# 1911 Encyclopædia Britannica/Ptolemy

**PTOLEMY**(Claudius Ptolemaeus), the celebrated mathematician, astronomer and geographer, was a native of Egypt, but there is an uncertainty as to the place of his birth. Some ancient manuscripts of his works describe him as of Pelusium, but Theodorus Meliteniota, a Greek writer on astronomy of the 12th century, says that he was born at Ptolemais Hermii, a Grecian city of the Thebaid. It is certain that he observed at Alexandria during the reigns of Hadrian and Antoninus Pius, and that he survived Antoninus. Olympiodorus, a philosopher of the Neoplatonic school who lived in the reign of the emperor Justinian, relates in his scholia on the

*Phaedo*of Plato that Ptolemy devoted his life to astronomy and lived for forty years in the so-called Πτερὰ τοῦ Κανώβου, probably elevated terraces of the temple of Serapis at Canopus near Alexandria, where they raised pillars with the results of his astronomical discoveries engraved upon them. This statement is probably correct; we have indeed the direct evidence of Ptolemy himself that he made astronomical observations during a long series of years; his first recorded observation was made in the eleventh year of Hadrian, 127 A.D.,

^{[1]}and his last in the fourteenth year of Antoninus, 151 A.D. Ptolemy, moreover, says, “ We make our observations in the parallel of Alexandria.” St Isidore of Seville asserts that he was of the royal race of the Ptolemies, and even calls him king of Alexandria; this assertion has been followed by others, but there is no ground for their opinion. Indeed Fabricius shows by numerous instances that the name Ptolemy was common in Egypt. Weidler, from whom this is taken, also tells us that according to Arabian tradition Ptolemy lived to the age of seventy-eight years; from the same source some description of his personal appearance has been handed down, which is generally considered as not trustworthy, but which may be seen in Weidler,

*Historia astronomiae*, p. 177, or in the preface to Halma's edition of the

*Almagest*, p. 61.

*Mathematics.*

Ptolemy's work as a geographer is discussed below, and an account of the discoveries in astronomy of Hipparchus and Ptolemy is given in the article Astronomy: *History*. Their contributions to pure mathematics, however, require to be noticed here. Of these the chief is the foundation of trigonometry, plane and spherical, including the formation of a table of chords, which served the same purpose as our table of sines. This branch of mathematics was created by Hipparchus for the use of astronomers, and its exposition was given by Ptolemy in a form so perfect that for 1400 years it was not surpassed. In this respect it may be compared with the doctrine as to the motion of the heavenly bodies so well known as the Ptolemaic system, which was paramount for about the same period of time. There is, however, this difference, that, whereas the Ptolemaic system was then overthrown, the theorems of Hipparchus and Ptolemy, on the other hand, will be, as Delambre says, for ever the basis of trigonometry. The astronomical and trigonometrical systems are contained in the great work of Ptolemy, Ὴ μαθηματικὴ σύνταξις, or, as Fabricius after Syncellus writes it, Μεγάλη σύνταξισ τῆς ἀστρονομίας; and in like manner Suidas says οὖτος [Πτολ.] ἔγραψε τὸν μέγαν ἀστρονόμον ἤτοι σύνταξις. The *Syntaxis* of Ptolemy was called Ὁ *μέγας ἀστρονόμος* to distinguish it from another collection called Ὁ μικρός ἀστρονόμος, also highly esteemed by the Alexandrian school, which contained some works of Autolycus, Euclid, Aristarchus, Theodosius of Tripolis, Hypsicles and Menelaus. To designate the great work of Ptolemy the Arabs used the superlative μεγίστη, from which, the article *al* being prefixed, the hybrid name *Almagest*, by which it is now universally known, is derived.

*Almagest*Ptolemy shows how to form a table of chords. He supposes the circumference divided into 360 equal parts (τμήματα), and then bisects each of these parts. Further, he divides the diameter into 120 equal parts, and then for the subdivisions of these he employs the sexagesimal method as most convenient in practice,

*i.e.*he divides each of the sixty parts of the radius into sixty equal parts, and each of these parts he further subdivides into sixty equal parts. In the Latin translation these subdivisions become “ partes minutae primae ” and “ partes minutae secundae, ” whence our “ minutes ” and “ seconds ” have arisen. It must not be supposed, however, that these sexagesimal divisions are due to Ptolemy; they must have been familiar to his predecessors, and were handed down from the Chaldaeans. Nor did the formation of the table of chords originate with Ptolemy; indeed, Theon of Alexandria, the father of Hypatia, who lived in the reign of Theodosius, in his commentary on the

*Almagest*says expressly that Hipparchus had already given the doctrine of chords inscribed in a circle in twelve books, and that Menelaus had done the same in six books, but, he continues, every one must be astonished at the ease with which Ptolemy, by means of a few simple theorems, has found their values; hence it is inferred that the method of calculation in the

*Almagest*is Ptolemy's own.

