2i2 THE POPULAR SCIENCE MONTHLY 



are the asteroids, about 800 discovered to date, which revolve around 

 the Sun, each in its own orbit, in from II 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 

 29-2 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, Xeptune, 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 dis- 

 tribution is most remarkable. Xearly 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 

 7 r> ] »arts 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 Y- of 1 per cent, is 

 divided up to make the planets, satellites, asteroids, comets and meteors. 

 The four outer planets, Jupiter, Saturn, Uranus and Xeptune 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 

 Avhose 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 in- 

 stalled in a museum; but many millions which collide with our atmos- 

 phere 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 Xeptune is 30; and 

 the diameter of Xeptune's orbit is 60. Xow 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 



