THE TRANSIT OF 'VENUS. 217 



can get for the eyes, the sun's face for the wall, and Venus for the 

 pencil, with a better idea of the way in which her coming between us 

 and the sun will help to find how far off it is. 



By the sun's horizontal parallax is meant that particular amount 

 of change in its direction which would be noted by our two observers 

 if they were half the diameter of the earth apart (as in Fig. 1, where the 

 observer at A sees "Fin the direction A B, the one at C in the direc- 

 tion C D, and where the difference of these directions is A V C, the 

 angle under which the earth's radius would be seen from V). At the 



Fig. 1. Horizontal Parallax. 



risk of needless repetition, the reader is again asked to keep in mind 

 that, finding an object's distance and finding its parallax are convert- 

 ible terms : that when the latter is large it is easily got, and implies 

 a short distance ; that when small, it is found with difficulty, and im- 

 plies great distance, and that the solar horizontal parallax is almost 

 immeasurably small less, whatever it is, than an angle of 9.", or 

 than that which would be filled by a human hair over five feet from 

 the eye. It was an error of one-thirtieth part of this last an error 

 less, that is, than a literal hair's-breadth seen fifty yards off which 

 caused the mistake of 3,000,000 miles, now known to have been made 

 in measuring the sun's distance in 1769 ; and, if the reader has heard 

 such a mistake cited to the discredit of astronomy, he is now in a 

 position to judge of the justice of the reproach. It may be added, in 

 the words of Sir John Herschel : " Moreover, this error has been de- 

 tected, and the correction applied, and the detection and correction 

 have originated with the friends, and not the enemies, of science." 



If we briefly review the history of human effort at this problem, 

 we find it occupying the mind of the ancient philosophy as well as the 

 modern. Ptolemy, following Hipparchus, estimated, by an unreliable 

 method, the solar parallax at 3.', or its distance at 1,210 semi-diameters 

 of the earth ; and this grossly erroneous value remained unimproved 

 to the time of Kepler, with whose age modern astronomy begins. 

 Kepler having, by life-long study, discovered a means of obtaining 

 the proportionate distances of the planets from the sun, saw clearly 

 that this led to a new method of findiug its absolute distance ; since, 

 whatever it is, it stands in a known relation to that of Venus and 

 Mars, either of which is easier found, owing to the comparative near- 

 ness of these planets, when in a line with the earth and sun. Venus, 

 at this time, commonly passes above or below the sun, and in either 



