even the most ferocious bull, when tied to a fig tree, becomes milder and allows itself to be touched, and, having now become as it were more languid, entirely lays aside its rage. This spirit, therefore, falling upon the flesh of victims or indeed upon any other flesh, permeates it, softens it, and makes it more tender. And so it is that from the sweet fig, unripe [figs], which are called *olynthoi* (grossi), draw to themselves the useless humidity of the plant and thus make it better; to which the holy Scripture alludes when it says: *The fig tree has brought forth her green figs* (14); that is, the soul, through purification, has cast out and drained away everything useless, as if it were an unripe fig.
OF PSELLUS TO THE EMPEROR AND LORD MICHAEL DUKAS: BRIEF SOLUTIONS TO NATURAL QUESTIONS
Plutarch, O divine and greatest Emperor, by setting forth diverse opinions, is so far from revealing the truth that he rather confuses the mind of his readers and leads them into ambiguity regarding the path of truth. Whoever, therefore, wishes to attain to the knowledge of things must needs know each thing well through demonstration, beginning from sensible things and proceeding by a method to ascend to the very principle of all things. For this reason, in this present work, I have taken a different path, and, making my beginning from the end, in the first chapter I shall demonstrate that the earth is spherical, and that it lies in the center of the universe, and how many stadia its circumference is, and how large and what kind of part of it we humans inhabit. Then we shall say how the seven climates are distributed, and up to what point humans dwell, and subsequently up to what point in
are divided, and how far human beings dwell, and what is the limit of the inhabitation in the west, and what in the north, and what in the south. In the second chapter, there will be discourse concerning water, and the sea, and air, and fire, and lightnings, and thunders, A and thunderbolts, and comets, and rains, and snows, and hail, and things like these. In the third, we shall then inquire concerning the celestial bodies and their nature, and the size of the sun and moon, and the motion of the stars. In the fourth, we shall treat of matter, and of the form of nature and of the soul and of the intellect. Finally, the fifth will give an account of the primordial cause of all things.
1. That the earth is spherical
There are many proofs regarding the spherical shape of the earth; the most significant, however, is that the sun shines upon the eastern parts before those in the west, and for this reason the day always arrives earlier among the Chinese and in Persia and among the nations there. For when it is midday among the Chinese, it is morning here, and when it is the fourth hour in Persia, it is the first here, and this is proportional to the distance of the east and the west. But if someone should ask whence it is evident that this is so, and that the hours differ so greatly that in one place it is day, while in another it is night, and that the hours arrive earlier or later, let him know that this is made manifest by solar and lunar eclipses. For it has been observed that when a solar eclipse occurs at the fourth hour in Persia, it B appears to us here, since the day arrives there earlier. The same is true regarding the hours of the night during lunar eclipses; for the curvature of the spherical shape of the earth obstructs the western parts until the sun also rises above them. For if the earth were not spherical, but flat and extended in length, the whole would be illuminated at once. That its surface is curved is clear from this: for when we sail on the expanse of the sea, the summits of the mountains are the first to appear to us; but as we approach the land, the lower parts gradually appear. And when we see a ship from the land while it is on the sea, we first see the sails, while the remaining parts of the ship are invisible to us; but as they approach, the lower parts also come into view. This would not happen if the surface of the water were not curved, possessing the form of a sphere together with the earth. If such were not the shape of the earth, all the stars would appear at once. But now, since we dwell towards the north, we indeed see the stars of the northern pole, C but we do not see those in the south. For behold, Canopus, which is a very great star, appears in Egypt, and not only this one, but also the other stars that are further to the south. If it were situated in the middle, but inclined toward the east, as the sun rose, the shadows of those things that are illuminated would become shorter, while those of things far away would be larger. Nay, all the stars rising in the east would appear larger, inasmuch as we were closer to them; while those setting would appear smaller. If, however, our region inclined toward the west, the exact opposite would occur; and if it inclined toward the north, D the shadows of the things illuminated by light would extend toward this side, and similarly if it were toward the south. But now, since the sun
oritur, ad occidentem vergunt umbræ, cum autem occidit, ad orientem. A And if it inclined toward the north, the shadows of illuminated objects would always extend toward that part; and the same applies if it inclined toward the south. But as it is, at the rising of the sun, the shadows incline toward the west, and at its setting, toward the east.
