The Ten Books on Architecture, 9.1

Vitruvius  translated by Joseph Gwilt

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Of the Universe and the Planets

1It is clearly by a divine and surprising arrangement, that the equinoctial gnomons are of different lengths in Athens, Alexandria, Rome, Piacenza, and in other parts of the earth. Hence the construction of dials varies according to the places in which they are to be erected; for from the size of the equinoctial shadow, are formed analemmata, by means of which the shadows of gnomons are adjusted to the situation of the place and the lines which mark the hours. By an analemma is meant a rule deduced from the sun’s course, and founded on observation of the increase of the shadow from the winter solstice, by means of which, with mechanical operations and the use of compasses, we arrive at an accurate knowledge of the true shape of the world.

2By the world is meant the whole system of nature together with the firmament and its stars. This continually turns round the earth and sea on the extreme points of its axis, for in those points the natural power is so contrived that they must be considered as centres, one above the earth and sea at the extremity of the heavens by the north stars, the other opposite and below the earth towards the south; moreover in these central points as round the centres of wheels, which the Greeks call πόλοι, the heavens perpetually revolve. Thus the earth and sea occupy the central space.

3Hence from the construction, the polar centre is raised above the earth in the northern part, whilst that in the southern part, which is underneath, is hidden from our view by the earth, and through the middle obliquely and inclined to the south, is a large band comprising the twelve signs, which, by the varied combination of the stars being divided into twelve equal parts, contains that number of representations of figures. These are luminous, and with the firmament and the other stars and constellations, make their circuit round the earth and sea;

4all these, visible as well as invisible, have their fixed seasons, six of the signs turning above the earth, the remaining six below it; which latter are hidden by the earth. Six of them, however, are always above the earth; for the portion of the last sign, which by the revolution is depressed below the earth and hidden by it, is on the opposite side equal to that of a fresh sign emerging from darkness by the force of the moving power; since it is the same power and motion which cause the rising and setting at the same moment.

5As these signs are twelve in number, each occupies a twelfth part of the heaven, and they move continually from east to west: and through them in a contrary course, the moon, Mercury, Venus, the sun itself, Mars, Jupiter and Saturn, as if ascending, pass through the heavens from west to east in different orbits. The moon making her circuit in twenty-eight days and about one hour, and thus returning to the sign from which she departed, completes the lunar month.

6The sun, in the course of a month, passes through the space of one sign which is a twelfth part of the heavens; hence in twelve months going through the twelve signs, when he has returned to that sign from which he set out, the period of a year is completed: but that circle which the moon passes through thirteen times in twelve months, the sun passes through only once in the same time. The planets Mercury and Venus nearest the rays of the sun, move round the sun as a centre, and appear sometimes retrograde and sometimes progressive, seeming occasionally, from the nature of their circuit, stationary in the signs.

7This may be observed in the planet Venus, which when it follows the sun, and appears in the heavens with great lustre after his setting, is called the evening star; at other times preceding him in the morning before sunrise, it is called the morning star. Wherefore these planets at times appear as if they remained many days in one sign, whilst at other times they pass rapidly from one to another; but though they do not remain an equal number of days in each sign, the longer they are delayed in one the quicker they pass through the succeeding one, and thus perform their appointed course: in this manner it happens that being delayed in some of the signs, when they escape from the retention, they quickly pass through the rest of their orbit.

8Mercury revolves in the heavens in such a manner, that passing through the several signs in three hundred and sixty days, he returns to that sign from which he set out, remaining about thirty days in each sign.

9The planet Venus, as soon as she escapes from the influence of the sun’s rays, runs through the space of one sign in forty days; and what she loses by stopping a long time in one sign, she makes up by her quick passage through others. She completes her circuit through the heavens in four hundred and eighty-five days; by which time she has returned to the sign from whence she set out.

