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(71.) A planet is saicl to be in conjunction with the sun, whentlie geocentric place of the planet is the same with that of the sun,and in opposition, when its place is opposite to that of the sun ;though an exact or central conjunction, can only take place whenthe line of its nodes passes through the earth, and the planet itselfis in one of its nodes at the time. It is however, in general,called a conjunction or opposition, when the same circle of lati-tude passes through the sun and planet at the same time. Whenthe geocentric place of a planet is 90°, or a quarter of a circle fromthe sun’s place, the planet is said to be in quadrature or in a quar-tile aspect with the sun; and these terms are used in a like sensewhen applied to any two of the heavenly bodies. Thus the sunand moon, or the moon and any planet, or any two planets, maybe in conjunction, opposition, or quadrature. Besides these, theancients reckoned two other aspects, the trine and the sextile; theformer when the bodies were distant 120°, and the latter whenonly half that distance. These aspects are marked as below:
Conjunct. Opposition. Quadrature. Trine. Sextile.
6 8 □ A
Thus, cl If I? denotes the conjunction of Jupiter and Saturn;g $ 2 the opposition of Herschel and Venus ; o0f the squareor quadrature of Sun and Herschel; A J $ the trine of Moon and Mars; $ the sextile of Jupiter and Mercury , &c. &c.
The full moon is represented in the two following ways: g © ]),and O and the new' moon either by d 0 5 , J ©» or ••
(72.) The inferior planets have two kinds of conjunction withthe sun ; one in the inferior part of their semicircles, the other inthe superior part. In the former, the planet is between the earthand the sun ; and in the latter, the sun is between the earth andplanet. The inferior planets can never be in opposition to thesun, nor even appear at a great distance from him. The lengththey go is called their elongation. Thus, in Plate XXIII, fig. 11. letO I’QRT be part of the ecliptic ; S the sun ; and the three cir-cles round him, the orbits of M ercury, Vemis, and the Earth . Sup-posing the earth to be at A, the sun’s geocentric place will beat Q. IfMercury be then at I, his geocentric place is likewise at Q; so thatlie is in conjunction with the sun in his inferior semicircle: if atM,liis geocentric place is likewise at Q ; so that he is in conjunctionin his superior semicircle. In like manner, Venus at E is in con-junction in her inferior semicircle, at C in her superior: but ifwe suppose the earth to lie at A, and Venus at II, her geo-centric. place is T, and her elongation Q T, which is the greatestpossible, when a straight line from the earth touches the orbit ofthe planet, as is evident from the figure; that is, provided theplanet be in its aphelion at the time. Thus the greatest possibleelongation of Mercury is QP when he is in his aphelion at L ; andthe quantity of this is found by astronomical observations to beabout 28 degrees, and that of Venus about 47. The inferiorplanets in their elongations are sometimes eastward and sometimeswestward of the sun; in the former case they appear in the even-ing, and in the latter in the morning. The smallness of Mercury and his nearness to the sun prevent him from being often takennotice of; but the largeness and beauty of Venus have made her,in all ages, celebrated as the evening and morning star.
(73.) The planets sometimes appear to go forward, sometimesbackward, and sometimes to stand still. These different condi-tions are by astronomers called direct, retrograde, and stationary.Were they to be viewed from the sun, they would always appeardirect; but when viewed from the earth, the inferior planets ap-pear direct while moving in their upper semicircles, and re-trograde when in their lower ones. 1 bus in tig. 11. suppose theearth at reslat A, while Mercury is going on in his orbit from N toI, and from 1 to L, his motion appears to an observer at A to beretrograde, or contrary to the order of the signs, namely, from Rto Q, and from Q to r ; but when in that part of his orbit whichlies between L and N, his motion appears direct, or from P to Q,and from Q to R.
