May 7, 1874] 



NATURE 



II 



THE COMING TRANSIT OF VENUS* 

 III. 

 T N the previous articles various methods have been in- 

 ■'• dicated by means of which we may discover the scale 

 upon which the plan of the solar system is drawn. The 

 last article concluded by illustrating the nature of the 

 methods of employing a transit of Venus, as proposed by 

 Ilalley. It will be noticed that this method can be 

 utilised in the way there indicated only when Venus 



passes nearly across the diameter ot the sun. Halley, in 

 fact, founding his calculations upon erroneous data, was 

 led to conclude that this would be the case in 1761. In 

 this he erred, and another slight but important mistake 

 having been made in his calculations, it followed that at 

 Hudson's Bay, his northern station, the transit was in- 

 visible. 



The present article will be devoted to a description of 

 the methods to be employed in the coming transit for de- 

 termining the solar parallax. In subsequent articles the 



preparations which have actually been made for observing 

 the transit of 1S74 will be described ; and the difficulties 

 encountered in this kind of observation enumerated. 



Let the reader now examine Fig. 1 1 and pay particular 

 attention to the description of it, and he will thus be 

 enabled better to understand what follows. The earth, 

 Venus, and the sun are here represented in their relative 

 positions ; and lines are drawn to show the directions in 

 which twn nh5;orvpr'; at'opposite sides of the earth will see 



Venus upo',1 the solar disc. It follows from this that an 

 observer on the southern portion of the earth will see 

 Venus tra:e a path D K F upon the sun's disc farther 

 north thai the path ABC which a northern observer on 

 the earth sees it trace. Now Venus will be three times 

 as far fro n the sun as from the earth on that date. From 

 this it follows that the dis'ance between the two lines 

 ABC and D E F will be three times as great as the dis- 

 tance N S. But the distance N S upon the earth can be 



easily f jund ou*. Suppose it to be 6,000 miles. la that 

 case t .e distance between ABC and D E F is knowa to 

 be I S,ooo miles. But it needs no demonstration to con- 

 vince us that if we have a distance of 18,000 miles mea- 

 sured out for us upon the sun's surface we can dete mine 

 the distance of the sun from the earth. 



' Continued from'vol, iv, p."i39 



Now the apparent distance between the tivo lines ABC 

 and D E F is the least observed distances between Venus' 

 centre and the sun's during the transit. If then, we can 

 vieasiire accurately the least distance between the centres 

 of Venus an. i the sun, at two stations suitably chosen, we 

 can determine the sun's distance. 



There are three methods by means of which this may 



