THE EARTH AS A PART OF THE SOLAR SYSTEM 533 



study. Learn to name first the constellations that can be seen 

 all the year, those near the North Star, such as the Great and 

 Little Dippers and Cassiopeia's Chair. During the winter, study 

 the winter constellations and during the last of the school year 

 study the summer constellations. In order to get them fixed 

 in mind, make a diagram in your notebook of each constellation 

 showing the position of the stars with reference to each other. 

 (This may also be given as a home project, in preparation 

 for which the instructor will make a diagram on the board of 

 the constellations, showing the position of the stars and ex- 

 plaining how to find them.) 



Distance of stars. The most marvelous and awe-inspir- 

 ing fact about the stars is the enormous distance at which 

 they are situated. They are so far away that the mile is 

 too small a unit to use in measuring these distances. So 

 astronomers have devised a new unit wKich is called the 

 light year. This is the distance that light travels in a year. 

 Light travels at the rate of 186,000 miles in a second ; that 

 is, it would travel around the earth seven times in a second. 

 It takes light only five hundred seconds to come to us from 

 the sun. In a year light would travel almost six trillion 

 miles. This distance, called the light year, is taken as the 

 unit in measuring distances of the stars. The very nearest 

 of all the stars is so far away that it takes light more than 

 four years to reach us. That is, the nearest star is twenty- 

 five trillion miles away. It takes light from the North Star 

 almost fifty years to reach us, so that the light you see from 

 this star started a short time after the close of the Civil War. 



A group of stars called the Pleiades is so far away that it 

 takes light one hundred and ninety years to reach us. The 

 light which we now see started fifty years before the Revolu- 

 tionary War. The rays of light that left the star at that 

 time traveled those fifty years preceding the war, during 

 the remainder of that century, during the first half of the 

 next century and up to the Civil War, and during the fifty 