2. Concerning the magnitude of the earth
Regarding the magnitude of the earth, there have been many opinions; but eventually all concluded that its entire circumference is 250,000 stadia, and they demonstrated this by means of geometric methods. Some measured such a distance on the earth that it corresponds to one degree in the heavens, and by multiplying this space by 360—that is, the quantity of the degrees of the heavens—they found the circumference of the earth to be as we have stated. B Our own habitation is situated in the northern part; for this reason the northern pole is always visible. The furthest known habitation on the earth toward the east ends at a certain city of the Chinese called Sera. Beyond this, as they say, there are marshes and reeds, and one cannot travel further east from there; these Chinese inhabit the easternmost parts of the world, and they are all Greeks in their doctrine, save that they are most just. Toward the west, the habitable earth ends beyond Spain at the western Ocean, into which our own sea flows. The islands called the Fortunate Islands are distant one thousand miles from the land; they were previously inhabited and sought after for the temperateness of the air and their fertility, but now they are uncultivated. Toward the northern part, the habitable earth ends at the island of Thule; for the regions beyond this are uninhabited because of the cold. C Toward the south, men dwell as far as the equinoctial circle; but the inhabitants of this region are barbarian tribes, whose skin is of a black color. Those who dwell under the equinoctial circle always have nights equal to their days. Therefore, what we call the Orient is not the Orient, but the Occident; and those cities which seem to most people to be the most eastern, such as those of the Iberians and the Medes, are themselves western, because they are in the western part of the earth. The beginning of the eastern parts is beyond Persia itself. D The earth was divided by the philosophers of old into seven climates in this manner. For they observed that all places situated further toward the north have a maximum day longer than those places situated toward the south; and moving from the equinoctial [line] toward the further parts, having set aside an increase of a half-hour, they called the regions that are separated from the equinoctial by that space the first climate, bestowing upon it the name Meroe. Now Meroe is an Ethiopian city. In this climate, the maximum day is of thirteen hours. Then, again observing where the maximum day was thirteen and a half hours, they called the days from that point the second climate, and they named it after Syene. Syene is a city between the Ethiopian and Egyptian territories. They did the same for the third climate, which is named from the name of lower Egypt, and which has a maximum day of fourteen hours. And similarly for the fourth, named after Rhodes, which has the greatest...
...fourteen hours; and likewise for the fourth, which is called that of Rhodes, since it has a longest day of fourteen hours; and for the fifth, in which people live, likewise, in which the longest day is fifteen hours. They named the sixth climate that which goes through the middle of the Euxine, in which the longest day is fifteen and a half hours. The seventh climate is that which goes through the Borysthenes, in which the longest day consists of sixteen hours. There are, however, regions in the northern part inhabited beyond those seven climates, in which the longest day is greater than the aforementioned hours. Indeed, in Thule, because the summer circle is entirely above the earth, the sun does not set for them for fifty days during the summer; but during the winter, for forty days, they do not see the sun, because the entire winter circle is under the earth and invisible. The distribution of the seven climates is, with regard to length, from east to west; and with regard to breadth, from the equinoctial line up to the northern regions which we mentioned. For the length of the inhabited world is a duration of twelve hours; for when the sun rises in the city of Sera, it sets in the Fortunate Isles. But its breadth goes from the equinoctial line to the island of Thule, so that the length is three times greater than the breadth; hence, those who depict the inhabited world as round are ridiculous. B
