The Primary Planets. 303 



8 seconds, as already intimated, and his distance 

 from the earth to be about ninety-five millions 

 of miles. 



The distance of the sun from the earth being 

 well ascertained, the distance of the other planets 

 may be easily calculated by the second law of 

 Kepler ; as their orbits or rather the time occu- 

 pied in traversing their orbits, is known by 

 observation. The following table will be found, 

 believe, to exhibit a fair statement of their re- 

 spective distances. 



the observer being carried in a direction apparently con- 

 trary to the former, the errors may counteract each other. 



'* Let the representations be as in the last figure. If the 

 sun has declination at the time of the transit, B (fig. 1 18.) 

 will represent the pole towards which the sun declines. 

 The observer at A, if at rest, would behold the transit 

 during the time Venus passes from V to Wj but being by 

 the earth's diurnal revolution carried from A through the 

 arc A E P to P, and arriving at P at the instant in which 

 Venus arrives at U, he will perceive the transit just finish- 

 ing at D ; consequently its duration will be as much longer 

 than the computed time as the heliocentric arc V U is 

 longer than V W. V U being found by the before-men- 

 tioned analogy, the difference between V U and V W is 

 W U or the parallax of A P, as before. 



" Now, in these two cases, a similar error will have a 

 contrary effect in the first to that which it has in the lat- 

 ter. For, if, by any error, the computed arc V W 

 (fig. 117-) be taken too large, the arc U W, and conse- 

 quently the parallax, will come out too great. But in the 

 latter observation, if the computed arc V W (fig. 1 18.) is 

 taken too large, the arc W U, and consequently the paral- 

 lax will come out too little. Therefore the mean between 

 two such observations will be much more to be depended 

 on than either singly. 



