THE TRANSIT OF VENUS. 215 



Mars over ten, whence we observe that Venus is our nearest neighbor, 

 and her distance from the sun two and a half times ours from her. 



As round numbers are given only for simplicity, and as we could 

 in fact draw such a map, with the actual elliptic orbits, in which no 

 error would exist which a microscope could detect, it may be asked, 

 " What more can be wanted ? " 



But there is a most important want unsupplied : our map has no 

 scale, and we do not know how much an inch on it represents in actual 

 distance. Our case, then, is like that of a person with an accurate 

 chart of his country before him, from which he wants to find his dis- 

 tance from the capital. If it have no scale attached, or an erroneous 

 one (and the latter is our own case), he cannot measure a single dis- 

 tance upon it. 



If, however, he can ascertain the actual number of miles between 

 any two points of the map, he will plainly know what an inch on it 

 stands for, and thus be able to construct the lacking scale ; and so we, 

 if we can measure the distance between any two primary planets, or 

 between any one of them (such as the earth) and the sun, have got at 

 the same time the means of determining all the dimensions of the 

 solar system. 



A determination of the distance of any remote object, which we 

 can see but cannot reach, whether celestial or terrestrial, the sun or a 

 mountain-top, requires that we should know either its size and the 

 angle it fills to the eye, or else how much the direction in which we 

 see it changes, as we change our own position by a known amount. 

 Thus, in the latter case, a surveyor, who wishes to determine his dis- 

 tance from an inaccessible object of unknown size, sends an assistant 

 to hold up a staff at the end of a line measured on the ground by a 

 chain. First he notes, with an instrument lor the purpose, the direc- 

 tion in which the object is seen as compared with that of the staff, and 

 then, the assistant and observer changing places, the latter notes again 

 the direction from the second point of view, and this will enable him 

 to calculate the distance desired. That first found by direct measure- 

 ment with the chain is called the " base-line," and it ought to be con- 

 siderable when the object is far away, since in that case its direction 

 will not, evidently, be much altered, without a corresponding altera- 

 tion in the observer's position. This difference of direction, caused by 

 a changed point of view, is called by astronomers parallax nearly 

 the only professional term with which the reader need be troubled, 

 but one which should be clearly understood. 



The principle involved in the method is probably familiar to him 

 already, but it is here recalled, to point out how its application must 

 be modified in finding the distance of the sun. As the earth sweeps 

 round that far-distant controller of her path, we can send no messen- 

 ger in advance along our orbit to distinguish the place we shall move 