3. Concerning the elements that are between the earth and the heaven, and the phenomena pertaining to them.
The earth, having obtained the middle position of the whole, immediately has the water above it; and the place of this is the ocean, into which all waters flow. There are, moreover, the greatest seas: first, the Erythraean, extending as far as the land of India, the region of the Seres, and up to Persia. The second is that which, in our region, flows toward Spain into the western ocean. The third is that which is called Great, because it contains no islands, and which bursts through the strait into our sea. The rest are rather lakes than seas. But seawater is salty for the following reason: since the sun constantly draws up the moisture that is most subtle within it, the thicker part is left behind, and it possesses such a taste, just as we observe in our own bodies; for when we drink water, nature uses its more subtle moisture for the substance of our body, but the thicker part becomes urine, which itself is salty to the taste. And this is observed to be an act of divine Providence. For if the seawater were not salty, it would suffer putrefaction, as it would not flow, and a pestilential odor would be emitted from it that would destroy animals. Now, the whole bulk of water is greater than the size of the earth; for it is necessary that the elements should be equal in power and weight. C
4. Concerning air, and fire, and clouds, and rains, and lightnings, and thunders.
Above the earth and the water is the air, most necessary for the constitution of animals. Above this is fire, not such as that which we make use of. For this is thick, having a burning power, and of the elementary D
it so exceeds the crystal the moisture of water. That [substance], however, is subtle and endowed with a temperate heat, not burning, and similar to what is innate in us. Clouds are formed from the earth and the sea, as vapors are carried upward by the heat of the sun and, in the high regions, are condensed by the cold there A and transformed into water. Some, however, are puzzled as to why the upper regions should be cold, even though they are closer to the sun. For they should be warm because of their proximity to the sun. But we say that the sun itself, in its own nature, is not hottest, as some suppose; rather, the heat we feel here is produced by the reflection of its rays toward the earth. This is clear from the summits of the highest mountains; for, being closer to the sun, they are colder than the region below. Rain, moreover, offers the greatest similarity to that vapor which is produced from waters heated in cauldrons. For if one closes these with a lid, the vapor rises to the covering body, and there it condenses, becomes water, and flows back down into the cauldron. Snow, however, is not frozen water, B but originates from a moist cloud which, before it is fully transformed into water, freezes and falls; and for this reason it has a spongy and thin composition. Hail is formed when rain freezes before it falls to the ground. For if the freezing occurs far from the earth, it falls providing a spherical shape, as its projections are dissolved over the great interval; but if it freezes near the earth, it falls having diverse shapes and varied projections. These things all occur in the air: clouds, rain, snow, hail. Similar things to these are formed near the earth: nebula corresponds to a cloud, dew to rain, hoarfrost to snow, and ice to hail. Lightnings and thunders are not produced by clouds colliding with each other, but when hot wind surrounding them—while they themselves are being condensed and cooled—is inflamed by the opposition. For as one prevails, the other flees. For even though fire tends upward, yet, C constrained by the cold that is above and by the density of the clouds, it tends toward the lower regions. If the heat is small and light, it produces only lightning and thunder; if it is great and fiery, a thunderbolt is produced. Moreover, the lightning precedes the thunder, even though they are produced simultaneously, for the following reason: Sight, which is the first of the senses, immediately perceives what is visible, but hearing perceives audible things after an interval of time. And for this reason we first see the light of the lightning, and then we hear the thunder. That sight is superior to hearing is clear also from the fact D that in rowing we see the oars striking the water, and after that we hear the sound emitted from the striking.
