CELESTIAL MEASURINGS AND WEIGHINGS. 195 



which the star is seen will undergo a semi-annual dis- 

 placement to and fro to the amount of the apparent (an- 

 gular) breadth of the orbit as it would be seen by a spec- 

 tator in the star. And this, in its equivalent form of 

 annual displacement, is the angle astronomers have to 

 measure for the purpose in question. One would natu- 

 rally suppose that so enormous a magnitude would be 

 something conspicuous from any distance short of the 

 fabulous; and that here at least we should have some- 

 thing to deal with palpable to very moderate means of 

 observation. Pent up and " chafing within the narrow 

 limit of the world" the astronomer in his measurement 

 of the sun's distance might complain, in the words which 

 the poet puts into the mouth of the great conqueror of 

 antiquity, of restricted elbow-room. Using the world 

 itself as a means of transport, and thus enabled to com- 

 mence anew on so vast a scale, he might expect to find 

 "ample room and verge enough" for his operations. 

 Quite the contrary ! The earth itself seen from the sun 

 would appear as large as the globe of Saturn at its medium 

 distance does to us a very conspicuous object in a 

 moderately good telescope. A globe large enough to 

 fill the earth's orbit round the sun would appear to a 

 spectator placed in the nearest fixed star, hardly larger 

 than the third satellite of Jupiter, as seen from the earth ; 

 which requires a very good telescope to be perceived to 

 have any size at all. 



(20.) Two methods only have been devised by which 

 this annual or parallactic displacement (as it is technically 



