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tion as would be requisite to revolve in one day round the earth ? Let us consider, also, what an immense power it would require to contain it, and to counteract its centrifugal force ; that is, to retain it in its orbit. Every one of the stars presents to us similar difficulties, which are all removed by admitting the revolution of the earth on its supposed axis.
We have already observed, that the pole of the equator seems to move slowly round that of the ecliptic, from whence results the precession of the equinoxes. If the earth is immoveable, the pole of the equator is equally so, since it always corresponds to the same point of the terrestrial surface: the ecliptic, therefore, moves round these poles, and in this motion carries all the heavenly bodies with it. Thus, the whole system, composed of so many heavenly bodies, differing from each other in their magnitudes, motions, and distances, would be again subject to a general motion, which disappears, and is reduced to a very simple law, if we suppose the terrestrial axis to move round the poles of the ecliptic. ; i.
According to the true theory, we are but in a similar situation, on a large scale, of a spectator placed in a ship that is in motion: we are carried on, it is true, with an immense velocity ; which velocity is, however, common to every thing that surrounds us. The person on ship-board fancies himself at rest, and, the shore, the hills, and all the objects placed out of the vessel, appear to him to move. But on comparing the extent of the shore, and the heights, perhaps, of surrounding hills or mountains, he begins to suspect that the apparent motion of these objects arises from the real motion of himself. The stars which fill, as it were, the celestial regions, are, relatively to the earth, what the shores and hills are to the vessel ; and the same reasons which convince the navigator of the reality of his own motion, prove, or ought to prove, to us the motion of the earth. It may be further observed, that in all the works of nature with
which we are acquainted, the Creator appears to act by the shortest, easiest, and simplest means. Now if the earth be at rest, and the stars to move, the velocity of these latter must be immense ; and yet all the purposes thereof might have been answered by a moderate motion of the earth alone.
The Moon's distance from the earth is 240,000 miles; of course, the length of the tract which it traverses, if it moves round the earth in 24 hours, is about 1,500,000; that is, at the rate of 62,500 miles. in an hour, instead of 2290 miles, which is really the case; consequently, in each second of time, the Moon, known to be the slowest of all the heavenly bodies, must move more than 17 miles. Again, the Sun's mean distance from the earth is about 95,000,000 miles ; consequently the diurnal path of that luminary, if it revolve about our globe in 24 hours, must be 580,000,000; and, therefore, in a single second, the beat of a watch, he must move nearly 7,000 miles.
Upon the same principle, that is, supposing the earth to be the centre of the system, and not the Sun, the planet Mars, in a second of time, must travel at the rate of more than 10,000 miles, Jupiter 36,000, and Saturn 62,000. And, lastly, the fixed stars being yet indefinitely more remote from the earth than the Sun or Saturn, their motion in or near the equator must be vastly swifter than this.
These arguments, and our author mentions many others, which are equally conclusive to the mathematician, though not so obvious to the general reader, are strengthened by analogy. A rotatory motion has been observed in several planets, and always from west to east, which is similar to that which the diurnal motion of the heavens seems to indicate in the earth. Jupiter, which is above a thousand times larger than the earth, moves, unquestionably, on its axis in less than 12 hours. An observer, on its surface, would see the heayens revolve round him in that time ; yet we, as mere observers, know, in the case
arth tanet Mars, ihan 10,000tly, the fi
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of Jupiter, that the motion of the heavens to that planet would be apparent, and not real. Is it not therefore reasonable to think, that it is the same with that which we observe on the earth? What confirms this analogy is, that both the earth and Jupiter are flattened at the poles, which is no doubt occasioned by their rotatory motions about imaginary axes. This circumstance should, in theory, diminish the force of gravity at the equator, and that this diminution does take place is proved by experiment. Hence every thing leads to the conclusion, that the earth has really a motion about its axis, and that the diurnal motion of the heavens is merely an illusion which is produced by it; an illusion not unlike that which represents the heavens as a blue vault, to which all the stars are fixed, and the earth as a plane on which it rests.
