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LEFT SOLAR SYSTEM 486 SOLBERG interval of two leap years. This cycle remained undisturbed till the end of the 19th century; but in consequence of the year 1900 not being reckoned as a leap year, the whole cycle was then over- thrown. It may, however, be recon- structed after 2000, that year being reck- oned as leap year; it will then last till 2100. SOLAR SYSTEM, the sun and all the bodies, by whatever name they may be ealled, which periodically revolve round the sun as a center. Visible to us are seven distinct orders or systems of re- volving worlds. They are the zodiacal light, whatever that may be, the planet- ary, the satellitic, the meteoric, the com- etary, the stellar, and the nebular sys- tems. All but the latter two belong to our solar system. The limits of the planetary system, as far as known, are Mercury, the nearest to the sun, and Neptune, the most distant. This limit does not include rings, which, while the earth is passing through them, produce the star showers. Every member of the solar system, be it planet, satellite, meteoroid or comet, moves in an orbit called an ellipse, of greater or lesser eccentricity. There is not a heavenly body known to man that moves in a circle. The planets and satel- lites revolve in elliptic orbits, while the comets move some in elliptic and others in parabolic and hyperbolic orbits. Those moving in either of the latter two can never return, as the two branches of their paths do not meet to form a closed curve as does the ellipse. For adopting the ellipse instead of the circle for plane- tary revolution the world is indebted to the genius of Kepler, who by the first of his three laws brought harmony out of confusion. The three laws of Kepler, as enunciated by him, are: (1) Th« planets move in ellipses, having the sun in one of the foci. (2) The radius vec- tor of each planet describes equal areas PRINCIPAL ELEMENTS OP THE SOLAR SYSTEM Diameter Planet (miles) Mercury 2,770 Venus 7.700 Earth 7,918 Planetoid Eros 20 Mars 4,230 Planetoid Ceres ... 480 Jupiter 86,500 Saturn 73,000 Uranus 31,900 Neptune 34,800 Mean distance Period of from sun revolu- Density Mass (millions of tion (earth's 1) (earth's 1) miles) (days) 0.56 0.024 35.7 88 0.82 0.82 67.2 225 1.00 1 92.9 365 9 9 135.1 643 0.71 0.105 141.5 687 ? ? 257.1 1,681 0.24 318' 483.3 4,333 0.13 93 886.0 10,759 0.22 15 1,781.9 30,688 0.29 17 2,791.6 60,181 the hypothetical intra-Mercurial planets, discovered during the total solar eclipses, in 1878, by Watson and Swift. Its com- etary extent is not known, and never will be, as several comets have computed peri- ods of over 1,000,000 years, and even their 'aphelia reach only a step toward even the nearest star. The sun's far-reaching power, of course, extends to half way to the stars, and it is not an unreasonable supposition to suppose that there are comets whose aphelia extend that far, and if so, their periodic times must be several million years. The bodies as far as known that are denizens of our solar system are the sun (the center), the planets of Mercury, Venus, earth with one satellite, Mars with 2 satellites, 428 asteroids, Jupiter and 5 satellites, Saturn with 5 rings and 8 satellites, Uranus with 4, and Neptune with 1, also Halley's, Pons', and Olbers' comets of long period, and about 25 of short period, ranging from 3.3 years (Encke's) to 13.78 years, commonly, but unjustly, called Tuttle's comet. It was discovered by Mechain in 1790. To the list must be added 200 or more meteoric in equal times. (3) The squares of their periodic times are proportional to the cubes of their mean distances from the sun. As all heavenly bodies hang sus* pended on nothing throughout all space, each being attracted by all the others, it results that they must move, and, while the motions of the planets are easily noticeable in a few hours, and even min- utes, it requires to detect motion in the stars long-continued observation with in- struments of great delicacy and power to ascertain that they have any motion at all, so far away are they. No planet whose orbit is interior to the earth's can ever assume the crescent phase, or rise when the sun sets, or be on the meridian at midnight, or transit the sun. On the other hand, those whose orbits are ex- terior to ours can never assume the cres- cent and half-moon phase, and are the only ones that can ever be in opposition, rising at sunset. SOLBERG, THORVALD, an Ameri- can public official, born at Manitowoc, Wis., in 1852. He served on the staff of the Library of Congress from 1876 to 1889