XVII.

it treats of

In fine, the third Book of the Principia treats of the Syftem of the World; Third book. in this Book, Newton applies the Propofitions of the two firft: in the fyftem this Application we shall endeavour to follow Newton, and point out the of the world.. Connection of his Principles, and fhew how naturally they unravel the Me

chanifm of the Univerfe.

XVIII.

The Term, Attraction, I employ in the Senfe in which Newton has defined What is it, understanding by it nothing more than that Force, by which Bodies tend meant bythetowards a Center, without pretending to affign the Caufe of this Tendency. tion.

THE

Principal Phenomena of the Syftem of the World.

I.

HE Knowledge of the Difpofition and Motions of the Celestial Bodies must precede a juft Enquiry into their Caufes. It will not therefore appear unneceffary to prepare our Readers by a fuccinct description of our planetary Syftem for our Account of the manner Newton demonftrates the powers which govern the Celestial Motions and produce their mutual Influences. This Defcription muft neceffarily comprize fome Truths, difcovered by that illustrious Philofopher, the Manner he attained them will be described in the Sequel.

II.

word attrac

Firftdivifion

The celeftial Bodies that compose our planetary Syftem, are divided into of the celef Primary Planets, that is, thofe which revolve round the Sun, as their Center tial bodies and Secondary Planets, otherwife, called Satellites, which revolve round their of our plane tary fyftem respective Primaries as Centers: There are fix Primary Planets whofe into princiNames and Characters are as follows,.

Mercury,
Venus,

The Earth,

3 Mars,
4 Jupiter,
ħ Saturn.

pal and fecon dary planets.

Names and characters of the principal flanets

Which are

enumeration

In enumerating the Primary Planets, we follow the Order of their Dif- the planets tances from the Sun, commencing with thofe which are nearest to him. that have The Eart!, Jupiter, and Saturn, are the only Planets which have been fatellites. discovered to be attended by Secondaries: The Earth has only one Satellite, of the celenamely, the Moon; Jupiter has four, and Saturn five, exclufive of his Ring, ftial bodies of fo that our Planetary Syftem is composed of eighteen celeftial Bodies, in- our planetacluding the Sun and the Ring of Saturn.

III.

ry fvftem. Second divifion of the

the planets into

The Primary Planets are divided into superior and inferior Planets, inferior Planets are those which are nearer the Sun than the Earth is; thefe fuperior and

interior,

which are

are Mercury and Venus; the Orbit (a) of Venus includes that of Mercury the inferior and alfo the Sun, and the Orbit of the Earth is exterior to thofe of Mercury planets and what is their and of Ve nus, and inclofes them and the Sun alfo.

arrainge

ment.

This order is difcovered, by Venus and Mercury fometimes appearing to be interpofed between the Sun and us, which could never happen unless thefe Planets revolved nearer the Sun than the Earth, and it is very perceivder has been able that Venus recedes farther from the Sun than Mercury does, and condifcovered. fequently its Orbit includes that of Mercury.

how this or

which are

The fuperior Planets are thofe which are more diftant from the Sun than the fuperior the Earth is, thefe are three in Number, Mars, Jupiter and Saturn; we planets and know that the Orbits of thefe Planets inclofe the Orbit of the Earth, bewhat is their caufe the Earth is fometimes interpofed between them and the Sun.

arrainge

ment.

how it has

The Orbit of Mars inclofes that of the Earth, the Orbit of Jupiter that of Mars, and the Orbit of Saturn that of Jupiter; fo that of the three fuperior Planets Saturn is the remoteft from the Earth, and Mars is the nearest.

This Arraingement is difcovered by those Planets which are nearer the been difco- Earth (b) fometimes coming between the Eye and the Remoter, and intervered. cepting them from our View.

IV.

All the Planets are opaque Bodies; this appears of Venus and Mercury, The planets becaufe when they pafs between us and the Sun, they refemble black Spots are opaque traverfing his Body, aud affiume all thofe various Appearances which are called Phafes, that is, the Quantity of their Illumination depends on their Pofition in refpect to the Sun and us.

bodies.

For the fame Reafon, fince Mars has Phafes we infer his Opacity, and the fame Conclufion is extended to Jupiter and Saturn, because their Satelites do not appear illuminated while their Primaries are between them, and the Sun which proves that that Hemifphere of thofe Planets which is turnThe planets ed from the Sun is opaque: Laftly, we know that the Planets are spheri are spherical cal Bodies, becaufe, whatever be their Pofition, in refpect of us, their Surface always appears to be terminated by a Curve.

