168 J. Lovering on Velocity of Light and the Sun’s Distance. 
value of aberration will be left unchanged. Is it possible, there- 
fore, that there can be an uncertainty to the extent of three 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 He 
locity of the earth from the velocity of light. For, Fouca 
has found the latter by geal more accurately than iz 
tronomy gives the former. If there is an error of three 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 
ear amounting to eleven days nearly. 
“The only other way of reaching the velocity of the earth is 
by diminishing the circumference of the earth’s orbit, and this, 
if diminished, changes , broportionally the mean radius of the 
orbit; that is, the sun’s mean distance. The question, there- 
riety resolves ne! into this. Can the distance of the sun from 
earth be considered uncertain to the extent of three per 
sent of the whole distance ? 
sare answer to this question will lead me into a brief discussion 
f the processes by which the sun’s distance from the earth has 
etn doientniad: and the limits of accuracy which belong to the 
received value. To see “ia distance of any body is an actof 
binocular vision. When t ly is near, the two eyes of t 
same individual converge pe it. The interval between the 
ie is the little base-line, and the angle which the optic axes of 
e ve eyes, when directed to the bod , make with each other 
> 
: 
; 
, 
llax; and by this simple triangulation, i in which an a 
inbactve 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 cial e : 
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 z 
orbit. Substitute then for the two eyes of the same observer — 
takes its stereoscopic view of the universe, sad plunges into i 
fon ts of space. In this way it is that the distance of the sum 
iods ‘of revolution, if the astronomer finds 