(74.) The planets appear stationary while changing their mo-tion from direct to retrograde, or from retrograde to direct. Ifthe earth stood still, their stationary appearances would bediilerentfrom what they are known to be. Tims in fig 11- suppose theearth to be at rest in A, Venus would appear stationary, her geo-centric place continuing at T all the while she is going in her orbitfrom a to b ; because her deviation from the visual line ATwould scarcely be perceptible so near the point of contact II.But if, while Venus is moving from a to b, we suppose the earth
to proceed toward B, Venus would appear to move in the heavensfrom T toward V ; in like manner, if Venus be in her greatestelongation at G, by the earth’s motion from A towards B she willappear to move from K towards P ; hence at her greatest elonga-tion she is always direct, and her stations are nearer the conjunc-tion. Were the earth to stand still in any part of its orbit, as atA, the places of conjunction, both in the superior and inferiorsemicircle, as also of the greatest elongation; and consequently,the places of direct and retrograde motion, and of the stations ofan inferior planet, would always be in the same part of the hea-vens. Thus, in Plate XX!! f. fig. 11. upon this supposition, theplaces of Mercury ’s stations would always be the points P and R,the arc of his direct motion P R, and of his retrograde motionR P : whereas, on account of the earth’s motion, the places wherethese appearances happen arc continually advancing forward intlie ecliptic, according to the order of the signs. In fig. 12, letABCDbe the orbit of the earth ; efg h that of Mercury , © thesun; GFK1 an arc of the ecliptic extended to the fixed stars.When the earth is at A, tlie sun’s geocentric place is at F; andMercury , in order to a conjunction, must be in the line AF;that is, m his orbit he must he at/or h. Suppose him to be at/,in his inferior semicircle : if the earth stood still at A, his nextconjunction would be when he is in his superior semicircle at h;the places of his greatest elongation also would be ate andg, andin the ecliptic at E and G : hut supposing the earth to go on in itsorbit from A to B, the sun's geocentric place is now at K ; andMercury , in order to be in conjunction, ought to be in the lineB K at m. As by the motion of the earth, tlie places of Mercury ’sconjunctions with the sun, are thus continually carried round inthe ecliptic, in consequentia, so the places of liis utmost elonga-tions must be carried in consequentia also. Thus, when thecartli is at A, the places of his greatest elongation from the sun arein the ecliptic E and G; the motion of the earth from A to Badvances them forward from G to L, and from E to I. In asimilar manner may the geocentric motions of Venus be ex-plained.
(75.) To explain the stationary appearances of the planets, itmust be remembered, that the diameter of the earth’s orbit, andeven that of Herschel, are but mere points in comparison of thedistance of the fixed stars; and, therefore, any two lines, abso-lutely parallel, though drawn at the distance of the diameter ofHerschel’s orbit from each other, would, if continued to the fixedstars, appear to ils to terminate in the same point. Let the twocircles, (Plate XXIH. fig. 13,) represent the orbits of Venus andof the Earth ; let the lines AE, BE, CG, DH, be parallel to SP;we may nevertheless affirm, that if continued to the distance ofthe fixed stars, they would all terminate in the same point withthe line SP. Suppose, then, Venus at E, while the earth is at A,the visual ray, by which she is seen, is the line AE. Supposeagain, that, while Venus goes from E to F, the Eartli goes from Ato B, the visual ray, by which Venus is now seen, is BE', parallelto AE ; and therefore, Venus will be all that time stationary, ap-pearing in that point of the heavens where SP extended wouldterminate: this station is at her changing from direct to retro-grade. Again, suppose, when the Earth is at C, Venus is at G,and the visual line CG; if, while the Earth goes from C to D,Venus goes from G to II, so that she is seen in the line Dll, pa-rallel to CG ; she will be all that time stationary, appearing inthe point where a line drawn from S through P would terminate.This station is at her changing from retrograde to direct; andboth are in her inferior semicircle.
(76.) An inferior planet, when in conjunction with the sun, inits inferior semicircle, is said to be in Perigee, and when in theother to be in Apogee, on account of its different distances fromthe Earth. Their real distances from the Earth , when in perigee,are variable, partly owing to the eccentricities of their orbits, aswell as that of the Earth ; and partly owing to the motions of thedifferent bodies, by which it happens that they are in perigee, indifferent parts of their orbits. The least possible distance is, whenthe perigee happens at the time that the Earth is in its perihelion,and when the planet is in its aphelion. The difference of distancebetween tlie Earth and inferior planets, at different times, makes aconsiderable variation in their apparent diameters, which indeed,is very observable in all the planets; and thus, they sometimeslook considerably larger than at others. This difference of dia-meter in Mercury, is nearly as 2/2 to 1 ; and in Venus , no les^