Page:Popular Science Monthly Volume 14.djvu/306

292 of Jupiter's first satellite with the velocity of light, as determined by terrestrial measurement, and so measures off the millions of miles between us and the source of almost all our energy. These four methods, notably the last two, give us 92,360,000 miles as a near approximation to this long-sought distance. We have thus reduced this space by 3,000,000 miles, or about of the entire amount. Across this interval the radiant energy of the sun dashes in eight minutes thirteen seconds. Thermal electricity, which might be presumed to exist at the sun in great quantity, would traverse the distance in one hour thirty-six minutes. Sensation travels along our nerves at the slow rate of about 150 feet a second. Imagine an infant with an arm long enough to reach the sun. It would have to live 102 years to know that it had burned its band in the solar fires. Counting three a second day and night, it would require an entire year to count the miles intervening between us and the sun; and to count the distance in feet, at the same rate, would consume 5,280 years, or nearly as much time as has elapsed since the introduction of man upon the earth, according to Biblical chronology.

The sun's distance being ascertained, its absolute diameter is determined from the apparent by the process applied to the moon, A near approximation to the sun's radius is 430,680 miles. Imagine the earth at the sun's centre: its surface would appear as far distant as does now the celestial vault, and the moon's orbit would fall nearly 200,000 miles within the surface, or little more than half-way from the centre out, A locomotive at thirty miles an hour would run from centre to surface in 1.63 years. Jules Verne got his traveler around the world in eighty days; at the same rate it would take him twenty-four years to make the circuit of the sun. Its volume is 1,334,000 times the earth's; but its mass, on account of its less density, is only 323,386 times as great. We say only, because the ratio of masses is so much less than that of volumes. But when we reflect that the spectroscope shows at least many terrestrial elements present in the sun, and that the sun contains enough of such substantial stuff as Mother Earth consists of to make more than 300,000 like her, we are prepared to admit that the ratio between the masses even is large enough for all practical purposes.

Vast and incomprehensible as we have found our distance from the sun to be, we have still to contemplate far greater reaches within the limits of the solar system. Jupiter holds on its silent course 5 as far away from the sun as the earth, and therefore receives only the intensity of solar radiant energy. Saturn is nearly twice as far distant as Jupiter; Uranus more than twice as far as Saturn; while Neptune glimmers at thirty times the earth's distance with light that has consumed eight and a quarter hours in flashing twice across this vast abyss since leaving the sun. At that distant boundary the light and heat of the sun have only the intensity that we enjoy, while its apparent diameter, observed from that position, would shrink to 64",