Page:Popular Science Monthly Volume 6.djvu/232

220 causes beyond human control; a contingency very remote, however, as affecting the entirety of the observers, and from which it is to be heartily hoped all will be exempted.

The results of the transit of 1769 were rendered uncertain, to some extent, by a curious attendant phenomenon called "the black drop," consisting in an apparent clinging of the planet to the limb, to which it is seemingly attached by a black ligament. The exact cause of this illusion is not quite agreed on, but there can be little doubt that it is in part a product of bad definition and inferior telescopes, and, as such, need be expected to give less trouble in our present observations of the times when the planet is really in contact with the edge. It may, however, cause an error of some seconds in noting the time, and in this particular seconds are all-important. Encke, who discussed these results, found from them that the parallax was 8."56, a value always known to be questionable; but whence the sun's distance of "95,000,000 miles," which found a place in our schoolbooks, was derived.

Within a few years past, it has become certain, by evidence from various quarters, that this is too much. Till toward the close of the last century, astronomers had no other means of finding the sun's distance than by observations on Venus and Mars; though, from those of the latter planet, indeed, a much closer approximation to the solar parallax than Kepler's value had long been obtained. Chiefly during the present century, other methods have been added, of which the most remarkable is that due to the French academician, Foucault.

Though the speed of the earth in its orbit, and that of light, were both unknown, yet the ratio of these two velocities had long been ascertained. From the assumed distance of the sun above given (95,000,000 miles), it was evidently possible to tell the circumference of the earth's orbit, and thence to say how many miles it went in a year, or a second, and, by a simple multiplication, a value for the velocity of light was obtained; since, as has just been said, the latter velocity bore a known proportion to the former. In this way, the value of 192,000 miles per second for the speed of light was found—a quantity which, being derived from an assumed distance of the sun, could not, of course, be used in turn to determine it. When, however, Foucault actually measured the velocity of light by a direct physical experiment, it became possible, by a reversal of the above process, to say how far the earth moved in a second; whence we learn how far it moves in a year, or, in other words, the length of its annual path; whence, again, the distance across it and the sun's distance obviously follow, the latter being thus found to be 92,260,000 miles, instead of 95,000,000.

From a discussion of all the different methods, Prof. Newcomb has concluded that the solar parallax cannot be far from 8".85; while Mr. Stone, from a rediscussion of the results of the transit of 1769,