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and will be the opposite pole; the line from which we infer, that the surface of the PR will be the axis of the sphere.
earth is not plane, as would appear to a su• To obtain precise views of the motions of perficial observer, but curved. the heavenly bodies, it is necessary to be able The heavenly bodies appear to describe to assigo precisely the place in which they are. a complete circle round the earth every 24 This is done by means of several imaginary hours; but besides these motions which are lines or circles, supposed to be described common to them all, there are several upon the surface of the sphere. These cir- which possess motions peculiar to themcles are divided into degrees, minutes, and selves. The sun is farther towards the seconds. The great circle of the sphere, south during winter than during summer; QE, which is perpendicular to the axis of he does not therefore keep the same stathe world, and of course 90° distant from tion in the heavens, nor describe the same either pole, is called the equator. The circle every day. The moon not only smaller circles which the stars describe in changes her form, diminishes and increases, consequence of their apparent diurnal mo- but, if she is observed in relation to certions, are called parallels, because they are tain fixed stars, it will be found that parallel to the equator. The equator divides she proceeds to the eastward, making the heavenly sphere into two equal parts, progress every day, till in about a month the north and south; but, to be able to as she makes a complete tour of the heavens. sign the position of the stars, it is necessary There are eight other stars which are conto have another circle passing through the tinually changing their place; sometimes poles, and cutting the equator perpendicu- they seem to be moving to the westward, Jarly;
this is called the meridian, which is sometimes to the eastward, and sometimes supposed to pass through the poles, and al- they appear stationary for a considerable so directly over the head of the observer M, time: these are called planets. There are and the point N exactly opposite to that other bodies which appear only occasionThe first of these points is called the zenith, ally, move for some time with very great and the second is denominated the nadir. velocity, and afterwards advance beyond The meridian divides the circles described the regions visible to us : these are comets, by the stars into two equal parts, and when of which one is now (November, 1807), apthey reach it they are either at their greatest parent. The greater number of the healieight above the horizon, or they are at venly bodies always retain the same, or their least height. The situation of the nearly the same relative distance from each pole is readily found, it being precisely half other, and are, on that account, called fixed way betweeu the greatest and least height stars. of those stars that never set. Since HMO, the visible part of the heavens, contains 180°, and it is 90° between the pole P, and the equator E Q; if, therefore, we The earth, as we have observed, was long take away P E from the semicircle HMO, considered as a large circular plane, spreadthere remains 90° for the other two arcs ing out on all sides to an indefinite distance; PH and E O, that is, the elevation of the but it is now ascertained that it is of a sphepole and the equator, are together equal to rical figure, nearly resembling that of a 90', so that the one being known, and sub- globe. The evidence for this fact is decitracted from 90°, the other also is found. sive, without having recourse to scientific Hence it is known, that “the elevation of principles, by considering that the celethe pole at any place, is the complement of brated navigators Magellan, Sir Francis the elevation of the equator:” or what that Drake, Lord Anson, and captain Cooke, elevation wants of 90°. Hence also the have all at different times sailed round the “ elevation of the equator is equal to the earth. They set out from European ports, distance from the pole P to the zenith M;" and, by steering their course westward, ar. for the elevation of the equator is the dif- rived at length at the very place from ference between that of the pole and 90o., whence they departed, which could not
When we travel towards the north, we have happened, had the earth been of any perceive that the north pole does not-re- other than a spherical or a globular figure. main stationary, but rises towards the ze- This form is also apparent, from the circunith, in proportion to the space that we passlar appearance of the sea itself, and the cirover. On the contrary, it sinks just as cumstances which attend large objects much when we travel towards the south, when seen at a distance on its surface. For VOL. I.
FIGURE AND MOTION OF THE
when a ship goes out to sea, we first lose this system the sun is supposed to be in the sight of the hull or body of the vessel, see centre, round which there are eleven plafig. 4; afterwards that of the rigging; and nets continually revolving. at last can discern only the top of the mast, If we can form a notion of the manner which is evidently owing to the convexity in which the earth moves, we shall easily of the water, between the eye and the ob. conceive the motions of all the rest of the ject; for otherwise the largest and most planets, and by that means obtain a comconspicuous part would be visible the plete idea of the order and economy of the longest. Another proof is taken from the whole system. And in order to this, noshadow of the earth upon the face of the thing more is necessary than to consider the moon, during the time of a lunar eclipse; common appearances of the leavens, which for the moon, having no light but what it are constantly presented to our view, and receives from the sun, and the earth being attend to the consequences. For since it is interposed between tbem, the moon must well known that the sun and stars appear either wholly or in part become obscure. to move daily from east to west, and to reAnd since in every eclipse of this kind, turn nearly to the same places in the heawhich is not total, the obscure part always vens again in twenty-four hours, it follows appears to be bounded by a circular line, that they must really move, as they appear the earth itself, for that reason, must be to do, or else that we ourselves must be spherical; it being evident that none but a moved, and attribute our motion to them: spherical body can, in all situations, cast a it being a self-evident principle, that if two circular shadow.