5. On earth-quakes
So that we may not be ignorant of the cause which produces earthquakes, it must be known that from the earth when heated, vapors and exhalations are emitted. A vapor is moist and intermediate between water and air; an exhalation is ca—
and dry, and very mobile, A by which the air is moved and the winds are produced. Therefore, when these exhalations are shut up in the earth and, being upward-tending, seek as it were a path to get out, they move all the surrounding earth, by divine command and providence. Let no one be surprised how such an exhalation, being light, moves the earth, which is denser; for behold, violent winds, even though they have their path above the earth, uproot trees and often cast down houses. Furthermore, such a condition of the earth is like a convulsion; for just as by this the body is moved, so also is the earth when the exhalation is forced to exit. Earthquakes are most numerous and greatest when the air is tranquil, and after earthquakes there usually blows a violent wind, the earth being broken apart by the violent movement of the spirit; and they occur at night more often than by day, B because at night the surface of the earth cools and thickens, and the exhalation is shut inside. Torrential rains precede earthquakes, and after this, droughts. They are particular and not over the whole earth. For the entire earth was shaken only once, at the time of the saving Passion of our Lord, when the very center of the earth was shaken. Earthquakes do not occur in every region, but in those where the earth is spongy and full of fissures, and in some [cases] continuously, in others rarely; and more in spring and autumn than in winter and summer. But even if these things are brought about by some natural process, they occur by the nod and providence of God. For just as neither rain, nor snow, nor ice, nor lightning descends without there being a cloud, but through these as a medium the divine decree is fulfilled in them, as the Creator intended from the beginning, so also no earthquake happens without a physical cause and a divine will.
6. Concerning comets.
Comets are formed from vapors changed into a sooty substance; this, becoming entangled in a certain part of that matter, provides it with nourishment and material. The shape of the comet is [determined] by the quantity of the vapors. Most of them, when they appear, are a deadly portent.
7. Concerning winds.
C Since, as we said, the exhalation is twofold, one part humid and the other dry, from the humid part clouds and water are produced, and from the dry part, the winds. They are twelve in number. For from the equinoctial sunrise blows the Subsolanus; opposite to this in the west is the Zephyrus; from the summer sunrise [blows] the Caecias, and opposite to this is the Iapyx; from the winter [sunrise blows] the Eurus, to which the Africus is opposite; from the south, the Notus; from the north, the septentrional wind [Boreas]. The one midway between the Notus and the Eurus is the Euronotus; the one midway between the Caecias and the septentrional wind is Boreas; D the one midway between the Notus and the Africus is the Africonotus; and the one midway between the septentrional wind and the Iapyx is the Thrascias.
8. On the Iris
A The so-called iris has no substance of its own, but is a certain reflection; for when the cloud contains some small droplets and is positioned opposite the sun, our sight is reflected back toward the sun, and there seems to be a part of a circle in the cloud. Aristotle demonstrates through geometric methods that it is neither a complete circle, nor a section greater than a semicircle.9. On the Halo
The halo is formed around the moon and the sun, and sometimes also around the most brilliant stars. It arises from the reflection of sight, which is reflected everywhere in like manner around the sun or the moon, as is shown through geometric form.10. On the Shape of the Heaven
The heaven is spherical, and this is clear from the fact that the fixed stars are seen rising and setting from the same places, and the northern pole is always seen by us unmoved, while the southern is perpetually hidden; which would not be the case if the heaven did not possess a spherical form. For every sphere, when it moves, has three immovable points; the center and the two poles. Moreover, this shape contains more than those which have the same circumference as it, and it is more perfect than other shapes.11. On the World
Some of the philosophers have opined that there are infinite worlds, and in each of them a earth similar to this one, and men and animals living in them. But these were ignorant that it is not the act of divine power to divide its creation C and distribute it into many things, but rather to collect and unite it.12. Whether the World is Animate and Governed by Providence
The Greeks indeed opined that the spheres of the heaven are animate, adding that the stars are also animate. For if earthly bodies are not lacking in soul, how much more so the celestial ones. We, however, say that their motion is physical, and not vital. For we see that the more a body is structurally organized, the greater vital activity it possesses. Therefore our body is more highly organized than the bodies of other animals, just as those are better organized than plants. D That which is spherical is entirely simple, and therefore devoid of life, like the four elements.13. Whether the World is Incorruptible
Aristotle indeed opines that the world is ungenerated and incorruptible; Plato, however, believes that it was generated, but will be eternal. We, however, say that since the world is a body, and every body possesses a limited power, it follows that it has a limited power itself, which is clear from the following. It revolves within twenty-four hours, as is confessed. If, therefore, it had greater power, it would move in fewer hours; so that, just as it is limited in magnitude, so too is it limited in power.14. Regarding whether the heavens are nourished.
