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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 see 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 the fhould always appear direct; as would all the Planets to a Spectator placed in the Sun.

When Copernicus firft propofed 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 the should appear to have Phases 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 planetvenus be difcovered fo to improve and enlarge the vifual Powers, as to render those 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

his anfwer

The Planes (1) 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 Sun would be in the Planes of all those Orbits.

angle the 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 Pointsderstood by of this Section, are called the Nodes 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 ofor an orbit Saturn is inclined to the Plane of the Ecliptic in an Angle of 2d, that of, Inclination Jupiter id, that of Mars in an angle fomewhat lefs than 2d, that of Venus of the Or fomewhat more than 3d, and that of Mercury about 7d.

X.

The Orbits of the primary Pianets 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 ftationary 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 said 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 fuppofed to move.

bits to the Ecliptic

excentricity The excentricity of all those Orbits have been measured, and have been of the pla found as follows, in decimal Parts of the femidiameter of the Earth's orbit, diameters fuppofed to be divided into 100,000 Parts,

nets in femi

of the earth That of Saturn,

excentricity

That of Jupiter,

That of Mars,

That of the Earth,

That of Venus,

And in fine, that of Mercury,

54207 Parts.
25058
14115
4692

500
8149 Parts.

The excentricity of the Planets measured in decimal Parts of the femidi

of the pla-ameter of their Orbits, fuppofed to be divided into 100,000 Parts, are nets in femi as follows,

diameters of That of Saturn,

5683 Parts.

Orbit

their great That of Jupiter,.

That of Mars,

That of the Earth,

That of Venus,

That of Mercury,

4822

9263

5700

694

21000 Parts

Proportion

meters of

Whence it appears that the Excentricity of Mercury is almoft infenfible.

XI.

The Planets are of different Magnitudes; of the Earth alone we know the of the dia abfolute Diameter, because this Planet is the only one whofe Circumference ahe planets, admits of actual Menfuration, but the relative Magnitudes of the Diameters of the other Planets have been difcovered, and the Diameter of the Sun being taken for a common Measure, and supposed to be divided into 1000 Parts: That of Saturn is

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Hence we fee that Mercury is the leaft of all the Planets, for Spheres are as the Cubes of their Diameters.

XII.

The Planets are placed at different Distances from the Sun, taking the Distances of Distance of the Earth from the Sun for a common Measure, and fuppofing from the funit divided into 100,000 Parts, the mean Distances of the Planets are as

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Distances of

The mean Distances of the Sun and the Planets from the Earth, have alfo been computed in Semidiameters of the Earth; the mean Distances of the the planets Sun, Mercury and Venus from the Earth are nearly equal, and amount to from the 22000 Semidiameters of the Earth, that of Mars is 33500, that of Jupiter earth 115000, and that of Saturn 210000.

XIII.

The Times of the Revolutions of the Planets round the Sun, are lefs in Periodic Proportion of their Proximity, thus Mercury the nearest revolves in 87 Days, times of the Venus next in Order revolves in 224, the Earth in 365, Mars in 686, Jupi-"the fun planetsround ter in 4332, and Saturn the remoteft from the Sun in 10759, the whole in round Numbers.

XIV.

difcover it

The Planets, befides their Motion of Tranflation round the Sun, have a- Rotation of nother Motion of Rotation round their Axis, called their Diurnal Revolution, the planets We only know the diurnal Revolution of the Sun and of four Planets, Means em namely of the Earth, Mars, Jupiter and Venus; this Revolution has been ployed to difcovered by Means of the Spots obferved on their Difcs, (m) and which fucceffively appear and vanish; Mars, Jupiter and Venus having Spots on In what pla their Surface, by the regular Return and fucceffive Difappearance of the fame nets this ro Spots it has been found, that these Planets turn round their Axes, and in what been per Time they compleat their Rotation; thus it has been observed, that Mars ceived makes his Rotation in 23h. 20m, and Jupiter in 9h. 56m.

tation has

to the time

Aftronomers are not agreed about the Time in which Venus revolves Incertitude round its Axis; moft fuppofe the Time of rotation to be about 23 h. But with regard Sign. Bianchini who obferved the Motions of this Planet with particular of the rota Attention, thinks the employs 24 Days in turning round; but as he was tion of Ve compelled to remove his Inftruments during the Time he was obferving, nus an House having intercepted Venus from his View; and as he loft an Hour in this Operation, 'tis probable that the Spot he was obferving during this Interval changed its Appearance; however this be his authority in Aftronomical Matters deferves we should fufpend our Judgment till more accurate Obfervations have decided the Point.

M. de la Hire obferved with a Telescope 16 Feet long, Mountains in Venus higher than thofe of the Moon.

The extraordinary brightnefs of Mercury arifing from his proximity to The rotation the Sun, prevents our discovering by Obfervation its Rotation; and Saturn of Mercury is too remote to have his Spots obferved.

and of Sa turn cannot

In the Year 1715 Caffini obferved with a Telescope 118 Feet long; be difcover three Belts in Saturn refembling thofe obferved in Jupiter, but probably ed by obfer thofe Obfervations could not be purfued with accuracy fufficient to con-way clude the Rotation of Saturn about its Axis.

