THE SOLAR SYSTEM. 105 



situated in the same region of the sky. In the case of Mars that 

 can always be done by making use of a neighbouring star, but 

 when Venus is near the earth she is also so close to the sun that 

 stars are not available, and consequently her parallax can be satis- 

 factorily measured only when her position can be accurately 

 referred to that of the sun, or, in other words, only during her 

 transits across the sun's disc. But even when the two bodies to 

 be compared are sufficiently near each other, we are still embar- 

 rassed by the fact that it is more difficult to measure the distance 

 between the limb of a planet and a star or the hmb of the sun, 

 than it is to measure the distance between two stars, and since the 

 discovery of so many asteroids, that circumstance has led to their 

 use for determinations of the solar parallax. Some of these bodies 

 approach within 75,230,000 miles of the earth's orbit, and as they 

 look precisely like stars, the increased accuracy of pointing on 

 them fully makes up for their greater distance, as compared with 

 Mars or Venus. 



After the Copernican system of the world and the New^tonian 

 theory of gravitation were accepted, it soon became evident that 

 trigonometrical measurements of the solar parallax might be sup- 

 plemented by determinations based on the theory of gravitation, 

 and the first attempts in that direction were made by Machin in 

 1729 and T. Mayer in 1753. The measurement of the velocity 

 of light between points on the earth's surface, first effected by 

 Fizeau in 1849, opened up still other possibilities, and thus for 

 determining the solar parallax we now have at our command no 

 less than three entirely distinct classes of methods which are 

 known respectively as the trigonometrical, the gravitational, and 

 the photo-tachymetrical. We have already given a summary 

 sketch of the trigonometrical methods, as applied by the ancient 

 astronomers to the dichotomy and shadow cone of the moon, and 

 by the moderns to Venus, Mars, and the asteroids, and we shall 

 next glance briefly at the gravitational and photo-tachymetrical 

 methods. 



>!< H« * ^ * 



The theory of probability and uniform experience alike show 

 that the limit of accuracy attainable with any instrument is soon 

 reached ; and yet we all know the fascination which continually 



