The following narrative seems to show that in a progressive science like Astronomy even the highest authority is not infallible.
Some sixty years ago, my attention having been accidentally drawn to a tide-mill for grinding corn, I began to consider what was the source of the power employed, and came to the conclusion that it was the momentum of the earth’s revolution on its axis. The next question I asked myself was—could such power be diverted, in however slight a degree, without drawing, as it were, on the stock? Further consideration showed me that the draught required for grinding the corn was trifling in comparison with that employed in grinding the pebbles on every seashore upon the earth’s surface; and, consequently, that the drain on the earth’s momentum might suffice in the course of ages to effect an appreciable retardation in the earth’s diurnal revolution.
I now, as usual in case of difficulty, applied to my father. He could detect no fault in my reasoning, but informed me that Laplace had demonstrated in his great work (“La Mécanique Céleste”) that the time occupied in the earth’s diurnal revolution is absolutely invariable. Of course both my father and I accepted the authority as unquestionable; but I never could fully satisfy my mind on the subject, and for the greater part of my life it was a standing puzzle.
It may be stated briefly that Laplace’s demonstration appears to have rested mainly on the fact that his Lunar Tables, if employed in calculating backwards certain eclipses of the Sun which happened about 2,000 years ago, give results agreeing so nearly with the ancient records as altogether to exclude the possibility of any appreciable increase in the length of the sidereal day during that long period.
But in the year 1866 Professor Adams (really the first discoverer of the planet Neptune) received the Gold Medal of the Astronomical Society for, among other recent claims, the discovery of an error in the data on which Laplace constructed his Lunar Tables which vitiates the above demonstration.
The details of this important discovery—and the co-operation therein of M. Delaunay—were fully and ably stated by Mr. Warren De La Rue, then President of the Society, on the presentation of the Medal.[375] And the position of the question two years later is concisely stated as follows by the Rev. Charles Pritchard, in an Addendum to his address as President in 1868:—“At present, then, the case stands thus,—the Lunar Tables, if calculated on the principles of gravitation alone, as expounded by Messrs. Adams and Delaunay, and as confirmed by other mathematicians, will not exactly represent the moon’s true place at intervals separated by 2,000 years, provided the length of the day is assumed to be uniform and unaltered during the whole of the intervening period. There are grounds, however, for at least suspecting that, owing to the effects of tidal action, the diurnal rotation is, and has been, in a state of extremely minute retardation; but the mathematical difficulties of the case, owing greatly to the interposition of terrestrial continents, are so great that no definite quantitative results have hitherto been attainable. The solution of the difficulty is one of those questions which are reserved for the Astronomy of the future.”[376]
I need not say that this confirmation of the truth of my early conjecture proved highly gratifying. I have only to add that the increase during the last 2,000 years in the length of the sidereal day is generally estimated at about the eightieth part of a second; but the estimate has, I apprehend, no better foundation than this—namely, that since the recent correction in the Lunar Tables an assumed increase to the extent in question has become necessary in order to make the backward calculation of the ancient eclipses agree with the records as to time.
I have found it very difficult at my age (little less than fourscore), and with my mental powers seriously impaired, to deal, however imperfectly, with a subject so abstruse as that now under consideration; and I think it by no means improbable that there may be some error in my statement of facts or in my argument thereon.
All that I can say is that I have done my best to render intelligible to ordinary readers an important advance in modern Astronomy—interesting in itself, irrespective of its remote and accidental connection with my own biography.