SOLAR SYSTEM. We have given the elements of the planetary motions minutely in the several articles MERCUR Y, VENUS, &c., together with such physical peculiarities as belong to them severally, and inde pendently of the System ; the general phenomena of their motions have been deduced from the great principle of GRAVITATION ; their history, as far as it is in the plan of this work to give it, has been treated in ASTRONOMY. It remains to bring together the dimensions of the various parts of the System, and to notice such points as could not properly find a place under any of the heads just mentioned.
By the Solar System is meant that collection of bodies which the Sun, the planets which revolve round him, their satellites, and such periodic comets as have had their returns successfully pre dicted. The system of the ancients includes the Earth as a fixed centre, with the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. That of the moderns includes at this day the Sun as a govern ing body (but not as a fixed centre), Mercury, Venus, the Earth (with the Moon), Mare, the group of minor planets (now amounting to sixty four); Jupiter and four satellites, Saturn (with his triple ring) and eight satellites, Uranus with four satellites, and Neptune with one satellite, besides a considerable number of periodic comets. The follow ing symbols are used to represent the ancient members of the system, to which we add the received explanation, without expressing any opinion about it :— The symbol for the Sun is all that modern abridgment has left bf a face surrounded by rays; Mercury has the caduceus, or rod, entwined by two serpents ; Venus, a circular looking-glass with a handle; the Earth (a modern symbol) has a sphere with an equator, and also with some) an inverted symbol of Venus. Those who first used it did not, we presume, know that they might be making a turned upside down represent their planet. The symbol of the31oon is obvious ; Mare has what remains of a spear and shield ; Jupiter, supposed to be a symbol of the thunder (arm and hand holding thunder 1); Saturn, an altered form of a mower's scythe, the emblem of time, But others have thought that Mercury was designated by putting cr and v together, the initials of Venus, from the first dud last letters of 4necrodpes. ; Jupiter, from the first and last letters of ZEbs. These signs are found on very old manuscripts and gems, variously figured, but all with some general resemblance to the modern printed forms.
The four minor planets discovered about the beginning of the pre sent century were also designated by symbols. Vesta had for its symbol t9i, or an altar with fire on it ; Juno was designated by a sceptre, ; Ceres, by a reaper's scythe, .?; and Pallas by the head of a lance, . This mole of designation has recently been abandoned. It is now usual to distinguish the asteroids merely bv a number, enclosed in a circle, indicating their place in the order of discovery.
Uranus has been distinguished by the initial letter of Ilerschel'a name, with the symbol of a planet attached, 1.1 ; Neptune has for its
symbol a trident, te.
We are now to state the relative dimensions of the Solar System in a rough manner. This, we think, it may le useful to do in such a manner that any two planets may be compared with one another without com putation. The planet Ceres is used to represent the group of minor planets.
And first, as to the relative distances from the Sun, we have the following table :— That is, if the mean distance of Venus were called 1000, its greatest distance would be only 1007, and its least 993.
For the excentricities of the minor planets, see ASTEILOIDS.
We now give a table for the times of revolution, similar to that given for the distances :— • This table represents the comparative mean distances of the planets from the Sun. In each column one of the distances is made 100 or 1000, and the rest are expressed accordingly. Thus we see by inspection that Uranus is about 12i times as far from the Sun as Mars ; about 19 times as far as the Earth ; about 26i times as far as Venus; and about 71 times the mean distance of the four small planets. Also, taking the mean distance of the small planets, we see that the distances from the Sun are as the numbers 15, 26, 36, 55, 100, 137, 347, 690; and if we take the first away from all the rest, we have 11, 21, 40, S5, 172, 332, 675, 1071, in which it will be observed that each is about double of the preceding, except in the case of the last two numbers. Kepler bad observed a pro gression, without assigning a law, and bad also noticed that one term appeared to be missing. Bodo assigned the law which has just been noticed, noticing also the apparently missing term. The existence of a planet between Mars and Jupiter was accordingly suspected ; and at last, to the astonishment of astronomers, four little bodies, looking more like fragments of a planet than planets, were discovered at a distance from the Sun so near to that which had been suspected, that their mean distance fills up its place in the series as well as that of any other planet. It was of courso immediately suspected (when only two bad been discovered) that these were remains of some planet which bad been shattered by explosion or other cause ; and the encourage ment which this idea gave to look for further fragments, was perhaps one of the main causes of the discovery of the remaining two. It has been already stated that the number of bodies constituting the group of minor planets now amounts to sixty-four. This law of Bode, as it has been called from the astronomer who first noticed it, may be thus expressed : if a be the distance of Mercury, and a+6 of Venus, then a+2 b is that of the Earth, a+4 b of Mars, of the small planets, a+166 of Jupiter, a+326 of Saturn, and a+646 of Uranus. The law falls, however, in the case of Neptune, the distance of which is a+ 096, instead of a+123 b.