12. Chap. 12. (15.)-Of The Motions Of The Planets And The General Laws Of Their Aspects .

CHAP. 12. (15.)—OF THE MOTIONS OF THE PLANETS AND THE GENERAL LAWS OF THEIR ASPECTS[1].

The three planets, which, as we have said, are situated above the sun[2], are visible when they come into conjunction with him. They rise visibly[3] in the morning, when they are not more than 11 degrees from the sun[4]; they are afterwards directed by the contact of his rays[5], and when they attain the trine aspect, at the distance of 120 degrees, they take their morning stationary positions[6], which are termed pri- mary; afterwards, when they are in opposition to the sun, they rise at the distance of 180 degrees from him. And again advancing on the other side to the 120th degree, they attain their evening stations, which are termed secondary, until the sun having arrived within 12 degrees of them, what is called their evening setting becomes no longer visible[7]. Mars, as being nearer to the sun, feels the influence of his rays in the quadrature, at the distance of 90 degrees, whence that motion receives its name, being termed, from the two risings, respectively the first and the second nonagenarian[8]. This planet passes from one station to another in six months, or is two months in each sign; the two other planets do not spend more than four months in passing from station to station.

The two inferior planets are, in like manner, concealed in their evening conjunction, and, when they have left the sun, they rise in the morning the same number of degrees distant from him. After having arrived at their point of greatest elongation[9], they then follow the sun, and having overtaken him at their morning setting, they become invisible and pass beyond him. They then rise in the evening, at the distances which were mentioned above. After this they return back to the sun and are concealed in their evening setting. The star Venus becomes stationary when at its two points of greatest elongation, that of the morning and of the evening, according to their respective risings. The stationary points of Mercury are so very brief, that they cannot be correctly observed.

1. "luminum canonica."

2. Mars, Jupiter, and Saturn.

3. They are then said, in astronomical language, to rise heliacally.

4. In the last chapter this distance was stated to be 7 degrees; see the remarks of Alexandre, in Lemaire, ii. 263.

5. "radiorum ejus contactu reguntur." The doctrine of the ancient astronomers was, that the motions of the planets are always governed by the rays of the sun, according to its position, attracting or repelling them.

6. A planet appears to be stationary, i. e. to be referred to the same point of the zodiac, when it is so situated with respect to the earth, that a straight line passing through the two bodies forms a tangent to the smaller orbit. The apparent motion of the planets, sometimes direct and at other times retrograde, with their stationary positions, is occasioned by the earth and the planets moving in concentric orbits, with different velocities. One hundred and twenty degrees is the mean distance at which the three superior planets become stationary. We have an elaborate dissertation by Marcus, on the unequal velocities of the planets, and on their stations and retrogradations, as well according to the system of Aristotle as to that of Copernicus; Ajasson, ii. 316 et seq. He remarks, and, I conceive, with justice, "...ce n'est pas dans les traités d'astronomie de nos savans que l'on doit puiser les détails destinés à éclaircir le texte des chapitres xii, xiii, xiv et xv du second livre de Pline...Je ne dis rien des commentaires de Poinsinet, d'Hardouin et d'autres savans peu versés en matière d'astronomie, qui ont fait dire à Pline les plus grandes absurdités."

7. "Occasus planetæ vespertinus dicitur, quo die desinit post occasum sois supra horizontem oculis se præbere manifestum;" Alexandre in Lemaire, ii. 265. It is then said to set heliacally.

8. The interpretation of this passage has given rise to much discussion among the commentators and translators; I may refer the reader to the remarks of Poinsinet, i. 70, 71; of Alexandre in Lemaire, ii. 266; and of Marcus in Ajasson, ii. 328. I conceive the meaning of the author to be, that while the other planets become stationary, when at 120 degrees from the sun, Mars becomes so at 90 degrees, being detained by the rays, which act upon him more powerfully, in consequence of his being nearer to their source.

9. I may refer to the remarks of Marcus on the respective distances from the sun at which Venus and Mercury become stationary, and when they attain their greatest elongations; Ajasson, ii. 328, 329. According to Ptolemy, Magn. Constr. lib. viii. cap. 7, the evening setting of Venus is at 5°40? from the sun, and that of Mercury at 11°30?.