To convert the above relative distances into actual ones, consider the distance from the Sun to the Earth as 23,934 mean semidiameters of the Earth, the mean sernidiameter being 3956 miles ; so that the dis tance in question is 95 millions of miles. The semidiatneter of the sun is times that of the earth ; so that the distance of the Earth from the Sun may be called 215 semidiameters of the Sun. One of our objects in this article is to correct the absurd notions derived from the playthings called orreries, and the diagrams exhibited in books and lectures. Let the capital letter 0 of the type which stands at the beginning of the article in this work represent the Sun; then the Earth is a speck which would need a good microscope to show it ; and its dis tance from the Sun is represented by 11 inches, or nearly two inches more than the length of one of our columns. Sir John Herschel describes the Solar System thus :— The explanation of this table resembles that of the preceding one— times, instead of distances, being the objects of comparison. Thus, if a revolution of Jupiter contain 1000 parts of time, that of Saturn has 2428 such parts ; and the revolution of Uranus is 34 01 as long as that of the Earth.
In the following table will be seen the absolute time of revolution in days of each planet, and also numbers expressive of the intervals (in days) between two successive conjunctions with the Sun :— Thus, Venus revolves in 225 days (all the numbers in this article are more roughly given than in the articles specially devoted to the planets); is in conjunction with the Sun at intervals of 584 days ; and, with its mean motion, would describe 274° in the heavens while Jupiter describes Saturn moves 2.01 minutes daily ; the Earth, 59' 8'1.
The minor elements (in a general consideration) may be best described by diagrams ; the inclinations of the orbits may be represented by the following lines, which show the slope or inclination of each orbit to the orbit of the Earth or plane of the ecliptic. None of the old planets have an inclination of more than 7°,while in the new planets the same element varies from 40' (Massilia) to 35° (Pallas).
In the next figure the plane of the ecliptic is represented : A ts is the line which points to the astronomical first point of Aries, or the vernal equinox, and the arrows represent the directions of the planetary The of a planet means the fractional part of a planet's mean distance by which its greatest or least distance exceeds or falls short of the mean distance. Arranging the larger planets in the order of their excentrieities, we have motions. On the outer circle are represented the longitudes of the ascending nodes of the larger planets, or the lines in which they are found when they rise through the ecliptic from the southern to the northern side of it. On the iuner circle are represented the longitudes
of their perihelia, or points of nearest approach to the Sun. The slow changes which take place in these elements are noted in the articles devoted to the different planets.
A general view of the elements of the minor planets has already been given. [ASTEROIDS.] Jupiter and his satellites might be inclosed in a sphere having a radius of about 2i times as great as that of the Sun; Saturn and his satellites in a sphere of if the radius of the Sun; Uranus and his satellites in a sphere of 4i the radius of the Sun ; and Neptune and his satellite in a sphere of which the radius is a little greater than the Sun's radius. The Earth and Moon [Moox] might be contained in a sphere of one-half the radius of the Sun.
In apparent diameter Mercury varies from 5" to 12"; Venus from 10'" to 61"; Mars from 4" to 18"; the small planets have diameters hardly measurable ; Jupiter from 30" to 46" ; Saturn from 14" to 18"; Uranus from a little less to a little greater than 4"; Neptune oscillates about 3".
If the radius of the Sun were divided into 1000 parts, there would be as follows in the radii of the several planets:—Mercury, 31; Venus, 81 ; the Earth, 9 ; Mars, 41; Jupiter, 971 ; Saturn, 85 ; and Uranus, 39. If the bulk of the Sun were divided intd a million of parts, Mercury would be a little less than oue-twentieth of one of the parts; Venus, two-thirds of a part ; the Earth, three-quarters of a part; Mars, one-tenth of a part ; Jupiter, 925 parts ; Saturn, 720 parts; Uranus, 53 parts ; Neptune, 120 parts. The Moon is about the third part of Mercury in bulk.
The masses of the planets vary very much from what they would be if they were nearly of the same substance. Froni the effects of the planets in attracting their satellites, compared with the effects of the Sun upon themselves, it is found that if, according to Cavendish's experiment, we take the Earth to be, at a mem, 51 times the density of water, or about half that of lead, the Sun may be considered, as to density, to be made of asphaltum, or rather heavy coal; Jupiter, of the same ; Uranus and Neptune, each of a material very little heavier; Saturn, of maple-wood ; the Moon, of diamond or topaz. By other modes, of course, than that of their satellites, Mercury is found to be three times as dense as the Earth ; Venus, of about the same density; and Mars, about three-quarters as dense as the Earth.