Page:Popular Science Monthly Volume 87.djvu/216

212 are the asteroids, about 800 discovered to date, which revolve around the Sun, each in its own orbit, in from 1 to 8 years, the orbits varying greatly in size, eccentricity and position of orbit planes; then we come to the giant Jupiter, its diameter 11 times the Earth's diameter, and 9 moons, the system completing a revolution about the Sun in 12 years; still farther out is Saturn, its diameter 9 times the Earth's, with its wonderful ring system and 9 moons, all revolving around the Sun in 29 years; next is Uranus, 4 times the Earth in diameter, with 4 moons, all revolving around the Sun once in 84 years; and finally we come to the outermost-known planet, Neptune, a shade larger than Uranus, and its one moon, this planet requiring 165 years to travel around the Sun.

Again, as to the material which composes the solar system: its distribution is most remarkable. Nearly all of it is in the Sun. If we add together the masses of the major planets, the hundreds of asteroids, the satellites, make liberal allowance for the comets, etc., and call the total 1. then the mass of the Sun on the same scale is 744; that is, of 745 parts of matter composing our Solar System, 744 parts are in the Sun and only 1 part is in the bodies revolving around it. To state it differently, 99% per cent. is in the Sun. and only of 1 per cent. is divided up to make the planets, satellites, asteroids, comets and meteors. The four outer planets, Jupiter, Saturn, Uranus and Neptune contain 225 times as much material as the four inner planets, Mercury, Venus, Earth and Mars. The Earth is fully 3,000 times as massive as the 800 asteroids combined. There is the zodiacal-light material, which, in a more or less finely-divided state, as dust grains or very small bodies, revolves around the Sun, each separate particle in effect a minute planet. This matter, distributed through a great volume of space somewhat the shape of a double-convex lens, whose center coincides with the Sun, and whose edge extends out at least as far as the Earth's orbit, reflects and scatters the Sun's rays falling upon it. and causes the illumination easily visible after sunset in the west and before sunrise in the east. Then there are the comets which pass in orbits usually very elongated around the Sun, their tails pointing approximately away from the Sun; and the meteoric matter, which, at least in part, and quite possibly all, revolves around the Sun in elliptic orbits. Occasionally a meteorite gets through our atmosphere to the Earth's surface, is found and is installed in a museum; but many millions which collide with our atmosphere every 24 hours are consumed by frictional heat in the atmosphere and lose their identity.

It is a most remarkable fact that all the planets revolve in orbits lying nearly in the same plane. Let us call the distance from the Sun to the Earth 1; then the distance from the Sun to Neptune is 30; and the diameter of Neptune's orbit is 60. Now our system lies so nearly in one plane that we could put it in a very flat band-box 60 units in diameter and only 1 unit thick, so that all the major planets and their