*Elements*of Euclid. Thus the side of the hexagon, or the chord of 60°, is equal to the radius, and therefore contains sixty parts. The side of the decagon, or the chord of 36°, is the greater segment of the radius cut in extreme and mean ratio, and therefore contains approximately 37

^{p}4′ 55″ parts, of which the diameter contains 120 parts. Further, the square on the side of the regular pentagon is equal to the sum of the squares on the sides of the regular hexagon and of the regular decagon, all being inscribed in the same circle (Eucl. XIII. 10); the chord of 72° can therefore be calculated, and contains approximately 70

^{p}32′ 3″. In like manner, the square on the chord of 90°, which is the side of the inscribed square, is twice the square on the radius; and the square on the chord of 120°, or the side of the equilateral triangle, is three times the square on the radius; these chords can thus be calculated approximately. Further, from the values of all these chords we can calculate at once the chords of the arcs which are their supplements.

sin (A−B)=sin A cos B−cos A sin B,

^{p}34′ 55″, and the chord of three-fourths of 1° contains 0

^{p}47′ 8″. A table of chords of arcs increasing by 1½° can thus be formed; but this is not sufficient for Pto1emy's purpose, which was to frame a table of chords increasing by half a degree. This could be effected if he knew the chord of one-half of 1°; but, since this chord cannot be found geometrically from the chord of 1½°, inasmuch as that would come to the trisection of an angle, he proceeds to seek in the first place the chord of 1°, which he finds approximately by means of a lemma of great elegance, due probably to Apollonius. It is as follows: If two unequal chords be inscribed in a circle, the greater will be to the less in a less ratio than the arc described on the greater will be to the arc described on the less. Having proved this theorem, he proceeds to employ it in order to find approximately the chord of 1°, which he does in the following manner—

*i.e.*

*i.e.*

^{p}47′ 8″ q.p., and the chord of 90′ is 1

^{p}34′ 15″ q.p.; hence it follows that approximately

^{p}2′ 50″ 40′″ and > 1

^{p}2′ 50″.

^{p}2′ 50″ as the approximate value of the chord of 1°. The chord of 1° being thus known, he finds the chord of one-half of a degree, the approximate value of which is of 31′ 25″, and he is at once in a position to complete his table of chords for arcs increasing by half a degree. Ptolemy then gives his table of chords, which is arranged in three columns; in the first he has entered the arcs, increasing by half degrees, from 0° to 180°; in the second he gives the values of the

^{[2]}

*Almagest*, whilst the solution of plane triangles is not treated separately in that work.

*Alm.*vi. ch. 7; i. 422, ed. Halma) Ptolemy solves a triangle in which the three sides are given by finding the segments of a side made by the perpendicular on it from the opposite vertex. It should be noticed also that the eleventh chapter of the first book of the

*Almagest*contains incidentally some theorems and problems in plane trigonometry. The problems which are met with correspond to the following: Divide a given arc into two parts so that the chords of the doubles of those arcs shall have a given ratio; the same problem for external section. Lastly, it may be mentioned that Ptolemy (

*Alm.*vi. ch. 7; i. 421, ed. Halma) takes 3° 8′ 30″,

*i.e.*3++=3·1416, as the value of the ratio of the circumference to the diameter of a circle, and adds that, as had been shown by Archimedes, it lies between 3 and 3.

*Almagest*. This error was corrected by Mersenne, who showed that it was known to Menelaus, an astronomer and geometer who lived in the reign of the emperor Trajan. The theorem now bears the name of Menelaus, though most probably it came down from Hipparchus; Chasles, indeed, thinks that Hipparchus deduced the property of the spherical triangle from that of the plane triangle, but throws the origin of the latter farther back and attributes it to Euclid, suggesting that it was given in his

*Porisms*.

