mony of the celes power of gravity decreases as the square of the dis- The har tance increases; and from this it follows, with mathematical certainty, that when two or more bodies tial mo move round another as their centre of motion, the tions. squares of their periodic times will be to one another in the same proportion as the cubes of their distances from the central body. This holds precisely with regard to the planets round the Sun, and the satellites round the planets; the relative distances of all which are well known. But, if we suppose the Sun - to move round the Earth, and compare its period with the Moon's by the above rule, it will be found that the Sun would take no less than 173,510 days to move round the Earth, in which case our year would be 475 times as long as it now is. To this we may add, that the aspects of increase and decrease of the planets, the times of their seeming to stand still, and to move direct and retrograde, answer precisely to the Earth's motion; but not at all to the Sun's, without introducing the most absurd and monstrous suppositions, which would destroy all harmony, order, and simplicity in the system. More. over, if the Earth be supposed to stand still, and the stars to revolve in free space about the Earth in 24 hours, it is certain that the forces by which the stars revolve in their orbits are not directed to the Earth, but to the centres of the several orbits; that is, of the several parallel circles which the stars on different The absides of the equator describe every day; and the like surdity of supposing inferences may be drawn from the supposed diurnal the stars motion of the planets, since they are never in the and pla equinoctial but twice in their courses with regard to move the starry heavens. But, that forces should be di- round the rected to no central body, on which they physically depend, but to innumerable imaginary points in the axis of the Earth produced to the poles of the heavens, is a hypothesis too absurd to be allowed of by any rational creature. And it is still more ab nets to Earth. L Objections against surd to imagine that these forces should increase exactly in proportion to the distances from this axis; for that is an indication of an increase to infinity; whereas the force of attraction is found to decrease in receding from the fountain from whence it flows. But the farther any star is from the quiescent pole, the greater must be the orbit which it describes; and yet it appears to go round in the same time as the nearest star to the pole does. And if we take into consideration the two-fold motion observed in the stars, one diurnal round the axis of the Earth in 24 hours, and the other round the axis of the ecliptic in 25920 years, 251, it would require an explication of such a perplexed composition of forces, as could by no means be reconciled with any physical theory. 112. There is but one objection of any weight that can be made against the Earth's motion round the the Sun, which is, that in opposite points of the Earth's Earth's orbit, its axis, which always keeps a paralswered. lel direction, would point to different fixed stars; motion an which is not found to be fact. But this objection is easily removed, by considering the immense distance of the stars in respect to the diameter of the Earth's orbit; the latter being no more than a point when compared to the former. If we lay a ruler on the side of a table, and along the edge of the ruler view the top of a spire at ten miles distance, and then lay the ruler on the opposite side of the table in a parallel situation to what it had before, the spire will still appear along the edge of the ruler, because our eyes, even when assisted by the best instruments, are incapable of distinguishing so small a change at so great a distance. 113. Dr. BRADLEY found, by a long series of the most accurate observations, that there is a small apparent motion of the fixed stars, occasioned by the aberration of their light, and so exactly answering to an annual motion of the Earth, as evinces the same, even to a mathematical demonstration. Those who are qualified to read the Doctor's modest account of this great discovery, may consult the Philosophical Transactions, No. 406. Or they may find it treated of at large by Drs. SMITH*, LONG†, DESAGULIERS, RUTHERFURTH, Mr. MACLAURIN, Mr. SIMPSON, and M. DE LA CAILLE**. Sun ap 114. It is true that the Sun seems to change his Why the place daily, so as to make a tour round the starry pears to heavens in a year. But whether the Sun or Earth change moves, this appearance will be the same; for, when his place. the Earth is in any part of the heavens, the Sun will appear in the opposite. And therefore this appearance can be no objection against the motion of the Earth. 