158 SEVENTEENTH CENTURY. FT. HI, 



irom c to D. This will give us the measurement neces- 

 sary to lay down the position of the two transits on 

 paper. 



Draw a circle any size you please, and, ruling a line 

 across the centre, divide it into six parts (as in Fig 28 1 ), to 

 represent the six hours which Venus would take in crossing 

 the centre ; each of those parts will then represent the dis- 

 tance which she travels in an hour ; 5^ of these, therefore, 

 will be the distance she travels in 5-J- hours. Take this 

 length in your compasses, and place it at any part of the 

 circle where it will meet the edge at both ends, and in that 

 position draw the line c D. Then take a second length of 

 five parts only, and placing it below the other, rule the line 

 A B parallel to c D. These two lines express the path of 

 Venus, as observed by the two men, and we already know 

 that the distance between them is 2.\ times 7200, or 18,000 

 miles. 



It is now easy to compare this interval with the sun's 

 diameter. Suppose, for instance, that 47 such spaces will 

 cover the whole diameter of the circle, as they would if the 

 lines were drawn accurately in the observed positions, then 

 18,000 x 47, or 846,000 miles, would be the measure of the 

 sun's diameter. Now, we saw (p. 155) that the sun's dis- 

 tance is 108 times his diameter; therefore 846,000 x 108, 

 or 91,368,000 miles would, by these measurements, 

 be the distance of the sun from the earth ; and this is as 

 near as we can arrive at the truth when taking whole 

 numbers. 



You will perhaps ask, if the measurement of the transit 

 is such a simple process, why it takes months to make the 

 proper calculations. But you must remember that in our 



1 It must be drawn very much larger to approach to anything like 

 accuracy. This figure merely indicates the method. 