(m) By the Difk of a Planet is understood that Part of its furface which is visible to us.

vation and

J

to conclude

but analogy As Mercury and Saturn are fubject to the fame Laws that dire& the authorifesus Courfes of the other Planets, and as far as has been difcovered appear to be Bodies of the fame Nature, Analogy authorizes us to conclude planets re- that they alto revolve, round their Axes; and perhaps future Aftronomers their Axes, may be able to obferve this Motion, and to determine its Period.

that thofe

volve round

its axis has

vered

XV.

There appear from Time to Time Spots upon the Sun, which have ferved to difcover that it has a rotatory Motion about its Axis.

How the ro It was long after the Discovery of those Spots, before Aftronomers could tation of the obferve any, fufficiently durable and permanent, to enable them to determine Sun about the Time of his Revolution. Keill in the 5th Lecture of his Aftronomy, been difco relates, that fome Spots have been obferved to pafs from the Western Limb of the Sun to the Eastern Margent in 13 Days and half, and after 13 Days and half to re-appear in the Western Verge of his Disk, from whence he infers that the Sun revolves round its Axis in the Space of about 27 Days from Weft to Eaft, that is in the fame Direction of the Planets; by means of those Spots it has been difcovered, that the Axis round which the Sun revo lves, is inc ined to the plane of the Ecliptic in an Angle of 78.

Jaquier, in his Commentary on Newton, has made fome Reflections on thefe Spots that deferve to be remarked; as no Obfervations prove the Times of their Occultation to be equal, but on the contraty, all the Obfervations he could collect, prove them to be unequal; and, that the Time during which they are concealed, has been always longer than that, during which they have been vifible, from hence he concluded (as alfo Wolf Art. 411. of his Aftronomy) that thofe Spots are not inherent to the Sun, but removed from his Surface to fome diftance.

The Solar Spots were firft difcovered in Germany, in the Year 1611, by John Fabricius, (n) who from thence concluded, the diurnal Revolution of the Sun. They were afterwards obferved by Scheiner, (o) who published the Refult of his Obfervations. The fame Discovery was made by Galileo in Italy.

Scheiner obferved more than fifty Spots on the Surface of the Sun; this may serve to account for a Phenomenon, related by many Hiftorians, that the Sun, fometimes for the Space of a whole Year, has appeared very Pale, as this Effect would naturally follow from a Number of Spots fufficiently large and permanent, to obfcure a confiderable Portion of his Surface.

(n) Wolf. Elementa Aftronomiæ Cap. 1.

(0) Scheiner having informed his Superior that he had difcovered Spots in the Sun, he gravely replied, "that is impoffible, I have read Ariftotle two or three times over, and have found not the leaft mention of it. "

It is no longer doubted that the Earth turns round her Axis in 23h 56m which compose our aftronomical Day; from this Rotation arise the changes of Day and Night, which all the Climates of the Earth enjoy.

XVI.

raifing their

what is the

the effects This Motion of the Celestial Bodies about their Centers alters their Fi-of the rogures, for it is known that Bodies revolving in Circles, acquire a Force motion of tatory which is fo much the greater, the Time of their Revolution being the the planete fame as the Circle which they defcribe is greater. This Force is called confift in Centrifugal Force; that is, the Force which repels them from the Center; equators. wherefore, from their diurnal Rotation, the Parts of the Planets acquire a Centrifugal Force, fo much greater as they are nearer the Equators of thefe centripetal Planets: (fince the Equator is the greateft Circle of the Sphere,) and fo force. much less as they are nearer the Poles (p); fuppofing therefore the Heavenly Bodies in their State of Reft, to have been perfect Spheres, their Rotation about their Axes must have elevated their equatorial and depreffed their polar Regions, and of Confequence changed their fpherical Figures into that of Oblate Spheroids, flat towards the Poles.

The Theory thus leads us to conclude, that all the Planets, in Confe- which are the planets quence of their Rotation, should be flat towards the Poles, but this is only in which the fenfible in Jupiter and the Earth. In the Sequel it will appear, that the elevation of Proportion of the Axes (q), in the Sun, is affignable from Theory, but is too inconfiderable to be obferved.

is perceived.. The Measures of Degrees of the Meridian, taken at the Polar Circle in France, and at the Equator, fix the Proportion of the Axes of the Earth to be as 173 to 174. By the Help of Telescopes the oblate Figure of Jupiter has been perceived And the Difproportion of his Diameters is much greater than that of the Earth, because this Planet is a great deal bigger, and revolves with greater Rapidity about its Axis than the Earth; the Proportion of the Axes of Jupiter is esteemed to be as 13 to 14.

XVII.

obfervation

proves that

As the Spots of Venus, Mars and Jupiter are variable, and frequently the Earth, Mars, Jupichange their Appearance, it is probable that these Planets, like our Earth, ter, Venus are furrounded by denfe Atmospheres, the Alterations in which, produce thefe and the Sun Phenomena in refpect of the Sun, as his Spots are not inherent on his Difk, are furrounded by and as they frequently appear and difappear, it is manifeft that he is furround-atmospheres ed by a grofs Atmosphere, contiguous to his Body, in which thefe Spots are fucceffively generated and dissolved.

(p) The Poles are the Points about which the Body revolves, and the Equator, the Circle equi diftant from those Points dividing the Sphere into two equal Parts.

(q) Axis or Diameter, in general, is a Line which paffes through the Center. and is termiaated at the Circumference, In the prefent Cafe, the Axes are two Lines which pass through the Center, one of which is terminated at the Poles, and the other at the Equator..

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