115. It is well known to every person who has sailed on smooth water, or been carried by a stream in a calm, that, however fast the vessel goes, he does not feel its progressive motion. The motion of the Earth is incomparably more smooth and uniform than that of a ship, or any machine made and moved by human art: and therefore it is not to be imagined that we can feel its motion. motion on strated, 116. We find that the Sun, and those planets The on which there are visible spots, turn round their axes for the spots move regularly over their discs. its axis From hence we may reasonably conclude, that demonthe other planets on which we see no spots, and the Earth, which is likewise a planet, have such rotations. But being incapable of leaving the Earth, and viewing it at a distance, and its rotation being smooth and uniform, we can neither see it move Optics, B. I. § 1178. † Astronomy, B. II. § 838. ton's Philosophical Discoveries, B. III. c. 2. § 3. Mathemat. Essays, p.1. ** Elements d'Astronomie, § 381. All bodies heavier at on its axis as we do the planets, nor feel ourselves affected by its motion. Yet there is one effect of such a motion, which will enable us to judge with certainty whether the Earth revolves on its axis or not. All globes which do not turn round their axes will be perfect spheres, on account of the equality of the weight of bodies on their surfaces; especially of the fluid parts. But all globes which turn on their axes will be oblate spheroids; that is, their surfaces will be higher or farther from the centre in the equatorial than in the polar regions; for, as the equatorial parts move quickest, they will recede farthest from the axis of motion, and enlarge the equatorial diameter. That our Earth is really of this figure, is demonstrable from the unequal vibrations of a pendulum, and the unequal lengths of degrees in different latitudes. Since then the Earth is higher at the equator than at the poles, the sea, which naturally runs downward, or toward the places which are nearest the centre, would run toward the polar regions, and leave the equatorial parts dry, if the centrifugal force of these parts by which the waters were carried thither did not keep them from returning. The Earth's equatorial diameter is 36 miles longer than its axis. 117. Bodies near the poles are heavier than those the poles toward the equator, because they are nearer the than they Earth's centre, where the whole force of the Earth's would be attraction is accumulated. They are also heavier, equator. because their centrifugal force is less, on account at the of their diurnal motion being slower. For both these reasons, bodies carried from the poles toward the equator gradually lose of their weight. Experiments prove that a pendulum which vibrates seconds near the poles, vibrates slower near the equator; which shews, that it is lighter or less attractive there. To make it oscillate in the same time, it is found necessary to diminish its length. By comparing the different lengths of pendulums swinging seconds at the equator and at London, it is found that a pendulum must be 2, lines, or 12th part of an inch shorter at the equator than at the poles. 160 118. If the Earth turned round its axis in 84 mi. How they might lose nutes 43 seconds, the centrifugal force would be all their equal to the power of gravity at the equator; and all weight. bodies there would entirely lose their weight. If the Earth revolved quicker, they would all fly off, and leave it. Earth's felt. 119. A person on the Earth can no more be sen- The sible of its undisturbed motion on its axis, than one motion in the cabin of a ship, on smooth water, can be sen- cannot be sible of the ship's motion when it turns gently and uniformly round. It is therefore no argument against the Earth's diurnal motion, that we do not feel it nor is the apparent revolutions of the celestial bodies every day a proof of the reality of these motions; for whether we or they revolve, the appearance is the very same. A person looking through the cabin-windows of a ship, as strongly fancies the objects on land to go round when the ship turns, as if they were actually in motion. 120. If we could translate ourselves from planet to planet, we should still find that the stars would appear of the same magnitudes, and at the same distances from each other, as they do to us on the Earth, because the diameter of the remotest planet's orbit bears no sensible proportion to the distance of the stars. But then, the heavens would seem to To the dif revolve about very different axes; and consequent- nets the ferent plaly, those quiescent points, which are our poles in heavens the heavens, would seem to revolve about other appear to points, which, though apparently in motion as seen on differfrom the Earth, would be at rest as seen from any other planet. Thus the axis of Venus which lies almost at right angles to the axis of the Earth, would have its motionless poles in two opposite points of the heavens, lying almost in our equi turn round ent axes. |
