and the Sun's Distance. 531 



cault's experiment, then we must at the same time diminish the 

 other term (the velocity of the earth) proportionally ; and the 

 old ratio will be preserved, and the value of aberration will be 

 left unchanged. Is it possible, therefore, that there can be an 

 uncertainty to the extent of 3 per cent, in the velocity of the 

 earth ? If so, the tables are turned ; and instead of employing 

 the ratio which aberration supplies to calculate the velocity of 

 light from the velocity of the earth, as the best known of the 

 two, we henceforth must calculate the velocity of the earth from 

 the velocity of light ; for Foucault has found the latter by ex- 

 periment more accurately than astronomy gives the former. If 

 there is an error of 3 per cent, in the velocity of the earth, it is an 

 error in space and not in time. To diminish the velocity of the 

 earth sufficiently by a change of time would demand an increase 

 in the length of the year amounting to eleven days nearly. 



The only other way of reducing the velocity of the earth is by 

 diminishing the circumference of the earth's orbit ; and this, if 

 diminished, changes proportionally the mean radius of the orbit, 

 that is, the sun's mean distance. The question therefore resolves 

 itself into this, Can the distance of the sun from the earth be 

 considered uncertain to the extent of 3 per cent, of the whole 

 distance ? 



The answer to this question will lead me into a brief discus- 

 sion of the processes by which the sun's distance from the earth 

 has been determined, and the limits of accuracy which belong to 

 the received value. To see the distance of any body is an act of 

 binocular vision. When the body is near, the two eyes of the 

 same individual converge upon it. The interval between the 

 eyes is the little base-line, and the angle which the optic axes of 

 the two eyes, when directed to the body, make with each other 

 is the parallax ; and by this simple triangulation, in which an 

 instinctive geometrical sense supersedes the use of sines and loga- 

 rithms, the distance of an object is roughly calculated. As the 

 distance of the object increases, the base-line must be enlarged; 

 but the geometrical method is the same, even when the object is 

 a star and the base of the triangle the diameter of the earth's 

 orbit. Substitute then for the two eyes of the same observer 

 the two telescopes of different astronomers, planted at the oppo- 

 site extremities of the earth's diameter, and any one will under- 

 stand the principle upon which the binocular eye of science takes 

 its stereoscopic view of the universe, and plunges into the depths 

 of space. In this way it is that the distance of the sun from 

 the earth is associated with the solar parallax, which is the 

 angle between the directions in which 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 