^{[3]}Carnot made this theorem the basis of his theory of transversals in his essay on that subject. It should be noticed that the theorem is not given in the Almagest in the general manner stated above; Ptolemy considers two cases only of the theorem, and Theon, in his commentary on the

*Almagest*, has added two more cases. The proofs, however, are general. Ptolemy then lays down two lemmas: lf the chord of an arc of a circle be cut in any ratio and a diameter be drawn through the point of section, the diameter will cut the arc into two parts the chords of whose doubles are in the same ratio as the segments of the chord; and a similar theorem in the case when the chord is cut externally in any ratio. By means of these two lemmas Ptolemy deduces in an ingenious manner—easy to follow, but difficult to discover—from the theorem of Menelaus for a plane triangle the corresponding theorem for a spherical triangle: If the sides of a spherical triangle be cut by an arc of a great circle, the chords of the doubles of the segments of any one side will be to each other in a ratio compounded of the ratios of the chords of the doubles of the segments of the other two sides. Here, too, the theorem is not stated generally; two cases only are considered, corresponding to the two cases given

*in plano*. Theon has added two cases. The proofs are general. By means of this theorem four of Napier’s formulae for the solution of right-angled spherical triangles can be easily established. Ptolemy does not give them, but in each case when required applies the theorem of Menelaus for spherics directly. This greatly increases the length of his demonstrations, which the modern reader finds still more cumbrous, inasmuch as in each case it was necessary to express the relation in terms of chords—the equivalents of sines—only, cosines and tangents being of later invention.

Such, then, was the trigonometry of the Greeks. Mathematics, indeed, has ever been, as it were, the handmaid of astronomy, and many important methods of the former arose from the needs of the latter. Moreover, by the foundation of trigonometry, astronomy attained its final general constitution, in which calculations took the place of diagrams, as these latter had been at an earlier period substituted for mechanical apparatus in solving the ordinary problems.^{[4]} Further, we find in the application of trigonometry to astronomy frequent examples and even a systematic use of the method of approximations—the basis, in fact, of all application of mathematics to practical questions. There was a disinclination on the part of the Greek geometer to be satisfied with a mere approximation, were it ever so close; and the unscientific *agrimensor* shirked the labour involved in acquiring the knowledge which was indispensable for learning trigonometrical calculations. Thus the development of the calculus of approximations fell to the lot of the astronomer, who was both scientific and practical.^{[5]}

*Almagest*. It is divided into thirteen books. The first book, which may be regarded as introductory to the whole work, opens with a short preface, in which Ptolemy, after some observations on the distinction between theory and practice, gives Aristotle’s division of the sciences and remarks on the certainty of mathematical knowledge, “ inasmuch as the demonstrations in it proceed by the incontrovertible ways of arithmetic and geometry.” He concludes his preface with the statement that he will make use of the discoveries of his predecessors, and relate briefly all that has been sufficiently explained by the ancients, but that he will treat with more care and development whatever has not been well understood or fully treated. Ptolemy unfortunately does not always bear this in mind, and it is sometimes difficult to distinguish what is due to him from that which he has borrowed from his predecessors.

*i.e.*the phenomena of the heavenly bodies—he says he will endeavour to explain in taking for principle that which is evident, real and certain, in resting everywhere on the surest observations and applying geometrical methods. He then enters on a summary exposition of the general principles on which his

*Syntaxis*is based, and adduces arguments to show that the heaven is of a spherical form and that it moves after the manner of a sphere, that the earth also is of a form which is sensibly spherical, that the earth is in the centre of the heavens, that it is but a point in comparison with the distances of the stars, and that it has not an motion of translation. With respect to the revolution of the earth round its axis, which he says some have held, Ptolemy, while admitting that this supposition renders the explanation of the phenomena of the heavens much more simple, yet regards it as altogether ridiculous. Lastly, he lays down that there are two principal and different motions in the heavens—one by which all the stars are carried from east to west uniformly about the poles of the equator; the other, which is peculiar to some of the stars, is in a contrary direction to the former motion and takes place round different poles. These preliminary notions, which are all older than Ptolemy, form the subjects of the second and following chapters. He next proceeds to the construction of his table of chords, of which we have given an account, and which is indispensable to practical astronomy. The employment of this table presupposes the evaluation of the obliquity of the ecliptic, the knowledge of which is indeed the foundation of all astronomical science. Ptoiemy in the next chapter indicates two means of determining this angle by observation, describes the instruments he employed for that purpose, and finds the same value which had already been found by Eratosthenes and used by Hipparchus. This “ is followed by spherical geometry and trigonometry enough for the determination of the connexion between the sun’s right ascension, declination and longitude, and for the formation of a table of declinations to each degree of longitude. Delambre says he found both this and the table of chords very exact.”

