THE SUN'S DIAMETER. 



i6i 



Fig. 27. 



hours crossing from a to b, Fig. 27, and the other that she 

 was 5^ hours crossing from c to d. This will give us the 

 measurement necessary 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. 27'), 

 to represent the 6 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 J 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 J times 7,200, 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 159) that the sun's dis- 



Transit of Venus. 



s, Face of the sun. v, Venus. A B, 

 Transit observed so as to occupy five 

 hours, c D, Same transit observed so 

 as to occupy five-and-a-quarter hours. 



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

 accuracy. This figure merely indicates the method. 