We conclude that the Earth is fpherical, because in Fclipfes her Shadow, always appears to be bounded by a Curve, and wher a Ship fails out of fight, it gradually disappears, firft the Hulk, next the Sails, and laftly the Maft, finking to the Eye and vanishing, and moreover, it the Earth was an extended Plane, Navigation would have difcovered its Limits and Boundaries the contrary of which is proved by many Voyagers, such as Drake, Forbith, and Lord Anfon, who have failed round the World.

(a) Orbit is the Curve which a Planet defcribes in revolving round the Body which ferves it as a Center.

(b)

Wolf's Elements of Aftronomy.

V.

All that we know therefore concerning the primary Planets, proves that The planets they are opaque, folid and fpherical Bodies.

appear to be all of the

The Sun appears to be a Body of a Nature entirely different from the Pla- fame nature, nets; we know not whether the Parts of which it is compofed be folid or It is probafluid; all that we can discover is, that thofe Parts emit light & heat, and burn ble that the when condenfed and assembled in fufficient Quantity; hence we may probably the Sun is a conclude, that the Sun is a Globe of Fire refembling terreftrial Fire, fince the globe of fire. Effects produced by this and the folar Rays, are exactly the fame.

VI.

leftial bodies

All the celestial Bodies compleat their Revolutions round the Sun in Ellip- In what' fes (c), more or lefs excentic, the Sun refiding in the common Focus of all curve the ce their Orbits; hence the Planets in their Revolutions fometimes approach revolve anearer, and fometimes recede farther from the Sun; a right Line paffing bout the fun. through the Sun and terminating in the two Points of the Orbit of a Planet, what is the which are nearest and remotest from the Sun, is called the Line of the Apfides, line of the the Point of the Orbit which is nearest the Sun is called the Peribelium; aptides the and the Point of the Orbit which is remotest from the Sun is called the and periheli aphelium Aphelium.

um.

The primary Planets in their Revolutions round the Sun, carry alfo their In what diSatellites, which at the fame Time revolve round them as their Centers. rection the All thefe Revolutions are performed in a direction from West to Eaft (d). planets reThere appear from Time to Time Stars that move in all Directions, and with aftonishing Rapidity, when they are fufficiently near to be visible, thefe are called Comets.

volve.

Of the cora

ets.

We have not yet collected Obfervations fufficient to determine their Number, all that we know concerning them, and 'tis but lately that the Difcovery has been made; is that they are Planets revolving round the Sun like The comets the other Bodies of our Syftem, and that they describe Ellipfes fo very excentric as to be vifible only while they are moving over a very fmall Part of their Orbit.

VII.

are planets.

and comets observe the

All the Planets in their Revolutions round the Sun, obferve the two Laws The planets of Kepler. Obfervations evince, that the Comets obferve the firft of thefe Laws, laws of Kep namely, that which makes the celeftial Bodies (e) describe equal Areas in e- ler

(c) A Species of Curve, which is the fame with what is commonly called an OVAL, the foci are the points in which Gardeners fix their pegs in order to trace this curve of which they make a frequent use.

(d) The Spectator is supposed to be placed on the Earth.

(e) By the Word Area, in general is understood a Surface, here it fignifies the Space in Juded between two Lines drawn from the Center to two Points where the Flanet is found;

the earth

qual Times; and in the fequel it will be fhewn, that all the Obfervations that have hitherto been made, concerning their Motions, render it highly probable that they are regulated by the second Law, that is, that their periodic (f) Times are in the fefquiplicate ratio of their mean Distances.

VIII.

Proofs of the Admitting these two Laws of Kepler, confirmed by all astronomical Obmotion of fervations, from them we may derive feveral convincing Proofs of the Motion of the Earth, a Point which had been fo long contested; for fuppofing the Earth to be the Center of the Celestial Motions, these two Laws are not observed; the Planets do not defcribe Areas proportional to the Times around the Earth, and the periodic Times of the Sun and the Moon, for inftance, round this Planet, are not as the Square Roots of the Cubes of their mean Distances from the Earth; for the periodic Time of the Sun around the Earth, being nearly thirteen Times greater than that of the Moon, its Diftance from the Earth would be, according to Kepler's Rule, between five and fix Times greater than that of the Moon, but Observations demonftrate, that this Distance is about four-hundred Times greater, therefore, admitting the Laws of Kepler, the Earth is not the Center of the celestial Revolutions.

The centripetal Force(g) which Newton has demonftrated to be the Cause of the Revolutions of the Planets renders the Curve they defcribe around their Center concave (h) towards it, fince this Force is exerted in drawing them off from the tangent (i); now the Orbits of Mercury and Venus, in fome Parts, are convex to the Earth; of confequence, the inferior Planets do not revolve round the Earth.