A Some have opined that the sun and the stars are nourished by the vapors rising from the earth, and that the sun follows an oblique path for this reason, because it is carried by these exhalations. But this is clearly false and stands in no need of refutation. For only animate bodies are nourished; the elements change into one another, while the celestial bodies perform only local motion, being incapable of any other movements.15. Regarding the order of the world.
The earth is what holds the center of the world, and it is said to be "below"; above the earth is water, and above the water is air, and above the air is fire; above the fire is the moon. Above the moon is the sphere of Mercury; above that is the sphere of Venus, and above that is the sun; then that of Mars; then that of Jupiter; and above that, the sphere of Saturn. Above Saturn is the sphere of the fixed stars; and above that is the starless sphere, which moves all the others from east to west.16. Regarding the inclination of the world.
The inclination of the world does not pertain to the world itself, but to us, because our dwelling place is not under the equator, but in the northern part. And for this reason, the northern pole is raised up toward us, while the southern is hidden, and the movement of the heaven does not appear straight, but inclined toward the south.17. Regarding whether there is a vacuum outside the world.
C That there is no vacuum within the world has been sufficiently demonstrated by Aristotle in his *Physics*; it was also demonstrated by him that there is no vacuum outside the world. Now, those who hold this opinion define a vacuum as a place deprived of body, which is capable of receiving a body. If, therefore, there were a vacuum beyond the heavens, it would have been produced by the Creator in vain, since no bodies are brought into existence within it; but neither God, nor nature, nor a prudent man does anything in vain. It is clear, therefore, that neither a plenum, nor a vacuum, nor any place at all exists beyond the heavens, but rather the intelligible world.18. Regarding what is the right side of the world, and what is the left.
D The eastern parts of the heavens are called the right parts of the world, and the southern are called the left. The eastern parts are called the "right" as being the point where movement begins. For just as in animals movement begins from the right parts (since we plant the left side on the ground and move the right), so the ancients assumed that the beginning of movement regarding us was in the right parts. With this assumed, the upper parts of the world are at the southern pole, which is always invisible to us, while the lower parts are at the northern, which is always visible to us; just as if you were to conceive of a man having his head inclined toward the southern pole, and his feet toward the nor-towards the southern pole, and his feet towards the northern; his right side will therefore be moving from east to west.
19. What is the essence of the world?
Aristotle says that the heaven is a fifth body, and different from the four elements: which he demonstrates from its motion. For those whose motions are different, their natures also are different. Now, the motions of the elements differ from the motion of the heaven, since the former move in a straight line, while the heaven moves in a circle; so that it is of a different genus from them. Philoponus, rising up against this demonstration, says: Alas, Aristotle, if those things whose motions are different also differ in their natures, it follows that those whose motions are the same, are also the same in nature. And indeed, B we see the earth moving downwards, and water likewise; therefore there is one nature for both. Also, air and fire are moved upwards, yet they are different in nature. But Plato taught that the celestial body is composed of the four elements. Proclus, agreeing with him, says that it is composed from the flower of the elements, and that there is in heaven both fire and the rest, not as they are here on earth, but in a celestial manner, just as fire on earth exists in a terrestrial manner. However, the demonstrations of those who thought that the heaven was composed of the flower of the elements, and especially of fire and air, prevailed against the demonstrations of those who asserted it to be a fifth body. C20. Of the circles excogitated in the heaven.
In the heaven, which is spherical, three diverse circles are excogitated: the equinoctial, the meridian, and the horizon. The equinoctial is that which divides the heaven from east to west; the meridian is that which divides it into two parts from north to south; and the horizon is that which divides the half-sphere that is above the earth from that which is under the earth. Also, in accordance with the summer and winter solstices, two circles are excogitated which determine the obliquity of the sun; and above these, other two circles, the arctic and the antarctic. These were posited so that the temperate zone might be separated from that which, because of the cold, is uninhabitable. D21. Of the nature of the stars.