^{[6]}

^{[7]}to equinoctial hours and vice versa, and of the nonagesimal point and the point of orientation of the ecliptic. In the following chapters of this book he determines the angles formed by the intersections of the ecliptic—first with the meridian, then with the horizon, and lastly with the vertical circle—and concludes by giving tables of the angles and arcs formed by the intersection of these circles, for the seven climates, from the parallel of Meroe (thirteen hours) to that of the mouth of the Borysthenes (sixteen hours). These tables, he adds, should be completed by the situation of the chief towns in all countries according to their latitudes and longitudes; this he promises to do in a separate treatise and has in fact done in his

*Geography*.

^{[8]}This fine principle, which is of universal application, may, we think—regard being paid to its place in the

*Almagest*—be justly attributed to Hipparchus. It is the first law of the “ philosophic prima ” of Comte.

^{[9]}We find in the same page another principle, or rather practical injunction, that in investigations founded on observations where great delicacy is required we should select those made at considerable intervals of time in order that the errors arising from the imperfection which is inherent in all observations, even in those made with the greatest care, may be lessened by being distributed over a large number of years. In the same chapter we find also the principle laid down that the object of mathematicians ought to be to represent all the celestial phenomena by uniform and circular motions. This principle is stated by Ptolemy in the manner which is unfortunately too common with him—that is to say, he does not give the least indication whence he derived it. We know, however, from Simplicius, on the authority of Sosigenes

^{[10]}that Plato is said to have proposed the following problem to astronomers: “ What regular and determined motions being assumed would full account for the phenomena of the motions of the planetary bodies?” We know, too, from the same source that Eudemus says in the second book of his

*History of Astronomy*that “ Eudoxus of Cnidus was the first of the Greeks to take in hand hypothesis of this kind,”

^{[11]}that he was in fact the first Greek astronomer who proposed a geometrical hypothesis for explaining the periodic motions of the planets—the famous system of concentric spheres. It thus appears that the principle laid down here by Ptolemy can be traced to Eudoxus and Plato; and it is probable that they derived it from the same source, namely, Archytas and the Pythagoreans. We have indeed the direct testimony of Geminus of Rhodes that the Pythagoreans endeavoured to explain the phenomena of the heavens by uniform and circular motions.

^{[12]}

^{[13]}Book iv., in which Ptolemy follows Hipparchus, treats of the first and principal inequality of the moon, which quite corresponds to the inequality of the sun treated of in the third book. As to the observations which should be employed for the investigation of the motion of the moon, Ptolemy tells us that lunar eclipses should be preferred, inasmuch as they give the moon’s place without any error on the score of parallax. The first thing to be determined is the time of the moon’s revolution; Hipparchus, by comparing the observations of the Chaldaeans with his own, discovered that the shortest period in which the lunar eclipses return in the same order was 126,007 days and 1 hour. In this period he finds 4267 lunations, 4573 restitutions of anomaly and 4612 tropical revolutions of the moon less 7½° q.p.; this quantity (7½°) is also wanting to complete the 345 revolutions which the sun makes in the same time with respect to the fixed stars. He concluded from this that the lunar month contains 29 days and 31′ 50″ 8′ ″ 20″ ″ of a day, very nearly, or 29 days 12 hours 44′ 3″ 20′ ″. These results are of the highest importance. In order to explain this inequality, or the equation of the centre, Ptolemy makes use of the hypothesis of an epicycle, which he prefers to that of the eccentric. The fifth book commences with the description of the astrolabe of Hipparchus, which Ptolemy made use of in following up the observations of that astronomer, and by means of which he made his most important discovery, that of the second inequality in the moon’s motion, now known by the name of the “ evection.” In order to explain this inequality he supposed the moon to move on an epicycle, which was carried by an eccentric whose centre turned about the earth in a direction contrary to that of the motion of the epicycle. This is the first instance in which we find the two hypotheses of eccentric and epicycle combined. The fifth book treats also of the parallaxes of the sun and moon, and gives a description of an instrument—called later by Theon the “ parallactic rods ” devised by Ptolemy for observing meridian altitudes with greater accuracy.

