The Primary Planets. 299 



mense distance, and in so small an angle, the 

 error of one second will create an error of seven 

 millions of miles. Hence the amazing difference 

 in the calculations of different astronomers. 

 Ptolemy and his followers, as well as Tycho 

 Brahe and Copernicus, conceived the sun's di- 

 stance to be 1200 semidiameters of the earth ; 

 Kepler nearly 3500, and Ricciolus doubles that 

 distance. 



The celebrated Dr. Halley first pointed out 

 the means of solving this difficult problem, which 

 he terms " the most noble in the sciences," upon 

 theoretical principles, though in the course of 

 nature he could never expect to see them re- 

 duced to practice. 



Venus passes the sun, or is, in the astro- 

 nomer's phrase^ in conjunction with it, very 

 often ; and if the plane of her orbit were coin- 

 cident with the plane of the ecliptic, she would 

 on such occasions appear like a spot on the sun 

 for about seven hours. But the orbit of Venus 

 only intersects the ecliptic in two points, which 

 are called its nodes. Venus, therefore, can never 

 be seen on the sun but at those inferior con- 

 junctions which happen in or near the nodes of 

 her orbit ; and though this circumstance seldom 

 happens, the time of its occurring is easily cal- 

 culated by astronomers. The last transit before 

 the time of Dr. Halley was in the year 1639, and 

 he calculated that one would again occur in 176J, 

 and another in 1769. 



