OF ARTS AND SCIENCES: APRIL 14, 1863. 123 



tance of the sun from the earth is associated with the solar parallax, 

 which is the ansjle between the directions in whicli two astronomers 

 point their telescopes when they are looking at the sun at the same 

 moment. To know the sun's distance, the astronomer studies the solar 

 pai'allax. As Kepler's third law establishes a relation between the 

 distances of the different planets from the sun and their periods of 

 revolution, if the astronomer finds either distance by observation, the 

 others can be computed from this law. As the horizontal solar parallax 

 is only about 8 seconds, and an eiTor of ^jj of a second includes an error 

 of more than a million of miles in the sun's distance, he takes advantage 

 of the law of Kepler, and selects a planet which comes occasionally 

 nearer to the earth than the sun. The choice lies between Venus at 

 inferior conjunction and Mars at opposition. The parallax of Mars 

 may vary from 20".7 to 19".l, according to the positions of Mars and 

 the Earth with respect to the perihelion of the orbit, at the time of 

 opposition. The parallax of Venus at conjunction may vary, for 

 the same reason, from 33".9 to 29".9. Venus, therefore, may be 

 nearer to the earth than Mars, and the parallax more favorable. But 

 Venus cannot be seen at conjunction except when its latitude is so 

 small that a transit across the sun's disc occurs. Then the two observ- 

 ers refer its place, not to a star, but to the sun, and the quantity they 

 determine is the difference of parallax between Venus and the sun, 

 which will vary from about 21" to 25". Moreover, the difference of 

 parallax is measured, not directly, but through the influence it produces 

 on the duration of the transit at the two stations, and therefore upon 

 a greatly enlarged scale. 



What are the results which have been obtained, 1st, by observa- 

 tions of the transits of Venus, and, 2d, by observations of Mars at 

 opposition ? 



1. Only two transits of Venus have occurred since the time when 

 the sagacious Dr. Halley invoked the attention of posterity to these 

 rare astronomical events, as pregnant with the grandest results to 

 science; viz. those of 1761 and 1769. The astronomers of the last 

 century did not neglect the charge which Halley consigned to them. 

 The transit of 1769 was eminently favorable, offering a chance which 

 comes only once in a millennium, as Professor Winthrop happily 

 explained in his lectures on the last transits. 



Whatever verdict posterity shall pronounce on the deductions from 

 the observations then made, they will never, says Encke, reproach 



