DRIFTING LIGHT-WAVES. 



325 



therefore the measures of the dimensions of the 

 solar system, in vogue before the recent transit. 

 These measures fall short to some degree of 

 those which result from the observations made in 

 December, 1874, on Venus in transit, the sun's 

 distance being estimated at about 91,400,000 

 miles instead of 92,000,000 miles, which would 

 seem to be nearer the real distance. Of course, 

 all the motions within the solar system would be 

 correspondingly under-estimated. On the other 

 hand, the new method would give all velocities 

 with absolute correctness if instrumental diffi- 

 culties could be overcome. The difference be- 

 tween the real velocities of Venus approaching 

 and receding, and those calculated according to 

 the present inexact estimate of the sun's distance, 

 is, however, much less than the observed dis- 

 crepancy, doubtless due to the difficulties in- 

 volved in the application of this most difficult 

 method. I note the point, chiefly for the sake 

 of mentioning the circumstance that theoretically 

 the method affords a new means of measuring the 

 dimensions of the solar system. Whensoever the 

 practical application of the method has been so 

 far improved that the rate of approach or reces- 

 sion of Venus, or Mercury, or Jupiter, or Saturn 

 (any of these planets), can be determined on any 

 occasion, with great nicety, we can at once infer 

 the sun's distance with corresponding exactness. 

 Considering that the method has not been in- 

 vented ten years (setting aside Doppler's first 

 vague ideas respecting it), and that spectroscopic 

 analysis as a method of exact observation is as 

 yet little more than a quarter of a century old, 

 we may fairly hope that in the years to come the 

 new method, already successfully applied to meas- 

 ure motions of recession and approach at the 

 rate of twenty or thirty miles per second, will be 

 employed successfully in measuring much smaller 

 velocities. Then will it give us a new method of 

 measuring the great base-line of astronomical 

 surveying — the distance of our world from the 

 centre of the solar system. 



That this will one day happen is rendered 

 highly probable, in my opinion, by the successes 

 next to be related. 



Besides the motions of the planets around the 

 sun, there are their motions of rotation, and the 

 rotation of the sun himself upon his axis. Some 

 among these turning motions are sufficiently 

 rapid to be dealt with by the new method. The 

 most rapid rotational motion with which we are 

 acquainted from actual observation is that of 

 the planet Jupiter. The circuit of his equator 

 amounts to about 267,000 miles, and he turns 



once on his axis in a few minutes less than ten 

 hours, so that his equatorial surface travels at the 

 rate of about 26,700 miles an hour, or nearly seven 

 and a half miles per second. Thus, between the 

 advancing and retreating sides of the equator 

 there is a difference of motion in the line of sight 

 amounting to nearly fifteen miles. But this is 

 not all. Jupiter shines by reflecting sunlight. 

 Now, it is easily seen that where his turning 

 equator meets the waves of light from the sun, 

 these are shortened, in the same sense that waves 

 are shortened for a swimmer traveling to meet 

 them, while these waves, already shortened in 

 this way, are further shortened when starting 

 from the same advancing surface of Jupiter, on 

 their journey to us after reflection. In this way, 

 the shortening of the waves is doubled, at least 

 when the earth is so placed that Jupiter lies in 

 the same direction from us as from the sun, the 

 very time, in fact, when Jupiter is most favorably 

 placed for ordinary observation, or at his highest 

 due south, when the sun is at his lowest below 

 the northern horizon — that is, at midnight. The 

 lengthening of the waves is similarly doubled at 

 this most favorable time for observation ; and 

 the actual difference between the motion of the 

 two sides of Jupiter's equator being nearly fifteen 

 miles per second, the effect on the light-waves is 

 equivalent to that due to a difference of nearly 

 thirty miles per second. Thus, the new method 

 may fairly be expected to indicate Jupiter's mo- 

 tion of rotation. The Greenwich observers have 

 succeeded in applying it, though Jupiter has not 

 been favorably situated for observation. Only 

 on one occasion, says Sir G. Airy, was the spec- 

 trum of Jupiter " seen fairly well," and on that 

 occasion " measures were obtained which gave a 

 result in remarkable agreement with the calcu- 

 lated value." It may well be hoped that when 

 in the course of a few years Jupiter returns to 

 that part of his course where he rises high above 

 the horizon, shining more brightly and through a 

 less perturbed air, the new method will- be still 

 more successfully applied. We may even hope 

 to see it extended to Saturn, not merely to con- 

 firm the measures already made of Saturn's rota- 

 tion, but to resolve the doubts which exist as to 

 the rotation of Saturn's ring-system. 



Lastly, there remains the rotation of the sun, 

 a movement much more difficult to detect by the 

 new method, because the actual rate of motion 

 even at the sun's equator amounts only to about 

 one mile per second. 



In dealins with this very difficult task, the 

 hardest which spectroscopists have yet attempted, 