The nature of the stars is igneous and aerial, yet it partakes more of the igneous nature than of the aerial. For the celestial spheres partake more of the aerial nature than of the igneous, but the stars are the contrary, as it seems to the Stagirite. The ancients disputed which were more worthy: the celestial spheres, as being lighter, or the stars which are within them, as being more splendid. Aristotle resolved this doubt in his work *On the Heaven*, saying: If that which is carried is better than that which carries, because the later was made for the sake of the former; and the spheres carry the stars, while they themselves are carried, therefore they [the stars] are more worthy.22. Of the forms of the stars.
Moreover, the forms of the stars are spherical; for with such a form God adorned the more universal figures, the fourelements and the heaven itself and whatever is contained within it. For this form is more perfect than all others. For all angular forms, when divided, produce a different form, but the circle, when divided, does not produce a different form. At the same time, it is also the simplest, inasmuch as it is encompassed by a single line and is therefore similar to unity. Moreover, the planets are configured toward one another by five forms: conjunction, sextile, quadrature, triangle, and diameter. For when the circle of the Zodiac is divided into twelve parts, when it happens that one star is the beginning of a certain division, and another is situated at the beginning of the third from that division, it is said to make a sextile; if at the beginning of the fourth from it, a quadrature; if at the beginning of the fifth from it, a triangle; if at the beginning of the seventh from it, a diameter. If they are in the same degree, they are said to be in conjunction. And the same holds for the other parts. These forms are derived from musical ratios, as Ptolemy shows in his four books.
23. Concerning the motion of the stars.
The motion of the stars is from west to east. The movement of the seven planets is indeed rapid. For the Moon completes its course in approximately twenty-eight days; Mercury in the months and a quarter of one day; Mars in two and a half years; Jupiter in twelve; Saturn in thirty years. But the fixed stars, which are in the highest sphere, move, most of them according to the ancients, in one hundred years, but according to the more recent, in 66 years. And, what is more C wonderful, they do not move around their own pole, but follow the oblique path of the sun. Whence it is manifest that God has placed the sun in the heaven as a leader, in order. Aristotle, then, does not admit that the stars move by themselves, fearing lest his hypotheses concerning the fifth body be overturned, as if the stars were cutting the celestial body as if it were some kind of moist substance; he supposes instead that there are rolling stars which move what is solid. But others who came after him thought that the stars moved by themselves, except for the planets, which they say move in a circle and outside their own center. For they imagine spheres, some not having the same center as the earth, but being close to the earth on one side and farther away on the other; these they called "excentric." And they imagined others, small revolutions upon the excentric itself, in which they say the planets proceed, and for this reason they named them "supracircular." They were compelled to suppose these things and to expend the greatest labor and pain for the confirmation of such hypotheses for the following reason. For the Greeks, believing that the stars were gods, and observing them as they traveled their proper path, standing immobile for certain days, then turning back, then again standing still and immediately proceeding—thinking that such wandering was unbecoming to divine bodies—imagined these hypotheses, to show that the stars do not suffer such things, but that they seem to us to be fixed and to turn back through the motion of the excentric and supracircular spheres, while they themselves suffer nothing of the kind. And while they wished to make uniform [the motion of] the celestial bodies
they ended up filling the heavens with much irregularity. But it seems to me that the stars themselves undergo such irregularity—as something intense, yet ordered; for the things here below are both irregular and disordered, while the things in the stars are both irregular and ordered, and the things above them are both ordered and uniform.
24. Whence the stars are illuminated.
Regarding this, there was much doubt among the ancient sages. For some, seeing the moon receiving its light from the sun, declared that all the stars also are illuminated by the sun. Others, however, seeing that Venus and Mercury are situated below the sun and do not depart far enough from it to receive light from it, held the opinion that these, and the others as well, possess their own light. B25. How the four seasons come to be.
