222 THE POPULAR SCIENCE MONTHLY. 



viously simple computation, just what part of a degree it went over in 

 passing, or in its shadow's passing, from one eye to the other; the ano-le 

 in other words, that the distance between his eyes would appear under 

 as seen from the light. But this is the parallax of the light, and it 

 gives him its distance at once (that between the eyes the base-line 

 being known). 



This suggests the principle of a method of obtaining the sun's par- 

 allax, on which the English astronomers will largely rely. 



For, neglecting matters of detail, and supposing Venus to pass 

 centrally across the sun, since she completes her revolution of 360 in 

 225 days, nearly, we find, on dividing 360 by the number of minutes 

 in that period, that in one minute she moves through an ai*c of 4." and 

 dividing 360 by the number of minutes in our year, that the earth 

 moves through 2".46 in the same time. Hence, as Venus is gaining 

 1".54 every minute, the case is the same as though the earth stood 

 still, and the shadow of Venus (could she throw one so far) passed 

 over the earth at that rate as seen from the sun. 



Suppose an observer on the left or eastern side of the o-l be had 

 his view of part of the left side of the sun intercepted by the in- 

 terior planet at nine o'clock, and one placed opposite the centre of the 

 globe (at half the earth's diameter west of the first), five and three- 

 quarter minutes later, then, since 5f times 1".54 is 8". 85, this angle 

 8".85 represents the difference of directions in which the sun would be 

 seen by the two observers, or, what is the same thing, the ano-le the 

 earth's semi-diameter would fill to an eye at the sun. This is the solar 

 parallax, and on reference to our tables we should find that such a 

 difference of direction could only be caused by an object nearly 

 92,000,000 miles off. In practice, observers are not stationed at the 

 extreme edge of the earth (as seen from the sun), because from such 

 a station the sun itself would be seen in the horizon, whei'e vision of it 

 is obscured and rendered unsteady by the vapors of our atmosphere. 

 Neither is it needful to place observers just half a diameter of the 

 earth apart, since it is easy to allow for the effect of greater or less 



Fig. 3. Showino the Displacement op Venus. 



distance, and, in reality, the time would be longer than that supposed, 

 because Venus's path lies aslant to the sun's edge, and it takes her lon- 

 ger to cross it. But it will, of course, be understood that such mat- 

 ters as these, and such complications as arise from the elliptical form 

 of the orbits, the real inequality of the motion, the fact of the earth's 