The fame may easily be proved of the fuperior Planets; for these are thofe Areas are proportional to the Times, that is, they are greater or lefs, as the Times in which they are described are longer or shorter.

(f) Periodical Time is the Time that a Planet employs in compleating its Revolution in its Orbit. An Example, of Sefquiplicate Ratio will render it more intelligible than a Definition; Suppofe then the mean Distance of Mercury from the Sun, to be 4, that of Venus 9, the periodical Time of Mercury 40 Days, and let the periodical Time of Venus be required, cubing the two first Numbers 4 and 9, there will result 64 and 729; afterwards extracting the Square-Roots of these two Numbers, there will be found 8 for that of the firft, and 27 for that of the second, and by the Rule of three you will have 8: 27: 40: 135, That is the Square-Root of the Cube of the mean Distance of Mercury from the Sun, is to the Square Root of the Cube of the mean Distance of Venus from the Sun, as the periodic Time of Mercury round the Sun is to the periodic Time fought of Venus round the Sun,which is found to be 135, according to the Suppofitions which have been made, and this is what is called Sefquiplicate Ratio.

(g) The Word CENTRIPETAL PORCE carries its Definition along with it, for it fignifies no more than that Force which makes a Body tend to a Center.

(h) The two Sides of the Crystal of a Watch may ferve to explain thofe Words CONCAVE and CONVEX; the Side exterior to the Watch is cONVEX, and that which is on the Side of the Dial-plate is coNCAVE.

(i) A Tangent is a right Line which touches a Curve, without cutting it.

fometimes obferved to be direct (k), fometimes ftationary, and afterwards retrograde; all thofe Irregularities are only apparent and would vanish if the Earth was the Center around which the heavenly Bodies revolved, for none of these Appearances would be observed by a Spectator placed in the Sun, fince they refult only from the Motion of the Earth in its Orbit combined with the Motion of thofe planets in their respective Orbits; from hence we may fee the Reason why the Sun and the Moon are the only heavenly Bodies that appear always direct; for as the Sun defcribes no Orbit, its Motion cannot be combined with that of the Earth, and as the Earth is the Center of the Moon's Motion, to us fhe fhould always appear direct; as would all the Planets to a Spectator placed in the Sun.

When Copernicus first proposed his Syftem, an Objection was raised against it, taken from the Planet Venus by fome who alledged, that if that Objection Planet revolved round the Sun she should appear to have Phafes as the Moon, made to Co to which Copernicus anfwered, if your Eyes were fufficiently acute you kenfrom the pernicus ta would actually obferve fuch Phafes, and that perhaps in Time fome Art may planet venus be discovered fo to improve and enlarge the vifual Powers, as to render those his anfwer Phafes perceivable: This Prediction of Copernicus was first verified by to this objec Galileo, and every Discovery that has been made fince on the Motion of tion the heavenly Bodies has confirmed it.

·IX

'The Planes (I) of the Orbits of all the Planets interfe&t in right Lines paffing through the center of the Sun, fo that a Spectator placed in the Center of the Under what angle the Sun would be in the Planes of all thofe Orbits. Orbits inter

The Right Line, which is the common Section of the Plane of each Or- fect bit, with the Plane of the Ecliptic, that is, the Plane in which the Earth What is un moves, is called the Line of the nodes of that Orbit, and the extreme Points derstood by of this Section, are called the Nades of that Orbit.

the nodes & the line of

The Quantities of the Inclination of the Planes of the different Orbits, the nodes with the Plane of the Ecliptic, are as follows, the Plane of the Orbit of of an orbit

Saturn is inclined to the Plane of the Ecliptic in an Angle of 2d, that of, Inclination Jupiter 18, that of Mars in an angle fomewhat less than 2d, that of Venus of the Or fomewhat more than 3d, and that of Mercury about 7d.

X.

The Orbits of the primary Planets being Ellipfes, having the Sun in one of their Foci, all these Orbits are confequently excentric, and are more or lefs fo,according to the Distance between their Centers and the Point where the Sun is placed.

(k) A Planet is faid to be DIRECT when it appears to move according to the Order of the Signs. that is, from Aries to Taurus, from Taurus to Gemini, &c. which is alfo faid to move in confequentia, it is stationary when it appears to correfpond for fome Time to the fame Points of the Heavens, and in fine it is RETROGRADE when it appears to move contrary to the Order of the Signs, which is also faid to move in Antecedentia, that is, from Gemini to Taurus, from Taurus to Aries, &c.

(1) The plane of the Orbit of a Planet is the furface on which it is supposed to move.

bits to the Ecliptic