All those things which God created are full of wisdom and worthy of admiration, but especially the obliquity of the solar path, as has been demonstrated by me in another work entitled *On the Cause of Celestial Phenomena*. For it does not traverse the mid-heaven along the equinoctial line, but inclines toward the north by about twenty-four degrees, and similarly toward the south. Therefore, when, returning from the south, it reaches the equinoctial line, it brings about the vernal season and approaches us, because, as we have said, our habitation is further to the north. And when it turns back again to the equinoctial, it brings about autumn, and when it arrives at the extreme of its southern oblique path, it brings about winter, because at that time it is far from us. Such an obliquity of the sun is the cause of the generation of the four seasons of the year; and not only of these, but also of other things most useful and necessary to the world, as has been demonstrated to us more extensively in the mentioned work. C26. On the magnitude of the sun and the moon.
The magnitude of the sun seems something incredible to those who are ignorant of geometry and uninitiated in philosophy. All mathematicians, therefore, have declared that it is greater than the earth, but they have disagreed as to how much greater and by how many multiples. But the most wise Ptolemy, through geometric demonstrations and methods, showed it to be one hundred and seventy times larger than the earth. And while to set forth and explain his demonstration is foreign to the present work, it will be shown here clearly and indubitably that the sun D is greater than the earth, in this way. Everything that illuminates is either greater than the thing illuminated, equal to it, or less than it. And if it is greater, it causes a shadow that begins from a broader base and extends to a sharp point; if it is equal, it causes a shadow that extends to infinity between parallel straight lines; if it is less, it produces a basket-shaped shadow, which from a narrow base expands to infinity. Since, therefore, the earth, being illuminated by the sun...it produces a conical shadow, it is smaller than it. That the earth produces a conical shadow is manifest from the eclipses of the moon. For when it suffers an eclipse at its apogee, it is far from the earth and is quickly illuminated again, because it passes through the narrower part of the shadow; but when it is at its perigee, it is sluggish in its eclipse, because it passes through the thicker part of the shadow. A Moreover, it is also shown by many other things that the shadow of the earth terminates. For if it proceeded to infinity, it would darken not a few of the stars and would cause them to suffer eclipses every night; but now nothing of the sort appears. Regarding this shadow, the poet also says: Into the Ocean fell the sun's bright light, Drawing black night over the fertile fields. But the size of the sun is also manifest from this: the moon lies under the summer tropic; when, therefore, the sun arrives there, B it casts a shadow of the bodies situated there for a space of three hundred stadia at midday. In other words, the circle which the sun traverses is ten thousand times greater than the earth, and the part of this circle which holds the sun’s size must be ten thousand times greater than the aforementioned space of the earth. Moreover, that the shape of the sun is spherical, likewise also that of the other bodies in the heaven, is evident, as has been said, and it will be spoken of more clearly in the treatise on shape.
27. Concerning the eclipse of the sun.
The eclipse of the sun occurs when the moon passes under the sun in conjunction, not having latitude relative to the zodiacal circle. C For at that time it blocks its rays, so that they do not reach the earth. For this reason, a solar eclipse always occurs in conjunction, except for that which occurred during the saving Passion of our Lord; for that happened miraculously at the time of the full moon. Now, eclipses of the sun are sometimes total, and sometimes partial. They are total when the entire solar circle is darkened, the centers of both the sun and the moon being situated along a perpendicular line; but they are partial when a part of it suffers an eclipse. However, the sun does not appear eclipsed to the whole inhabited world, D but only to certain parts. For even here, during the reign of the Comnenus, it was totally eclipsed in the west, while in Egypt it did not suffer an eclipse at all, as I observed when I was there at that time. Mathematicians, therefore, based on appearances, have divided the surface of the sun and the moon into twelve digits; and if the eclipse is half, they call it six digits; but if it is a third part of the sun, four digits, and similarly for the other divisions.28. Concerning the substance, light, and shape of the moon.
