412 THE BEAUTIES OF NATURE CHAP. 



naked eye, and the millions which make up 

 the Milky Way, are considered to be on an 

 average fully equal to the Sun in lustre. 



Arcturus is, so far as we know at present, 

 the swiftest, brightest, and largest of all. Its 

 speed is over 300 miles a second, it is said to 

 be 8000 times as bright . as the Sun, and 80 

 times as large, while its distance is so great 

 that its light takes 200 years in reaching us. 



The distances of the heavenly bodies are 

 ascertained by w r hat is known as " parallax." 

 Suppose the ellipse (Fig. 54), marked Jan., 

 Apr., July, Oct., represents the course of the 

 Earth round the Sun, and that A B are two 

 stars. If in January we look at the star A, 

 we see it projected against the front of the 

 sky marked 1. Three months later it would 

 appear to be at 2, and thus as we move round 

 our orbit the star itself appears to move in 

 the ellipse 1, 2, 3, 4. The more distant star 

 B also appears to move in a similar, but 

 smaller, ellipse ; the difference arising from 

 the greater distance. The size of the ellipse 

 is inversely proportional to the distance, and 

 hence as we know the magnitude of the 