*Almagest*, and the method of calculating eclipses is there given. The author says nothing in it which was not known before his time.

^{[14]}It has been remarked that Ptolemy, living at Alexandria, at which city the altitude of the pole is 5° less than at Rhodes, where Hipparchus observed, could have seen stars which are not visible at Rhodes; none of these stars, however, are in Ptolemy’s catalogue. The eighth book contains, moreover, a description of the milky way and the manner

^{[15]}So far all astronomers are agreed. This is not the case, he says, with respect to the two remaining planets, Mercury and Venus, which the old astronomers placed between the sun and earth, whereas more recent writers

^{[16]}have placed them beyond the sun, because they were never seen on the sun.

^{[17]}He shows that this reasoning is not sound, for they might be nearer to us than the sun and not in the same plane, and consequently never seen on the sun. He decides in favour of the former opinion, which was indeed that of most mathematicians. The ground of the arrangement of the planets in order of distance was the relative length of their periodic times; the greater the circle, the greater, it was thought, would be the time required for its description. Hence we see the origin of the difficulty and the difference of opinion as to the arrangement of the sun, Mercury and Venus, since the times in which, as seen from the earth, they appear to complete the circuit of the zodiac are nearly the same—a year.

^{[18]}Delambre thinks it strange that Ptolemy did not see that these contrary opinions could be reconciled by supposing that the two planets moved in epicycles about the sun; this would be stranger still, he adds, if it is true that this idea, which is older than Ptolemy, since it is referred to by Cicero,

^{[19]}had been that of the Egyptians

^{[20]}It may be added, as strangest of all, that this doctrine was held by Theon of Smyrna,

^{[21]}who was a contemporary of Ptolemy or somewhat senior to him. From this system to that of Tycho Brahe there is, as Delambre observes, only a single step.

^{[22]}and shows how the motions in longitude of the planets can be represented in a general manner by means of the hypothesis of the eccentric combined with that of the epicycle. He next applies his theory to each planet and concludes the ninth book by the explanation of the various phenomena of the planet Mercury. In the tenth and eleventh books he treats, in like manner, of the various phenomena of the planets Venus, Mars, Jupiter and Saturn.

*Almagest*intelligible, he (Delambre) was obliged to recast them. This statement of Ptolemy is important, as it shows that the mathematical theory of the planetary motions was in a tolerably forward state long before his time. Finally, book xiii. treats of the motions of the planets in latitude, also of the inclinations of their orbits and of the magnitude of these inclinations.

*Syntaxis*was translated into Arabic at Bagdad by order of the enlightened caliph Al-Mamūm, who was himself an astronomer, about 827 A.D., and the Arabic translation was revised in the following century by Tobit ben Korra. The

*Almagest*was translated from the Arabic into Latin by Gerard of Cremona (

*q.v.*). In the 15th century it was translated from a Greek manuscript in the Vatican by George of Trebizond. In the same century an epitome of the

*Almagest*was commenced by Purbach (d. 1461) and completed by his pupil and successor in the professorship of astronomy in the university of Vienna, Regiomontanus. The earliest edition of this epitome is that of Venice (1496), and this was the first appearance of the

*Almagest*in print. The first complete edition of the

*Almagest*is that of P. Liechtenstein (Venice, 1515)—a Latin version from the Arabic. The Latin translation of George of Trebizond was first printed in 1528, at Venice. The Greek text, which was not known in Europe until the 15th century, was first published in the 16th by Simon Grynaeus, who was also the first editor of the Greek text of Euclid, at Basel (1538). This edition was from a manuscript in the library of Nuremberg—where it is no longer to be found—which had been presented by Regiomontanus, to whom it was given by Cardinal Bessarion.