things change their situation with respect It is not ascertained who was the first to each other, one of theni, at least, must person that asserted the figure of the earth have moved. But if this change be owing to be spherical, but the opinion is of very
to the revolution of the stars, we must supgreat antiquity. For when Babylon was pose them to be endowed with a motion so taken by Alexander the Great, it was exceedingly swift, as to exceed all concepknown that the philosophers in that city tion; since it is now known, by calculaBad been long in the habit of calculating tions founded on the surest observations, eclipses, which they could not have accom that their distances from us are so immense, plished without a knowledge of the true fi- and the orbits they have to run round so gure of the earth. Thales, who flourished prosligiously great, that tlie nearest of thein six centuries before the birth of Christ, pre- would move at least one hundred thousand dicted, according to the testimony of Hero- miles in a minute. Now as nature never dotus, an eclipse of the sun. Hence it does that in a complicated and laborious should seem, that in those early days, the manner, which may be done in a more sim. globular figure of the earth had been by the ple and easy one, it is certainly more agreelearned investigated and credited. This able to reason, as well as to the power and being known, its magnitude would also soon wisdom of the Creator, that these effects be discovered : the solution of this appa should be produced by the motion of the rently difficult problem engaged the atien earth ; especially as such a motion will best tion of many great men about the same pe account for all the celestial appearances, . riod; and thongh the measures which they and at tlie same time preserve that beautihave given are wide of the truth, and even ful simplicity and harmony which is found very different from one another, yet this to prevail in every other part of the cre. may be imputed to the inaccuracy of their ation. And this argument will appear still instruinents, and the want of matheniatical more forcible, if we compare the vast bulk knowledge rather than to the impractica of the celestial bodies with the bulk of the bility in the thing itself. Without, how- earth. For it is now well known, that the ever, entering upon this subject, we may sun is above a million of times larger tiap observe, that the universe in general, as the earth; and from the best modern obserwell as the solar system in particular, are various it appears, that many of the stars in some measure comected with the mo are at least equally large. It is much more tion of the globe that we inhabit. By the probable, therefore, that the earth revolves universe may be understood the whole round its axis, with an easy natural motion, frame of nature, to the utmost extent of the once in twenty-four hours, than that those creation, and by the solar system is meant, immense bodies should be carried from one that portion of it which comprehends the place to another, with such incredible 59129, planets, satellites, and comets. Of swittuess. Nor is it any objection to this
rotation of the earth that we are unable to the earth which we inhabit, and also the perceive it. For as the motion of a ship at moon, revolve round the sun, which is fixed sea, when she sails swiftly over the smooth in the centre of the system. There are two surface of the water, is almost impercepti- kinds of planets, primary and secondary. ble to the passengers and company on The first move round the sun, and respect board; much more so must it be with such a him only as the centre of their motions, large body as the earth, which has no impedi- The secondary planets, called also satellites ments or obstacles of any kind to meet with or moons, are smaller planets, revolving in its way, or to disturbits motion. And in a round the primary, while they, with the mamer equally easy may another objec- primary planets about which they move, are tion be removed, which has frequently been carried round the sun. The planets move brought against this doctrine. It has been round the sun at various distances, some asserted, that if the earth moved, a stone being much nearer to him than our earth, and dropped from the top of a tower, or any others being much farther off. There are 11 other high building, would not fall just at primary planets, which are situated with rethe bottom of it, as the building must have spect to their distances from the sun as foladvanced considerably forward during the lows: Mercury $ ; Venus &; the Earth ; time of the fall. But this is evidently a
Mars ; Ceres, Pallas, Juno, Vesta, Jupimistake; for it is well known, by repeated ter 4; Saturn h; and the Herschel planet, experiments, that if a body be projected or the Georgium Sidus H. (See Plate I. from another body in motion, it will always Astronomy.) of these our earth is accompartake of the motion of that other body. panied by one moon, Jupiter has four moons, Thus, a stone dropped from the top of a Saturn has seven moons, and the Herschel mast, while the ship is under sail, is not planet has six moons. None of these moons, left by the vessel, but falls exactly at the except our own, can be seen without a good foot of the mast. And if a bottle of wa. telescope. The other five planets do not ter be hung up in the cabin, with its neck appear to have any satellites, or moons. dowuwards, it will empty itself, drop by All the planets move rouud the sun from drop, into another bottle placed exactly west to east, and in the same direction do underneath it, though the ship shall have the moons revolve round their primaries, run many feet whilst each drop was in the excepting those of the Herschel planet, air. This motion of the earth round its which seem to move in a contrary direcaxis, which, from the instances already tion. The paths in which they move round given, has been sufficiently proved, is called the sun are called their orbits. These orits diurnal or daily motion, and is that which bits are elliptical; but the eccentricity of occasions the regular return of day and the ellipses is so small, that they approach night, and all the celestial appearances be very nearly to circles. They perform their fore mentioned. But there is also another revolutions also in very different periods of motion of the earth, called its annual or
time. The time of performing their revoluyearly motion, which occasions the various tions is called their year. The planets are vicissitudes of the seasons, summer, winter, evidently opaque bodies, and they shine spring, and autumn. And the proofs of only by reflecting the light which they rethis second motion may be easily gathered ceive from the sun ; for Mercury and Venus, from celestial appearances, in nearly the when viewed by a telescope, often appear to same manner as the former. For as that be only partly illuninated, and have the apluminary seems to move round the earth, pearance of our moon when she is cusped or from east to west, in the space of a day, horned, having the illumined part always which is really owing to the diurnal revolu- turned towards the sun. From the appeartion of the earth upon its axis, in a con
ance of the boundary of light and shadow trary direction ; so likewise he seems to have upon their surfaces, we conclude that they an annual motion in the heavens, and to rise are spherical; which is confirmed by some and set continually in different parts of of them having been found to turn periodithem ; which is certainly occasioned by the cally on their axes. Venus and Mercury daily motion of the earth in its orbit, or being nearer to the sun than our earth, are path round the sun, which it completes in called inferior planets, and all the rest, the space of a year.
