THE TRANSIT OF VENUS. 221 



lieves that it is nearer 8".91. The first value corresponds to a dis- 

 tance of 92,380,000 miles, the second to one of 91,730,000. It follows 

 that we have heretofore made an error of about three per cent, in esti- 

 mating the distances, and about ten per cent, in estimating the masses 

 of the solar system. Neither authority regards his result as more 

 than approximative, Prof. Newcomb, for instance, considering that 

 his own may, as likely as not, be over a hundred thousand miles from 

 the truth. 



We get no idea from these large-sounding numbers of the all but 

 inconceivable minuteness of the error of observation which would 

 cause them ; and such a measure of uncertainty, far from casting any 

 discredit on the exactness of modern astronomy, is an evidence of its 

 surprising advance toward absolute truth. Modern astronomy began 

 witL the age of Kepler ; but, while the angle which represents the 

 erroi in the parallax Kepler found, would correspond to that filled by 

 the vi idth of one of the pages of this magazine at a distance of 2,000 

 feet from the eye, the error now admitted as probable by Prof. New- 

 comb is represented by a less angle than that filled at the same dis- 

 tance by the same leaf turned edgewise. 



Now that we have considered the delicacy of the measurements 

 which have already been made, we are prepared to appreciate the 

 task of those who, on the 8th of this month, are about to try to better 

 them, and to examine the principles underlying the methods which 

 will be actually used in the trial. To do this, we may, perhaps, here 

 recur to a former illustration. If we suppose a person looking at a 

 remote object let us say a lighted window from a distance which 

 is quite half a mile, the distance between his eyes bears nearly the 

 same relation to that of the light, that the distance between any two 

 stations practically usable on the earth does to that of the sun. Ac- 

 cordingly, the difficulty of obtaining the sun's parallax, without mov- 

 ing: from off the earth, is the same in degree that the observer would 

 experience in measuring the distance of the light without moving 

 from his place, and by means of the small virtual change of his point 

 of view, obtained by looking at it with either eye ; and it is under 

 such all but insuperably hard conditions that astronomers will actually 

 be working this month. 



To see how Venus comes to their aid, we may represent her motion 

 by a car moving at a uniform rate on a circular ti*ack, between the 

 light and the observer. If the car pass across the light from left to 

 right (as Venus crosses the sun), it will of course cut off the observer's 

 view of the left side of the window from the left eye first, and, if the 

 motion be slow enough, we may suppose him to note the exact time 

 before the sight of the same point by the right eye is intercepted. 



If he know from previous watching how long it takes the car to 

 make its whole circuit of 360, he knows from his watch, by an ob- 



