74 LOCAL PARTICULARS OF THE TRANSIT OF VENUS, 1874. 



next the sun's limb, taking the form fii'st of the letter D, or a 

 dome, then that of a circle with a piece stretched out on one 

 side ; then as the ingress went on, becomiug like a pear, and 

 finally becoming suddenly round, but some distance within the 

 sun's edge. Now at what time during these phenomena did 

 ingress take place ? No one could answer the question, and a 

 very serious uncertainty, amounting to fifteen or twenty seconds, 

 existed in many of the observations. Fortunately the observers 

 recorded what they did see very carefully, and a century after- 

 wards all the difiiculty was cleared up by Mr. Stone, who was 

 able, by interpreting the observers' notes, to ascertain when the 

 ingress took place, at least for all the principal places, and he 

 thus cleared up a difficulty which had long been a sore puzzle to 

 astronomers. All the complete observations were found to agree 

 within small quantities, which were not greater than probable 

 errors of observation. 



Of course, in the coming transit, with the experience of the 

 past before us, it is hoped that observations made for Halley's 

 method will be much more successful and accurate than those of 

 1769. One of the essential conditions, however, of Halley's 

 method, viz., that the observer should see both beginning and 

 end of transit, necessarily limits the possible stations for its 

 application. The uncertainty of the weather is also introduced, 

 by which ingi'ess or egress may be lost, and it becomes necessary 

 to spread the stations over as wide a space as possible, to lessen 

 the chances of cloudy and bad weather. 



Still, another method of solving the question of the sun's 

 distance by means of a transit of Yenus was proposed by the 

 astronomer Delisle. It only requires the observer to see one 

 phase, either ingress or egress, and his observations are at once 

 of value. The principle upon which this method is based is 

 simply this — that if two observers are so situated as to see the 

 ingress or egress of Yenus at different times, the difference of 

 time represents a certain space over which Yenus has moved. 

 By reference to figure 6 it will be seen that A and B see Yenus 

 leave the sun one after the other — that B looking in the direction 

 B g sees it leaving when it crosses that line, and that A look- 

 ing in the direction A n sees it leaving some time after B. 

 Now it is evident that Yenus moved from the line B 9- to A n 

 in the interval ; and if A and B pick their station, they may both see 

 Yenus leave the sun at the same point, and therefore the lines 

 B g and A n will meet at the sun and form a triangle of which we 

 have a known base, A B, and the relation between the distances, 

 Sun to Yenus and- Yenus to Earth. Hence we find the linear 

 distance over which Yenus moved in the interval. And since we 

 know the time in which the given space was moved, we find the 

 whole length of orbit, and hence its radius, and by proportion 

 the earth's distance. . 



