LIBER II · CAPUT 17
WHY THE SAME STARS HAVE DIFFERENT MOTIONS
Latine · Mayhoff 1906
Primum igitur dicatur, cur Veneris stella numquam longius XLVI partibus, Mercurii XX ab sole abscedant, saepe citra eas ad solem reciprocent. conversas habent. utraeque apsidas ut infra solem sitae, tantumque circulis earum subter est quantum superne praedictarum, et ideo non possunt abesse amplius, quoniam curvatura apsidum ibi non habet longitudinem maiorem. ergo utrique simili ratione modum statuunt apsidum suarum margines, ac spatia longitudinis latitudinum evagatione pensant. at enim cur non semper ad quadraginta sex et ad partes viginti perveniunt? immo vero, sed ratio canonicos fallit. namque apparet apsidas quoque earum moveri, quod numquam transeant solem. itaque cum in partem ipsam eius incidere margines alterutro latere, tum et stellae ad longissima sua intervalla pervenire intelleguntur. cum citra fuere margines, totidem partibus et ipsae ocius redire cogurtur, cum sit illa semper utrique extremitas summa.
Hinc et ratio motuumconversa intellegitur. superiores enim celerrime feruntur in occasu vespertino, hae tardissime; illae a terra altissime absunt, cum tardissime moventur, hae, cum ocissime, quia, sicut in illis propinquitas centri adcelerat, ita in his extremitas circuli. illae ab exortu matutino minuere celeritatem incipiunt, hae vero augere; illae retro cursum agunt a statione matutina usque ad vespertinam, Veneris a vespertina usque ad matutinam. incipit autem ab exortu matutino latitudinem scandere, altitudinem vero ac solem insequi a statione matutina, ocissima in occasu matutino et altissima, degredi autem latitudine motumque minuere ab exortu vespertino, retro quidem ire simulque altitudine degredi ab statione vespertina, Mercurii rursus stella utroque modo scandere ab exortu matutino, degredi vero latitudine a vespertino, consecutoque sole ad quindecim partium intervallum consistit quadriduo prope inmobilis. mox ab altitudine descendit retroque graditur ab occasu vespertino usque ad exortum matutinum, tantumque haec et luna totidem diebus, quot subiere, descendunt. Veneris quindecies pluribus subit, rursus Saturni et Iovis duplicato degrediuntur, Martis etiam quadruplicato. tanta est naturae varietas, sed ratio evidens. nam quae in vaporem solis nituntur, etiam descendunt aegre.
Multa promi amplius circa haec possunt secreta naturae legesque, quibus ipsa serviat, exempli gratia in Martis sidere, cuius est maxime inobservabilis cursus, numquam id stationem facere Iovis sidere triquetro, raro admodum LX partibus discreto, qui numerus sexangulas mundi efficit formas, nec exortus nisi in duobus signis tantum, cancri et leonis, simul edere, Mercurii vero sidus exortus vespertinos in piscibus raros facere, creberrimos in virgine, in libra matutinos, item matutinos in aquario, rarissimos in leone, retrogradum in tauro et geminis non fieri, in cancro vero non citra vicesimam quintam partem, lunam bis coitum cum sole in nullo alio signo facere quam geminis, non coire aliquando in sagittario tantum, novissimam vero primamque eadem die vel nocte nullo alio in signo quam ariete conspici—id quoque paucis mortalium contigit, et inde fama cernendi Lynceo—, non conparere in caelo Saturni sidus et Martis, cum plurimum, diebus CLXX, lovis XXXVI aut, cum minimum, denis detractis diebus, Veneris LXVIIII aut, cum minimum, LII, 5 Mercuri XIII aut, cum plurimum, XVII.
English · Bostock & Riley 1855–57
I must first state the cause, why the star Venus never recedes from the sun more than 46 degrees, nor Mercury more than 23, while they frequently return to the sun within this distance. As they are situated below the sun, they have both of them their apsides turned in the contrary direction; their orbits are as much below the earth as those of the stars above mentioned are above it, and therefore they cannot recede any farther, since the curve of their apsides has no greater longitude. The extreme parts of their apsides therefore assign the limits to each of them in the same manner, and compensate, as it were, for the small extent of their longitudes, by the great divergence of their latitudes. It may be asked, why do they not always proceed as far as the 46th and the 23rd degrees respectively? They in reality do so, but the theory fails us here. For it would appear that the apsides are themselves moved, as they never pass over the sun. When therefore they have arrived at the extremities of their orbits on either side, the stars are then supposed to have proceeded to their greatest distance; when they have been a certain number of degrees within their orbits, they are then supposed to return more rapidly, since the extreme point in each is the same. And on this account it is that the direction of their motion appears to be changed. For the superior planets are carried along the most quickly in their evening setting, while these move the most slowly; the former are at their greatest distance from the earth when they move the most slowly, the latter when they move the most quickly. The former are accelerated when nearest to the earth, the latter when at the extremity of the circle; in the former the rapidity of the motion begins to diminish at their morning risings, in the latter it begins to increase; the former are retrograde from their morning to their evening station, while Venus is retrograde from the evening to the morning station. She begins to increase her latitude from her morning rising, her altitude follows the sun from her morning station, her motion being the quickest and her altitude the greatest in her morning setting. Her latitude decreases and her altitude diminishes from her evening rising, she becomes retrograde, and at the same time decreases in her altitude from her evening station.
Again, the star Mercury, in the same way, mounts up in both directions from his morning rising, and having followed the sun through a space of 15 degrees, he becomes almost stationary for four days. Presently he diminishes his altitude, and recedes from his evening setting to his morning rising. Mercury and the Moon are the only planets which descend for the same number of days that they ascend. Venus ascends for fifteen days and somewhat more; Saturn and Jupiter descend in twice that number of days, and Mars in four times. So great is the variety of nature! The reason of it is, however, evident; for those planets which are forced up by the vapour of the sun likewise descend with difficulty.
There are many other secrets of nature in these points, as well as the laws to which they are subject, which might be mentioned. For example, the planet Mars, whose course is the most difficult to observe, never becomes stationary when Jupiter is in the trine aspect, very rarely when he is 60 degrees from the sun, which number is one-sixth of the circuit of the heavens; nor does he ever rise in the same sign with Jupiter, except in Cancer and Leo. The star Mercury seldom has his evening risings in Pisces, but very frequently in Virgo, and his morning risings in Libra; he has also his morning rising in Aquarius, very rarely in Leo. He never becomes retrograde either in Taurus or in Gemini, nor until the 25th degree of Cancer. The Moon makes her double conjunction with the sun in no other sign except Gemini, while Sagittarius is the only sign in which she has sometimes no conjunction at all. The old and the new moon are visible on the same day or night in no other sign except Aries, and indeed it has happened very seldom to any one to have witnessed it. Prom this circumstance it was that the tale of Lynceus’s quick-sightedness originated. Saturn and Mars are invisible at most for 170 days; Jupiter for 36, or, at the least, for 10 days less than this; Venus for 69, or, at the least, for 52; Mercury for 13, or, at the most, for 18.