Although the Aristotelians supposed the heaven to be a fifth body, yet they agree that some of its parts are thicker, especially those situated near the earth, while others are thinner, namely those that are above and further removed from the earth. The moon, therefore, being close to this world subject to generation and corruption, was not left without a share or part in its evils, but has a thick and, as it were, earthy substance. And it is not entirelyA uniform throughout its whole body, but there are certain parts in it that are incapable of light and are dark—namely, those through its middle. And just as things subject to generation and corruption have a beginning, an increase, a prime, and a decline, so too the illumination of this [body] completes its course. And not only in these respects does it fall short of the other celestial bodies, but it sometimes also falls into the shadow of the earth; and it suffers an eclipse not merely in regard to our vision, as the sun does, but it [loses its light] in accordance with its own proper light, because it receives light from the sun, which is evident even to those who have observed it in any manner whatsoever; for it always keeps its illuminated part toward the sun, wherever it may be. For being lower than the sun, when it is in conjunction with it, its parts toward us are unilluminated, while those toward the sun are illuminated; but when it departs ever so little from the sun, a certain portion of the part toward us begins to be brightened, and it becomes crescent-shaped; then, departing further, it is more illuminated and becomes halved, and again gibbous, and when it is diametrically opposite, it is full. Then, having passed the diameter, it is seen again gibbous, and again halved, and after this crescent-shaped, and finally, upon being in conjunction with the sun, it becomes invisible to us. The ancients used to doubt how it B receives and sends forth light here. And some said that light occurs in it by reflection, just as it happens in mirrors and smooth bodies; others, however, taught that it is illuminated by reception and union; and that it is illuminated by reflection proceeding from it, most people indeed suppose. Which, however, is false. It has been observed that reflections proceeding from the largest and smallest bodies do not pass farther than two stades. But it is also demonstrated otherwise: all reflections, as is shown in the field of optics, occur by equal angles; and if this were the case with the moon, it would be necessary that, when it is situated according to the diameter of the sun at the full moon, the rays should not be refracted toward the earth, but toward the sun. And many other proofs refute this opinion. It remains, therefore, that it is illuminated by the reception and union with the solar light, and that it shines upon those things here. But one might raise a difficulty: if the solar light is mingled with it, it ought to be illuminated in its entirety and have full light continuously. And we respond to this that it has great depth and a very large diameter, and therefore the sun's rays do not pass through the whole of it, but after they have passed through the greatest part of it, they are terminated. That its form is spherical is evident from its light; for if it had the form of a disk, as soon as the solar rays fell upon it, it would be illuminated, with no curvature of light obstructing it; but now, C since it is spherical, it is illuminated in parts by the rays because the remaining parts obstruct [the light] by the curvature of such a shape. Moreover, the magnitude of the moon is smaller than that of the earth by thirty-nine times, though according to others, by twenty-nine times. D Deficit autem luna quando directe ex opposito. That is to say, the moon suffers an eclipse when it is located directly opposite the sun.
when it has no latitude, inclining neither to the North nor to the South, but travels along the same path as the sun. For at that time it falls into the shadow of the earth. And if it is at its apogee, it passes through the shadow more quickly, as it is very thin at the top, as we said before; but if it is at its perigee, it makes a stay and is slow to emerge, because the shadow is thicker below. From this, the wise men of old took the occasion for understanding the distance from the earth to the sky. For by observing that the sun, at its rising, makes shadows sixty times larger than the objects casting them, they demonstrated that the length of the earth's shadow must be sixty times its diameter. And since its point ends at the lunar sphere, then if, by hypothesis, one were to stack sixty earths one upon another, they would reach to the solar sphere; and reasoning thence, they understood that the interval between the earth and the lunar sphere is as much as about forty times the measure of the earth's diameter. The moon, moreover, is eclipsed sometimes wholly, sometimes partially. And when it is eclipsed wholly, it sometimes makes a stay, because it happens to be in the thicker part of the shadow; at other times, as soon as it is covered, it begins to emerge, when it is eclipsed above in the thinner part of the shadow. But it does not happen with the moon as with the sun, which can be seen eclipsed in one place and not in another; for this eclipse occurs everywhere on earth, unless it is day for some people, for indeed, the sun is at that time above the earth for them; for it has been shown to us that while it is day for some, it is night for others.