MIRACLES OF SCIENCE 



is fortunate for our system as Vega is a hundred times 

 brighter than our sun. 



MEASURING STAR DISTANCES AND SPEEDS 



It will be obvious that the amount of proper 

 motion or backward drift of a star gives us at least 

 a general notion of the distance of the star. A more 

 accurate determination of the distances, however, may 

 be made by viewing the star from exactly opposite 

 sides of the earth's orbit, and thus utilizing the inter- 

 vening space of 186,000,000 miles as a base line, some- 

 what as the surveyor utilizes base lines of carefully 

 measured length in his triangles. It is obvious that 

 the shift of position of a star due to its being viewed 

 from opposite sides of the earth's orbit will be oscil- 

 latory in character, as contrasted with background 

 stars that show no change of position. 



It is also obvious that the amount of this shift 

 will be much smaller than the annual proper motion, 

 since the sun's flight carries us over an annual dis- 

 tance almost double the diameter of the earth's orbit ; 

 nor can we magnify the result by taking observa- 

 tions at intervals of years as in the other case. It 

 follows that the oscillation due to the sun's annual 

 revolution is so slight, even in the case of the very 

 nearest stars, as to be exceedingly difficult of detec- 

 tion. For the vast body even of the stars that have 

 proper motion it is entirely beyond reach of observa- 

 tion with any existing instruments. 



When this semi-annual oscillatory shift can be 

 detected, however, it affords the most accurate 

 means of determining the exact distance of a star. 



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