move weftward 45 feconds in a year; and areas are defcribed proportional to the times.

OF THE SUPERIOR PLANETS.

The fuperior planets exhibit phenomena confiderably different from thofe exhibited by Mercury and Venus.

They come to our meridian both at noon and midnight; when they come to our meridian at noon, and are in the ecliptic, they are never feen croffing the fun's difc.

They are always retrograde when in oppofition, and direct when in conjunction.

I have already obferved to you, that the greatest clongation of either of the inferior planets is lefs than 90 degrees, or a quarter of a circle; fo that they are never far from the fun, but conftantly attend it. But the fuperior planets do not always accompany the fun, as thé inferior ones do: they are indeed fometimes in conjunction with it, but then they are alfo fometimes in oppofition to, or 180 degrees from it.

To be more particular, let S, fig. 3, pl. 6, be the fun; ABCD the orbit of any fuperior planet, Mars, for inftance; EFG the carth's orbit. If the earth be at E, the fun at S, and the planet at D, the fun and the planet will be both on the fame fide of the earth; and confequently the planet will appear in conjunction with the fun. But as the orbit of the earth is between the fun and the orbit of the fuperior planet, it is poffible for the earth to be between the fun and the planet, and confequently for the planet and the fun to be on oppoite fides of the earth, or the planet to be in oppofition; thus, if when the earth is at E, Mars be at A, he is then in oppofition to the fun.

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A fuperior planet is in quadrature with the fun, when it's geocentric place is 90° from the geocentric place of the fun; thus if the earth be at E, and Mars at B or C, he is in quadrature with the fun; for the lines A E, E B, form a right angle, as do alfo the lines EA, E C.

OF THE DIRECT, STATIONARY, AND RETROGRADE MOTION OF THE SUPERIOR PLANETS.

As the earth goes round the fun in lefs time, and in a lefs orbit than any of the fuperior planets, it will not be amifs to fuppofe a fuperior planet to ftand ftill in fome part of it's orbit, while the earth goes once round the fun in her's, and confider the appearances the planets would then have, which are thefe: 1. While the earth is in her most distant femicircle, the apparent motion of the planet would be direct. 2. While the earth is in her nearest femicircle, the planet would be retrograde. 3. While the earth is near the points of contact of a line drawn from the planet, fo as to be a tangent to the carth's orbit, the planet would be flationary.

To illuftrate this, let ABCDEFGH, pl. 7, fig. 1, be the orbit of the earth, S the fun, PQOV the orbit of Mars, LMNT an arc of the ecliptic. Let us fuppofe the planet Mars to continue at P, while the earth goes round in her orbit, according to the order of the letters ABC, &c. A B C D E FGH may be confidered as fo many ftations, from whence an inhabitant of the earth would view Mars at different times of the year; and if ftrait lines be drawn from each of thefe ftations, through Mars at P, and continued to the ecliptic, they will point out the apparent place of Mars, at thefe different Stations.

Thus fuppofing the earth at A, the planet will be feen among the ftars at L; when the earth is ar

rived at B, the planet will appear at M; and in the fame manner when at CD and E, it will be seen among the ftars at NRT; therefore, while the earth moves over the large part of the orbit A B C DE, the planet will have an apparent motion from L to T, and this motion is from weft to caft, or the fame way with the earth; and the planet is faid to move direct, or according to the order of the figns. When the earth is near to A and E, the point of contact of the tangent to the earth's orbit, the planet will be flationary for a fhort space

of time.

When the earth moves from E to H, the planet feems to return from T to N; and while it moves from H to A, it will be retrograde to L, where it will again be ftationary: and fince the part of the orbit which the earth defcribes in paffing from A to E, is much greater than the part EHP, though the space TL which the planet defcribes in direct and retrograde motion is the fame, the direct motion from L to T must be much flower than the retrograde motion from T to L.

When the earth is at C, a line drawn from C through S and P to the ecliptic, fhews that Mars is then in conjunction with the fun. But when the earth is at H, a line drawn from H through P, and continued to the ecliptic, would terminate in a point oppofite to S; therefore in this fituation Mars would be in oppofition to the fun. Thus it appears that the motion of Mars is direct when in conjunction, and retrograde when in oppofition.

The retrograde motions of the fuperior planets happen oftener, the flower their motions are; as the retrograde motions of the inferior planets happen oftener, the fwifter their angular motions; becaufe the retrograde motions of the fuperior planets depend upon the motions of the earth; but thofe of the inferior on their own angular motion. A fuperior

fuperior one is retrograde once in each revolution of the earth; an inferior one in every revolution of it's own.

OTHER PHENOMENA OF THE SUPERIOR PLANETS.

The fuperior planets are fometimes nearer the earth than at other times; they also appear larger, or fmaller, according to their different diftances from us. Thus fuppofe the earth to be at C; if Mars be at P, he is the whole diameter of the earth's orbit nearer to us, than if he were at V, and confequently his difc muft appear larger at V than it would be at P. In other places, the diftances of Mars from the earth are intermediate.

The fuperior planets going round the fun in larger orbits than the earth, turn much the greater part of their enlightened hemifphere towards it, and therefore appear round like the full moon, except Mars, who fometimes appears like the moon at a little distance from the full.

They alfo move in an ellipfe, having the fun in the center; the areas defcribed are proportional to the times.

They are fometimes nearer to, fometimes further from the earth, and their apparent diameter is found to vary according to the difference in their, diftance,

OF THE SECONDARY PLANETS, OR SATELLITES.

As the moon turns round the earth, enlightening our nights, by reflecting the light fhe receives from the fun, fo do other fatellites enlighten the planets to which they belong; and as it keeps company with the earth in it's annual revolution round the fun, fo do they feverally accompany the VOL. IV. planets

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planets to which they belong in their several courfes round that luminary. Jupiter has four fatellites, Saturn seven, the Georgium Sidus two.

The existence of all the fatellites except the moon would have been unknown to us without the use of the telescope.

The fatellites are diftinguifhed according to their places, into firft, fecond, &c. the firft being that which is nearest the planet.

The fatellites revolve round their primaries in elliptic orbits, the primary planets being in the focus.

The orbits of all Jupiter's fatellites are nearly but not exactly in the fame plane, which produced makes an angle with the plane of Jupiter's orbit of about 3°. The fecond deviates a little from the reft.

The planes of the orbits of the fecondary planets produced, interfect the heliocentric orbits of their primaries in two oppofite points, which are called their nodes. The planes of the orbits of the fatellites of Jupiter and Saturn produced, interfect the ecliptic in two oppofite points: thefe points of interfection, to diftinguish them from the other, may be called the geocentric nodes of the fatellites. The orbits of Jupiter and Saturn are so small in comparison of the sphere of the fixed stars, that the places of their fatellites nodes are not fenfibly altered by their primaries being in different parts of their orbits.

The orbits of all Saturn's fatellites, except the 5th, which deviates from the reft feveral degrees, are nearly in the fame plane. They are nearly parallel to the plane of the equator. The orbit of Saturn's 5th fatellite makes an angle with the orbit of his primary of 13° 8'.

A fatellite in one of it's nodes, feen from it's primary, appears in the orbit of it's primary in

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