Mr. Hind, the astronomer, in a communication to the Times, September 17, 1863, observes: “It may occasion surprise to many who are accustomed to read of the precision now attained in the science and practice of Astronomy, when it is stated that there are strong grounds for supposing the generally received value of that great unit of celestial measures—the mean Distance of the Earth from the Sun—to be materially in error; and that, in fact, we are nearer to the central luminary by some 4,000,000 miles than for many years past has been commonly believed. The results of various researches during the last ten years appear, however, to point to the same conclusion.”
Mr. Hind then proceeds to describe the actual state of our knowledge respecting it, extending through two entire columns of the above Journal. We have only space for the results:
“To recapitulate briefly: a diminution in the measure of the sun’s distance now adopted is implied by—1st, the theory of the moon, as regards the parallactic equation, agreeably to the researches of Professor Hansen and the Astronomer Royal; 2nd, the lunar equation in the theory of the earth, newly investigated by M. Le Verrier; 3rd, the excess in the motion of the node of the orbit of Venus beyond what can be due to the received values of the planetary masses; 4th, the similar excess in the motion of the perihelion of Mars, also detected within the past few years by the same mathematician; 5th, the experiments of M. Foucault on the velocity of light; and 6th, the results of observations of Mars when near the earth about the opposition of 1862.
“Subjoined are a few of the numerical changes which will follow upon the substitution of M. Le Verrier’s solar parallax (8´´·95) for that of Professor Encke, on which reliance has so long been placed. The earth’s mean distance from the sun becomes 91,328,600 miles, being a reduction of 4,036,000. The circumference of her orbit, 599,194,000 miles, being a diminution of 25,360,000. Her mean hourly velocity, 65,460 miles. The diameter of the sun 850,100 miles, which is smaller by nearly 38,000. The distances, velocities, and dimensions of all the members of the planetary system of course require similar corrections if we wish to express them in miles; in the case of Neptune, the mean distance is diminished by 30 times the amount of correction to that of the earth, or about 122,000,000 miles. The velocity of light is decreased by nearly 8000 miles per second, and becomes 183,470 if based upon astronomical data alone. These numbers will illustrate the great importance that attaches to a precise knowledge of the sun’s parallax, in our appreciation of the various distances and dimensions in the solar system.
“The evidence which has been adduced since the publication of M. Le Verrier’s investigations, would rather induce us to adopt a diminished measure of the earth’s distance from the sun, as the most probable solution of the difficulty.
“M. Léon Foucault, of Paris, has succeeded in measuring the absolute velocity of light by means of the ‘turning mirror’—an experimental determination of no little interest and significance. He concludes that it cannot differ much from 298,000,000 of French metres per second, or 185,170 English miles, which is a notable diminution upon the velocity previously derived from astronomical data alone. The time which light requires to travel from the sun to the earth is known with great precision; at the mean distance of the latter it is rather less than 8´ 18´´, and if this number be combined with M. Foucault’s measure of the velocity, it will be evident that the received distance is too great by about one-thirtieth part—that light, in fact, has not so far to travel before it reaches the earth as generally supposed. The corresponding solar parallax is 8´ 86´´, which approaches much nearer to M. Le Verrier’s theoretical value than to the one depending on the transits of 1761 and 1769. So curious a corroboration of the former deserves especial remark.”
Blue Colour of the Sky.
Mr. Glaisher, in his Report of Scientific Balloon ascents made by him and Mr. Coxwell, in 1863, remarks that the Colour of the Sky in 1862 was of a deeper blue generally than in 1863. On the 31st of March the sky was of a deep Prussian blue, and on the 18th of April it was of a faint blue only, exhibiting another great contrast to the appearance of last year. Sir Isaac Newton considers this colour as a “blue of the first order, though very faint and little, for all vapours, when they begin to condense and coalesce into small parcels, become first of that bigness, whereby such an azure must be reflected.” Professor Clausius considers the vapours to be vesicles or bladders, and ascribes the blue colour of the first order to reflection from the thin pellicle of water. In reference to these opinions the following facts are important:—1. The azure colour of the sky, though resembling the blue of the first order when the sky is viewed from the earth’s surface, becomes, as observed by Mr. Glaisher in his balloon ascents, an exceedingly deep Prussian blue, as we ascend to the height of five or six miles, which is a deep blue of the second or third order. 2. The maximum polarizing angle of the atmosphere being 45 deg. is that of air, and not that of water, which is 55 deg. 3. At the greatest height to which Mr. Glaisher ascended—namely, at the height of five, six, and seven miles, where the blue is the brightest—“the air is almost deprived of moisture.”
Hence it follows that the exceedingly deep Prussian blue cannot be produced by vesicles of water, but must be caused by reflection from the molecules of air, whose polarizing angle is 45 deg. The faint blue which the sky exhibits at the earth’s surface is therefore not the blue of the first order, and is merely the blue of the second or third order, rendered paler by the light reflected from the aqueous vapour in the lower regions of the atmosphere.
Mr. Glaisher speaks of the curious changes in colour that he and Mr. Coxwell experienced in ascending, and remarked that they could now easily go a mile higher without turning quite so blue as before. In one descent they very nearly got into the sea, and only escaped that fate by coming down at the rate of four miles in two minutes.