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 what 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. Ifin 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 
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