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which Veni;s appeared to occupy would be 18,000 miles apart. It 

 is only necessary to measure the proportion tliis distance bears to 

 the whole diameter of the Sun, to arrive at the true value of 

 the latter ; and the diameter of the Sun known, its distance can 

 be readily inferred. But in practice it is difficult to measure the 

 position of Venus on the Sun, indeed until the invention of 

 photography it was impossible to do it with the requisite accuracy. 

 Photography, though enabling us to compare the observations 

 made at different stations with the greatest ease, has yet its own 

 peculiar difficulties and drawbacks, and though the American 

 astronomers place great reliance on the method, English astronomers 

 are as a rule disposed to regard it as inferior in precision to the 

 methods which were used in the Transits of last century. 



The first of these, called Halley's method, depends on the fact 

 that Venus as seen from some stations must seem to pass along a 

 shorter line on the Sun's disk, than when seen from others. The 

 Transit, therefore, does not take so long a time when seen from 

 some places as from others. The earth's rotation also makes 

 Venus seem to travel more quickly, as viewed from some places, 

 than from others ; and it is often possible to choose stations such 

 that both causes work the same way. In this case, if the 

 duration of the Transit be carefully noted at each station, the 

 difference betAveen these intervals would, by the proportion it bears 

 to the whole duration, give a means of determining the proportion 

 between the distance separating the two apparent paths of Venus, 

 and the entire diameter of the Sun, the very thing sought to be 

 accomplished by direct measurement in the photographic method. 

 The other, or Delisle's method, depends on the fact that the 

 Transit seems to begin or to end at different times when viewed 

 from different places, and the differences of the times of the 

 planet's entering on or leaving the Sun's disk are used in a some- 

 what similar manner as the differences of durations on HaUey's 

 method, to give the needed proportion. 



Both methods depend therefore for success on the accuracy 

 with which the time of Venus's entry on and exit from the Sun's 

 disk can be determined. Unfortunately, a good deal of difficulty 

 is experienced in making this determination, partly owing to what 

 is known as the "black drop." 



