ASTRONOMICAL PHENOMENA AND PROGRESS. 



153 



Encke declares they will be so unfavorable for 

 observation that the reduction of error in the 

 solar parallax by this means to within a limit 

 of j-ffoth of a second, is hopeless for at least 

 two centuries to come. The solar parallax, as 

 derived from that of Mars, in 1740, by Lacaille, 

 was 10". 20, with a possible error of 0".25 ; and 

 in 1832, by Henderson, -9".028. Dr. B. A. 

 Gould computed it from the first opposition of 

 Mars observed by Lieut. J. M. Gilliss, 1849-'52, 

 in Chili, at 8".50. Various determinations of 

 the solar parallax, from the law of gravitation, 

 and based chiefly on perturbations of the 

 moon's longitude, have placed it at from 7".80 

 to 8".84; while the most recent by methods 

 of the sort here referred to, are, that of Han- 

 sen in his new " Tables of the Moon," 8".8762, 

 and of Leverrier, 8".95. 



[Evidently, with a fixed base line, the greater 

 the actual parallax, the less is the true distance 

 of the sun from the earth. In a recent account 

 of this same question, Mr. Hind calls attention 

 to the fact that the deductions from the first 

 transit of Venus, taken alone, gave a solar par- 

 allax of 8".9142, which is nearly as great as 

 that of the most recent calculations based on 

 other methods. He speaks of the increase of 

 the solar parallax from the value found by 

 Encke, and usually adopted, to about 8".95, as 

 being now demanded by the concurrent results 

 of six distinct authorities, among whom are 

 some of those just named ; but he thinks that 

 in their calculations astronomers will retain 

 the old value till the next transits of Venus ; 

 and, doubtless with a view to observations of 

 these, he remarks that the important question 

 which has recently been raised as to the exist- 

 ence of a large error in the estimated distance 

 of the sun, may perhaps be set at rest in twen- 

 ty years, though hardly in less time.] 



Romer, as is well known, by observations of 

 eclipses of Jupiter's satellites, first determined 

 the fact that the transmission of light through 

 space requires time, and calculated from the 

 time apparently occupied by the sun's light in 

 crossing the orbit of the earth the velocity 

 with which it must move. Delambre, from a 

 discussion of 1,000 of those eclipses, deduced 

 the time of the passage of light over the mean 

 distance from tbe sun to the earth as 493.2 

 seconds ; and 96,300,000 miles divided by this 

 number gives 193,350 miles per second as the 

 velocity of light. Again, taking the sun's dis- 

 tance as now given, the velocity of the earth 

 in its orbit is 18.977 miles per second ; and the 

 velocity of light calculated by Bradley's prin- 

 ciple of aberration the amount of the aber- 

 ration according to Struve being 20".45 is 

 10,088 times that of the earth in its orbit = 

 191,513 miles a second. From Fizeau's experi- 

 ment in 1849, with a revolving toothed wheel, 

 the radiating teeth and included spaces of which 

 alternately interrupt or transmit, according to 

 the moments in which it is returned to the 

 edge of the wheel, a ray of light reflected be- 

 tween distant mirrors, the velocity of light was 



calculated at 194,067 miles a second. Finally, 

 Foucault has carried out, and apparently with 

 entire success, the application suggested by 

 Arago of Wheatstone's revolving mirror (with 

 the addition of needful apparatus) to the pur- 

 pose of determining the absolute velocity of 

 light ; and the result at which he arrives is a 

 velocity equal only to 185,177 miles in a second. 



Now, this velocity is more than three per 

 cent, less than the lowest usually accepted 

 (192,000 miles), as deduced from the sun's ac- 

 cepted parallax and distance. But Foucault 

 states that the extreme difference of results in 

 the various trials made by him did not exceed 

 T ff oth of the entire value ; and he believes that 

 the mean result can be trusted to the jg-oth part 

 of the value; while the aberration adopted 

 (20".45) in Bradley's method cannot be supposed 

 at fault beyond the TTf l 5 gth part of the whole. 

 How is the new velocity of light to be recon- 

 ciled with the old value of aberration ? This 

 latter establishes the ratio of the velocity of 

 light to that of the earth. And if this ratio 

 must still be accepted, while one term of it (the 

 velocity of light) must be diminished by three 

 per cent., then inevitably the other term of 

 the ratio must also be diminished, and propor- 

 tionally. Is it possible that there is an uncer- 

 tainty, to the amount of three per cent., in the 

 orbital motion of the earth ? Shall we have 

 to reverse hereafter the usual mode, and deter- 

 mine the velocity of the earth in its orbit from 

 that of light, accepting the latter as the better 

 known of the two ? The error in the earth's 

 movement, if it be such, is an error not in 

 time but in space. "We cannot, as the slower 

 terrestrial velocity would (on that side of the 

 question) require, lengthen the year by about 

 11 days ; hence we must, as the only other way 

 of satisfying the new facts, diminish our esti- 

 mate of the circumference of the earth's orbit ; 

 and of course, therefore, in like proportion les- 

 sen the mean radius of the orbit, i. e., the sun's 

 mean distance. [It will be noticed that the re- 

 cent tendency to a larger solar parallax has the 

 same general effect as Foucault's reduction of 

 the velocity of light; namely, to place the 

 earth nearer to the sun.] 



Foucault's experiment on the velocity of 

 light has been popularly announced as making 

 a " revolution in astronomical science." Prof. 

 Lovering judges that it has only attracted 

 popular attention to an old difficulty, and per- 

 haps given a solution of it, " Neither the ve- 

 locity of light, aberration, nor the sun's dis- 

 tance can be [was ?] suspected of an error to 

 the extent of 3 or 4 per cent. ; and yet one at 

 least must be wrong to this degree, as the best 

 values of the three elements are irreconcilable 

 with each other. Which shall be changed ? 



" It may excite surprise in those who have 

 heard of the accuracy of astronomy, without 

 weighing the exact significance of the word as 

 applied to so large a subject, that there should 

 still be a lingering uncertainty, to the extent 

 of three or four millions of miles, in the sun's 



