Page:Popular Science Monthly Volume 6.djvu/238

226 transit (to the whole earth), and again at its close, with the effects of the earth's rotation in the interval. These diagrams are made from those prepared by Mr. Proctor (to whose admirably lucid illustrations the writer is otherwise indebted), and by Mr. Hill, under the charge of Prof. Coffin, of our own "Nautical Almanac" office; and on them have been marked the eight stations occupied by American parties. The next transit, in December, 1882, will be visible, it may be observed, from beginning to end, in the United States.

On the whole, it will appear, from what has been stated, that a transit of Venus, though not the only means of determining the sun's distance, and not possessing the relative importance it once did, remains probably the best, as it is the best known, and, if it may be so called, the most classic method.

Judging from what appears to be the probable error of our best independent determinations of the solar parallax (those from Mars), and the presumption that the majority of astronomers regard those obtainable by modern methods from Venus as still better, it is no unreasonable anticipation that the probable error of the coming result will not exceed one-hundredth of a second. In other words, it may be. expected to be at least an even wager that the error of angular measurement in the final result—made up, let us remember, from the independent results of observers working in distant parts of the globe—will not exceed that which would be represented by the breadth of a hair, seen at the distance of one mile. So slight is that error, which will seem so large when carried out in the enormous numbers which represent the distance of the sun, and those numbers still more inconceivable which represent his own distance from his brother stars.

In one of the most remarkable writings which have descended to us from ancient philosophy, the "Arenaria" of Archimedes, that geometer undertakes to show his contemporaries that it is in the power of number to reckon not only every grain of sand upon the sea-shore or even in the whole earth, but more than would fill a solid sphere extending beyond the sun; and, in the course of his demonstration, he describes to us how he attempted to find its diameter by measurements carried on with a staff and a rod, when the morning and evening mists rendered its light bearable to the eye. If the striking picture of this "Newton of the ancient world" gazing at the setting sun, to attempt, with such rude means, a portion of the task which remains unsolved after two thousand years, be recalled here, it is because it seems fittingly to remind us of the early steps of that ascent on which man's long effort has raised him to the power of questioning Nature with means of the wonderful exactness just described, and to remind us also how long human thought has rested on the great problem to which we hope this present month may bring an answer.