which are withont the earth's orbit, are cal
led superior planets. That the first go round OF THE SOLAR SYSTEM.
the sun is certain, because they are scen It is fully proved that the planets, with sometimes passing between us and the sun,
and sometimes they go behind it. That according to some, the mean distance is a their orbits are within that of the earth is mean proportional between the two axes of evident, because they are never seen in op- that planet's orbit. The distance of either position to the sun, that is, appearing to rise focus from the centre of the orbit is called from the horizon in the east when the sun is its eccentricity. The two points in a plasetting in the west, which is another proof nets orbit which are farthest and nearest to that the earth is not the centre of celestial the body round which it moves are called motions. On the contrary, the orbits of all the apsides; the former of which is called the other planets surround that of the earth; the higher apsis, or aphelion; the latter is for they sometimes are seen in opposition to called the lower apsis, or perihelion. The the sun, and they never appear to be horn. diameter which joins these two points is ed, but always nearly or quite full, though called the line of the apsides. When the sometimes Mars appears a little gibbous, or sun and moon are nearest to the earth, they somewhat deficient from full.
are said to be in perigee. When at their Since all the planets move round the sun greatest distance from the earth they are in elliptical orbits, the sun itself is situated said to be in apogee. When a planet is in one of the foci of each ellipse. That fo- situated so as to be between the sun and cus is called the lower focus. If we sup. the earth, or so that the sun is between the pose the plane of the earth's orbit, which earth and the planet, then that planet is said passes through the centre of the sun, to bé to be in conjunction with the sun. When extended in every direction as far as the the earth is between the sun and any planet fixed stars, it will mark out among them a then that planet is said to be in opposition, great circle, which is the ecliptic; and with It is evident that the two inferior planets this the situations of the orbits of all the must have two conjunctions with the sun, other planets are compared. The planes of and the superior planets can have only one, the orbits of all the other planets must ne- because they can never come between the cessarily pass through the centre of the sun; earth and the sun. When a planet comes but if extended as far as the fixed stars, directly between us and the sun, it appears they form circles different from one another, to pass over the sun's disc, or surface, and as also from the ecliptic; one part of each this is called the transit of the planet. When orbit being on the north, and the other on a planet moves from west to east, viz. acthe south side of the ecliptic. Therefore cording to the order of the signs, it is said the orbit of each planet cuts the ecliptic in to have direct motion, or to be in consequentwo opposite points, which are called the tia. Its retrogade motion, or motion in annodes of that particular planet, and the tecedentia, is when it appears to move from nodes of one planet cut the ecliptic in planes east to west, viz. contrary to the order of different from the nodes of another planet. the signs. The place that any planet apA line passing from one node of a planet to pears to occupy in the celestial hemisphere the opposite node, or the line in which the when seen by an observer supposed to be plane of the orbit cuts the ecliptic, is called placed in the sun, is called its heliocentric the line of nodes. That node where the place. The place it occupies when seen planet passes from the south to the north from the earth is called its geocentric side of the ecliptic is called the ascending place. gode, and the other is the descending node. The planets do not move with equal veloThe angle which the plane of a planet's city in every part of their orbits, but they orbit makes with the plane of the ecliptic move faster when they are nearest to the is called the inclination of that planet's orbit. sun; and slower in the remotest part of Thus fig. 2, Plate II. where F represents their orbits; and they all observe this rethe sun, the points A and B represent the markable law, that if a straight line be nodes, and the line A B the line of nodes drawn from the planet to the sun, and this formed by the intersection of the planes of line be supposed to be carried along by the the orbits C and D. The angle EFG is periodical motion of the planet, then the the angle of inclination of the planes of the areas which are described by this right line two orbits to each other. A line drawn and the path of the planet are proportional from the lower focus of a planet's orbit (viz. to the times of the planet's motion. That where the sun is) to either end of the conju- is, the area described in two days is double gate axis of its orbit, (which line is equal to that which is described in one day, and a half the transverse axis) is called the mean third part of that which is described in six distance of the planet from the sun. But days, though the arcs or portions of the