Page:Popular Science Monthly Volume 10.djvu/431

Rh already been said, to a distance of 92,250,000 miles, with a probable error of about one-half per cent., or 450,000 miles.

But, though the distance can thus easily be stated in figures, it is not so easy to give any real idea of a space so enormous; it is quite beyond our power of conception. If one were to try to walk such a, distance, supposing even that he could walk 4 miles an hour, and keep it up for 10 hours every day, it would take 68 years to make a single million of miles, and more than 6,300 years to traverse the whole.

If some celestial railway could be imagined, the journey to the sun, even if our trains ran 60 miles an hour, day and night and without a stop, would require over 175 years. Sensation, even, would not travel so far in a human lifetime. To borrow the curious illustration of Prof. Mendenhall, if we could imagine an infant with an arm long enough to enable him to touch the sun and burn himself, he would die of old age before the pain could reach him, since, according to the experiments of Helmholtz and others, a nervous shock is communicated only at the rate of about 100 feet per second, or 1,637 miles a day, and would need more than 150 years to make the journey. Sound would do it in about 14 years if it could be transmitted through celestial space, and a cannon-ball in about 9, if it were to move uniformly with the same speed as when it left the muzzle of the gun. If the earth could be suddenly stopped in her orbit, and allowed to fall unobstructed toward the sun under the accelerating influence of his attraction, she would reach the central fire in about four months. I have said if she could be stopped, but such is the compass of her orbit that, to make its circuit in a year, she has to move nearly 19 miles a second, or more than fifty times faster than the swiftest rifle-ball; and in moving 20 miles her path deviates from perfect straightness by less than one-eighth of an inch. And yet, over all the circumference of this tremendous orbit, the sun exercises his dominion, and every pulsation of his surface receives its response from the subject earth.

By observing the slight changes in the sun's apparent diameter, we find that its distance varies somewhat at different times of the year, about 3,000,000 miles in all; and minute investigation shows that the earth's orbit is almost an exact ellipse, whose nearest point to the sun, or perihelion, is passed by the earth about the 1st of January, at which time she is 90,750,000 miles distant.

The distance of the sun being once known, its dimensions are easily ascertained—at least, within certain narrow limits of accuracy. The angular semi-diameter of the sun when at the mean distance is almost exactly 962", the uncertainty not exceeding of the whole. The result of twelve years' observations at Greenwich (1836 to 1847) gives 961.82", and other determinations oscillate around the value first mentioned, which is that adopted in the "American Nautical