*On the Apparitions of the Fixed Stars and a Collection of Prognostics*. It is a calendar of a kind common amongst the Greeks under the name of παράπηγμα, or a collection of the risings and settings of the stars in the morning or evening twilight, which were so many visible signs of the seasons, with prognostics of the principal changes of temperature with relation to each climate, after the observations of the best meteorologists, as, for example, Meton, Democritus, Eudoxus, Hipparchus, &c. Ptolemy, in order to make his

*Parapegma*useful to all the Greeks scattered over the enlightened world of his time, gives the apparitions of the stars not for one parallel only but for each of the five parallels in which the length of the longest day varies from 13½ hours to 15½ hours—that is, from the latitude of Syene to that of the middle of the Euxine. This work was printed by Petavius in his

*Uranologium*(Paris, 1630), and by Halma in his edition of the works of Ptolemy, vol. iii. (Paris, 1819). (2) Ὑποθέσεις τῶν πλανωμένων ῆ τῶν οὐρανίων κύκλων κινήσεις,

*On the Planetary Hypothesis*. This is a summary of a portion of the

*Almagest*, and contains a brief statement of the principal hypotheses for the explanation of the motions of the heavenly bodies. It was first published (Gr., Lat.) by Bainbridge, the Savilian professor of astronomy at Oxford, with the Sphere of Proclus and the Κανὼν βασιλειῶν (London, 1620), and afterwards by Halma, vol. iv. (Paris, 1820). (3) Κανὼν βασιλειῶν,

*A Table of Reigns*. This is a chronological table of Assyrian, Persian, Greek and Roman sovereigns, with the length of their reigns, from Nabonasar to Antoninus Pius. This table (cf. G. Syncellus,

*Chronogr*. ed. Dind. i. 388 seq.) was printed by Scaliger, Calvisius, Petavius, Bainbridge and by Halma,

*Treatise on Music*was published in Greek and Latin by Wallis at Oxford (1682). It was afterwards reprinted with Porphyry’s commentary in the third volume of Wallis’s works (Oxford, 1699). (5) Τετράβιβλος σύνταξις,

*Tetrabiblon*or

*Quadripartitum*. This work is astrological, as is also the small collection of aphorisms, called Καρπός or

*Centiloquium*, by which it is followed. It is doubtful whether these works are genuine, but the doubt merely arises from the feeling that they are unworthy of Ptolemy. They were both published in Greek and Latin by Camerarius (Nuremberg, 1535), and by Melanchthon (Basel, 1553). (6)

*De analemmate*. The original of this work of Ptolemy is lost. It was translated from the Arabic and published by Commandine (Rome, 1562). The Analemma is the description of the sphere on a plane. We find in it the sections of the different circles, as the diurnal parallels, and everything which can facilitate the intelligence of gnomonics. This description is made by perpendiculars let fall on the plane; whence it has been called by the moderns “ orthographic projection.” (7)

*Planisphaerium*,

*The Planisphere*. The Greek text of this work also is lost, and we have only a Latin translation of it from the Arabic. The “ planisphere ” is a projection of the sphere on the equator, the eye being at the pole—in fact what is now called “ stereographic ” projection. The best edition of this work is that of Commandine (Venice, 1558). (8)

*Optics*. This work is known to us only by imperfect manuscripts in Paris and Oxford, which are Latin translations from the Arabic. The Optics consists of five books, of which the fifth presents most interest: it treats of the refraction of luminous rays in their passage through media of different densities, and also of astronomical refractions, on which subject the theory is more complete than that of any astronomer before the time of Cassini. De Morgan doubts whether this work is genuine on account of the absence of allusion to the

*Almagest*or to the subject of refraction in the

*Almagest*itself; but his chief reason for doubting its authenticity is that the author of the Optics was a poor geometer. (G. J. A.)

*Claudii Ptolemaei opera quae exstant omnia*, was recently undertaken at Leipzig. The first volume (in two parts, 1898, 1903) contains the Greek text of the

*Almagest*edited by J. L. Heiberg. Consult also J. E. Mentucla,

*Histoire des mathématiques*, i. 293; J. B. J. Delambre,

*Connaissance des temps*(1816); and

*Histoire de l’astronomie ancienne*, vol. 2; J. J. A. Caussin,

*Nouvelles mémoires de l’acad. des inscriptions*, t. vi.; P. Tannery,

*Recherches sur l’histoire de l’astronomie ancienne*, ch. vi.–xv.; Narrieu,

*History of Astronomy*(1833); Fabricius,

*Bibliotheca graeca*, ed. Harles, vol. 5; Halma’s 1813–1816 edition of his

*Almagest*(Greek with French translation); A. Berry,

*A Short History of Astronomy*, pp. 62–73;

*British Museum Catalogue*.

