ASTRONOMY 



to convey any meaning. We therefore employ 

 other units more suitable for measuring the 

 heavens. Consider for a moment the distance 

 of a Centauri, the star nearest to us. If we con- 

 struct a model of the universe on the scale of 

 i foot to 20 million miles, our sun will be re- 

 presented by a sphere less than half an inch in 

 diameter, and the earth will be represented by a 

 speck of dust one two-hundredth of an inch in 

 diameter revolving round the sun at a distance of 

 a little over 4^ feet. The base line which astro- 

 nomers have to use for measuring the distances of 

 the stars is the diameter of the earth's orbit, and 

 in our model this is 9^ feet long, a Centauri, 

 which is a double star, will then be represented by 

 two spheres equal in size to the sphere representing 

 the sun, placed at Newcastle, 240 miles away. 

 This model enables us to form some idea of the 

 vastness of space, and of the extreme sparseness 

 of the matter which it contains. The actual 

 distance of a Centauri is of the order of ten 

 millions of millions of miles, and therefore we find 

 it more convenient to take as our unit for measuring 

 celestial distances the distance travelled by light, 

 whose speed is 186,600 miles per second, in one 

 year. This length is called a light-year. Now the 

 above figure represents a pathway leading to a point 

 on the distant horizon, the nearer part being paved 

 with square blocks whose sides are 10 light-years 



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