with a certain star at each of its three 

 angles will have the same shape and size whenever you 

 look at it. However, some stars are actually moving 

 very swiftly through space perhaps hundreds of 

 miles every second. This motion is not visible to you 

 because the stars are so far away that their move- 

 ments will not change their apparent positions within 

 your lifetime. Do you understand how this can be 

 true ? Suppose a near object and a very distant object 

 were each moved a certain distance, say a foot or a rod. 

 Which one would have its direction from you most 

 changed ? 



Twinkling of the stars. If you look through the air 

 that is rising above a hot stove, the things you see appear 

 to quiver. The twinkling of the stars is due to the 

 light from them having to pass through the air that 

 is about the earth before it gets to our eyes. When 

 there are strong currents rising in the air above the earth, 

 as in the air heated by a stove, the stars seem more un- 

 steady than at other times. Stars near the horizon 

 twinkle more than do those near the zenith, because 

 light from them must then come through more air 

 before it reaches our eyes. The planets shine with a 

 steady light because they are comparatively near us, 

 and in a telescope a planet shows a disk of some size 

 instead of a mere point like a star. It sends light to us 



