When discovered to be a planet, Herschel dignified it with the name of the Georgian Sidus, or Georgian star, in honour of his royal patron, George the Third, and by this name it was known in the Nautical Almanack. Foreign astronomers call it Herschel, from the name of its discoverer; but the Royal Academy of Prussia, and some others, called it Uranus, by which name it is now usually known.

At different times Herschel discovered six satellites moving round this planet, for the same purpose probably as those of Jupiter and Saturn move round those bodies; but these satellites are distinguishable only by the highest telescopic means afforded by art. Two were easily marked out by Sir W. Herschel; but the others, together with the two innermost moons of Saturn, are the most difficult objects in the solar system to get a sight of. It is considered to be a very remarkable fact, that the planes of the orbits of the satellites of Uranus are nearly perpendicular to the ecliptic, and that the motions of these satellites in these orbits are retrograde, or from east to west, which is contrary to the order of the signs: but, owing to the very great difficulty of observing these attendant bodies, we have not any decided knowledge respecting their masses and motions.

The discovery of this planet and his satellites was one of the triumphs of the Reflecting Telescope, thus beautifully alluded to by the poet :—

Delighted Herschel, with reflected light,
Pursues his radiant journey through the night;
Detects new guards, that roll their orbs afar,

In lucid ringlets round the Georgian star.-DArwin.

P

CHAPTER X.

LAW OF ATTRACTION OF PLANETS. POWERS OF NUMBERS.
PROPORTIONAL SIZES AND DISTANCES OF THE PLANETS.
CELESTIAL GLOBE-ITS MOTION AND USES. AMPLITUDE
AND AZIMUTH. FIXED STARS CANNOT BE MAGNIFIED.
APPARENT DECREASE OF SIZE BY DISTANCE. ZODIACAL
LIGHT. CONTEMPLATION OF THE SOLAR SYSTEM.

THERE are some important circumstances connected
with the planets, as a system, to which we must now
allude. In our second chapter, when speaking of the
attraction of gravitation, we stated that each body
attracts others in proportion to its mass or quantity of
matter. Now, one consequence of this is, that a given
bulk of any substance would weigh differently at the
surfaces of the different planets. That planet which is
larger will attract a body which is upon its surface,
towards the centre, with more force than a planet
which is smaller; and it is this attraction from the
surface towards the centre which constitutes the whole
of what we mean by weight: consequently, a given
mass of matter will weigh more on a large planet than
on a small one, supposing the density to remain the
same; but as we have already seen that the densities
of the planets vary, the forces of their relative attrac-
tions must be estimated by the increased or diminished
density of the mass, as well as by the actual extent or
diminution of size.

There has been noticed a remarkable ratio between the distances of the planets from the Sun. As we shall have to speak of the power of a number, we may

1

as well here state that it means a number multiplied by itself one or more times. Thus 23 means that 2 is to be multiplied into itself twice, or that three twos are to be multiplied together. If, now, we call the mean distance of the Earth from the Sun 10, then the mean distances of the other planets are nearly as here follow:

Now, we must observe that this tabular law was formed by Professor Bode of Berlin, in the year 1772, before the discovery of the Asteroids and Uranus; and that the void occurring between Mars and Jupiter led several German astronomers to suspect the existence of a planet in that point of space; which surmises received a confirmation by the discovery of the four ultra-zodiacal planets at the beginning of this century; while the law itself had been already confirmed by the discovery of Uranus, as a planet, many years before.

As a curious but faithful illustration of the proportional sizes and distances of the planets, referred to the Sun, we quote the following from Sir John Herschel.

"Choose any well levelled field or bowling-green. On it place a globe, two feet in diameter: this will represent the Sun; Mercury will be represented by a grain of mustard-seed, on the circumference of a circle 164 feet in diameter for its orbit; Venus, a pea, on a

circle 284 feet in diameter; the Earth, also a pea, on a circle of 430 feet; Mars, a rather large pin's head, on a circle of 654 feet; Juno, Ceres, Vesta, and Pallas, grains of sand, in orbits of from 1000 to 1200 feet; Jupiter, a moderate-sized orange, in a circle nearly half a mile across; Saturn, a small orange, on a circle of four-fifths of a mile; and Uranus, a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half in diameter."

But when the student of Astronomy shall have come to form a just estimate of the extent of the solar system, and the magnitudes of its component masses, he will yet refer all the members of it, together with the other celestial bodies, to the great concave sphere of the heavens, on which he will trace their various paths, real and apparent. Hence we arrive at the motion and uses of the celestial globe; on which the fixed stars are laid down in their several constellations, and by which the course of the Sun, and the paths of the Moon and planets among the stars, may be readily and conveniently traced.

For this purpose, the following figure, (fig. 43,) furnishes the rudiments of the celestial globe; for, as when a person uses this globe, he must fancy himself placed at its centre, so, when we direct our attention to the great circles of the heavens, by means of which we estimate the positions and motions of the heavenly bodies, we consider the Earth as the central point, as shown in the figure. The axis of the Earth being produced both ways, meets the surface of the great sphere of the heavens, at p and v. These are the poles of the heavens, about which the stars have their apparent or diurnal paths, at right angles to the terrestrial axis, as

is seen in the motion of the star at s. If the Earth's equator, E C, be carried out to the heavens, the great circle thus formed, EQ, is the equinoctial. The declination circles are those which have their centre at the

centre of the Earth, and which are at right angles to the equinoctial; such as PS M. The use of these circles is to determine the situation of a star, &c.; as the latitude of a place on the Earth's surface is reckoned on a meridian. This is the computation for north or south; and as on the terrestrial globe it is necessary to know the east or west distance from a meridian, so, on the celestial globe, this is noted from the first point of