But there is another motion of the earth, which, as we have seen, occasions the vicissitudes of the seasons, from winter to spring, summer, and autumn. The proofs of this second motion may be easily gathered from celestial appearances, in nearly the same manner as the former. For as the Sun seems to move round the earth from east to west in the space of 24 hours, which is owing to the diurnal revolution of the earth upon its axis, from west to east, so, likewise, he seems to have an annual motion in the heavens, and to rise and set continually in different parts of them; which is certainly occasioned by the daily motion of the earth in its orbit or path round the Sun, which it completes in the space of a year. On either hypothesis the appearance is the same; that is, the Sun will appear in motion, and the earth as standing still, to a spectator on the earth, whether the Sun really move round the earth at rest, or the earth move while the Sun is at rest.
To the inhabitants of any one of the planets, the mundane sphere, with the Sun, stars, and all the other planets, will, as to us, appear to move round them from east to west ; and accordingly the inhabitants of our planet, the earth, are only liable to the same delusive appearance with those of the rest. Now it is ascertained from observation, that the orbits of the planets include the Sun as the common centre of them all ; but it is only the orbits of the superior planets that include the earth, which, however, is not the centre of any one of them. The earth's orbit being ascertained, by observation, to be between those of Venus and Mars, it follows that the earth must turn round the Sun; for as it lies within the orbits of the superior planets, their motion would appear unequal and irregular (on account of the earth not being in the centre), but they would never appear either stationary, or retrograde, without this supposition.Again, from the orbits and periods of the several planets about the Sun, and the Moon and satellites round the earth, Jupiter, Saturn, and the Georgian planet, it is evident that the law of gravitation is the same towards the earth, Jupiter, and Saturn, as towards the Sun; and the periodical times of the several bodies moving round each, are in the same ratio to their several distances from them. On the supposition of the earth's annual motion, her periodical time exactly suits this law, bearing such a proportion between those of Mars and Venus, as the several other bodies directed by the same law do bear; that is, the squares of the periodical times are, in all, as the cubes of the distances from the centre of their orbits.
If the earth does not move round the Sun, the Sun must move with the Moon round the earth; now, the distance of the Sun to that of the Moon is nearly 400 to 1, and the period of the Moon being about 28 days (see the article for July), the Sun's period should be, by the law above mentioned, full 600 years, whereas it is, in fact, but a single year. This con
1 It will be 13: 4003: 282: P?; that is to the square of the periodical time of the Sun; the square root of which being taken, is equal to about 220,000 days, or 600 years..
sideration was, of itself, thought of weight enough to determine the controversy between the two opinions,
and to establish the motion of the earth in its orbit for I ever.
The masses of the Sun and several of the planets are considerably greater than that of the earth; it is therefore much more simple to make the latter revolve round the Sun, than to put the whole solar system in motion round the earth. What a complication in the heavenly, motions would the immobility of the earth suppose! What a rapidity of motion must be given to Jupiter ; to Saturn, which is ten times farther from the Sun than the earth is; and to the Georgian planet, which is double the distance of Saturn even, to make them every year revolve round us, at the same time they are revolving round the Sun! This complication, and this rapidity of motion, disappear by transferring the motion to the earth; a motion conformable to the general law, by which the small celestial bodies revolve round the large ones which are placed in their vicinity.
We may further observe that the analogy of the earth with several of the planets confirms this theory : like Jupiter, for instance, it revolves on its axis, and is accompanied by a satellite. An observer on the surface of Jupiter would conclude that the solar sys. tem was in motion round him, and the magnitude of that planet would render this illusion less improbable than for the earth. Is it, not, therefore, reasonable to imagine, that the motion of the solar system round us is likewise only an illusion ?
To an inhabitant of the Sun, (if there be inhabit. ants on that body, which Dr. Herschel thinks not at all improbable), these bodies will appear to move from west to east: this identity in the direction indicates a motion of the earth, but that which demonstrates it is evidently the law which exists between the times of the revolutions of the planets and their distances from the Sun. They revolve round it slower, as