10Mars, on about the six hundred and eighty-third day, completes the circuit of the signs, and returns to his place; and if, in any sign, he move with a greater velocity, his stationary state in others equalizes the motion, so as to bring him round in the proper number of days. Jupiter moving also in contrary rotation, but with less velocity, takes three hundred and sixty days to pass through one sign; thus lengthening the duration of his circuit to eleven years and three hundred and twenty-three days before he returns to the sign in which he was seen twelve years before his setting out. Lastly, Saturn, remaining thirty-one months and some days in each sign, returns to his point of departure at the end of twenty-nine years, and about one hundred and sixty days, or nearly thirty years.Hence, the nearer he is to the extremity of the universe, the larger does his circuit appear, as well as the slower his motion.

11All those which make their circuit above that of the sun, especially when they are in trine aspect, do not advance, but, on the contrary, are retrograde, and seem to stop till the sun passes from the trinal sign into another. Some are of opinion, that this happens on account of their great distance from the sun, on which account their paths not being sufficiently lighted, they are retarded by the darkness. But I am not of that opinion, since the brightness of the sun is perceptible, evident and unobscured throughout the system, just as it appears to us, as well when the planets are retrograde as when they are stationary. If, then, our vision extends to such a distance, how can we imagine it possible to obscure the glorious splendour of the planets?

12It appears more probable, that it is the heat which draws and attracts all things towards itself: we, in fact, see the heat raise the fruits of the earth to a considerable height, and the spray of waters from fountains ascend to the clouds by the rainbow: in the same manner the excessive power of the sun spreading his rays in a triangular form, attracts the planets which follow him, and, as it were, stops and restrains those which precede him, preventing them from leaving him, and, indeed, forcing them to return to him, and to remain in the other trinal sign.

13One may perhaps ask, whence it happens that the sun, by its heat, causes a detention in the fifth sign from itself, rather than in the second or third, which are nearer. This may be thus explained. Its rays diverge through the heavens in lines which form a triangle whose sides are equal. Those sides fall exactly in the fifth sign. For if the rays fell circularly throughout the system, and were not bounded by a triangular figure, the nearer places would be absolutely burnt. This seems to have struck the Greek poet, Euripides; for he observes, that those places more distant from the sun are more intensely heated than those temperate ones that are nearer to him: hence, in the tragedy of Phaëthon, he says,Καίει τὰ πόῤῥω, τὰ δ᾽ ἐγγὺς εὔκατ᾽ ἔχει. (The distant places burn, those that are near are temperate.)

14If, therefore, experience, reason, and the testimony of an antient poet, prove it, I do not see how it can be otherwise than I have above shewn. Jupiter performs his circuit between those of Mars and Saturn: thus it is greater than that of Mars, but less than that of Saturn. In short, all the planets, the more distant they are from the extremity of the heaven, and the nearer their orbit is to the earth, seem to move swifter; for those which have a smaller orbit, often pass those above them.

15Thus, on a wheel similar to those in use among potters, if seven ants be placed in as many channels round the centre, which are necessarily greater in proportion to their distance therefrom, and the ants are forced to make their circuits in these channels, whilst the wheel moves round in an opposite direction, they will assuredly complete their circuit, notwithstanding the contrary motion of the wheel; and, moreover, that nearest the centre will perform his journey sooner than he who is travelling in the outer channel of the wheel, who, though he move with equal velocity, yet, from the greater extent of his circuit, will require a longer time for its completion. It is even so with the planets, which, each in its particular orbit, revolve in a direction contrary to the motion of the heavens, although, in their diurnal motion, they are carried backwards by its rotation.

16The reason why some planets are temperate, some hot, and others cold, appears to be this; that all fire has a flame, whose tendency is upward. Hence the sun warms, by his rays, the air above him, wherein Mars moves, and that planet is therefore heated thereby. Saturn, on the contrary, who is near the extremity of the universe, and comes in contact with the frozen regions of the heavens, is exceedingly cold. Jupiter, however, whose orbit lies between those of the two just mentioned, is tempered by the cold and heat, and has an agreeable and moderate temperature. Of the band comprising the twelve signs, of the seven planets, and their contrary motions and orbits, also of the manner and time in which they pass from one sign into another, and complete their circuits, I have set forth all that I have learnt from authors. I will now speak of the moon’s increase and wane, as taught by the antients.

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