*Geography.*

*Geographikē syntaxis*exercised as great an influence on geographical progress (especially during the period of the Classical Renaissance), as did his

*Almagest*on astronomical. This exceptional position was largely due to its scientific form, which rendered it convenient and easy of reference; but, apart from this, it was really the most considerable attempt of the ancient world to place the study of geography on a scientific basis. The astronomer Hipparchus had indeed pointed out, three centuries before Ptolemy, that the only way to construct a trustworthy map of the inhabited world would be by observations of the latitude and longitude of all the principal points on its surface. But the materials for such a map were almost wholly wanting, and, though Hipparchus made some approach to a correct division of the known world into zones of latitude, “ climates ” or

*klimata*, as he termed them, trustworthy observations of latitude were then very few, while the means of determining longitudes hardly existed. Hence probably it arose that no attempt was made to follow up the suggestion of Hipparchus until Marinus of Tyre, who lived shortly before Ptolemy, and whose work is known to us only through the latter. Marinus' scientific materials being inadequate, he contented himself mostly with determinations derived from itineraries and other rough methods, such as are still employed where more accurate means of determination are not available. The greater part of Marinus’ treatise was occupied with the discussion of his authorities, and it is impossible, in the absence of the original work, to decide how far his results attained a scientific form. But Ptolemy himself considered them, on the whole, so satisfactory that he made his predecessor’s work the basis of his own in regard to all the Mediterranean countries, that is, in regard to almost all those regions of which he had definite knowledge. In the more remote regions of the world, Ptolemy availed himself of Marinus' information, but with reserve, and himself explains the reasons that induced him sometimes to depart from his predecessor’s conclusions. It is unjust to term Ptolemy a plagiarist from Marinus, as he himself fully acknowledges his obligations to that writer, from whom he derived the whole mass of his materials, which he undertook to arrange and present to his readers in a scientific form. It is this form, unique among those ancient geographical treatises which have survived, that constitutes one great merit of Ptolemy’s work. At the same time it shows the increased knowledge of Asia and Africa acquired since Strabo and Pliny.

*Mathematical*

*Geography*.—As an astronomer, Ptolemy was of course better qualified to explain the mathematical conditions of the earth and its relations to the celestial bodies than most preceding geographers. His general views had much in common with those of Eratosthenes and Strabo. Thus he assumed that the earth was a globe, the surface of which was divided by certain great circles— the equator and the tropics—parallel to one another, dividing the earth into five zones, the relations of which with astronomical phenomena were of course clear to his mind as a matter of theory, though in regard to the regions bordering on the equator, as well as to those adjoining the polar circle, he could have had no confirmation of his conclusions from actual observation. He adopted also from Hipparchus the division of the equatorial circle into 360 parts (degrees, as they were subsequently called, though the word does not occur in this sense in Ptolemy), and supposed other circles to be drawn through this, from the equator to the pole, to which he gave the name of meridians. He thus, like modern geographers, conceived the whole surface of the earth as covered with a network of parallels of latitude and meridians of longitude, terms which he himself was the first extant writer to employ in this technical sense. Within the network thus constructed it was his task to place the outline of the world, so far as known to him.

*i.e.*25,000 (25,200) geographical miles, was generally adopted by subsequent geographers, including Strabo. Poseidonius, however (

*c.*135-50 B.C.), reduced this to 180,000, and the latter computation was inexplicably adopted by Marinus and Ptolemy. This error made every degree of latitude or longitude (measured at the equator) equal to only 500 stadia (50 geographical miles), instead of its true equivalent of 600 stadia. The mistake would have been somewhat neutralized had there existed a sufficient number of points of which the position was fixed by observation; but we learn from Ptolemy himself that such observations for latitude were very few, while the means of determining longitudes were almost wholly wanting.

^{[23]}Hence the positions laid down by him were, with few exceptions, the result of computations from itineraries and the statements of travellers, liable to much greater error in ancient times than at the present day, from the want of any accurate mode of observing bearings, of measuring time (by portable instruments), or of estimating distances at sea, except by the rough estimate of the time employed in sailing from point to point. Even the use of the log was unknown to the ancients. But, great as were the errors resulting from such imperfect means of calculation, they were increased by the permanent error arising from Ptolemy’s system of graduation. Thus if he concluded (from itineraries) that two places were 5000 stadia distant, he would place them 10° apart, and thus in fact separate them by 6000 stadia.

*i.e.*Cape St Vincent, regarded by Marinus and Ptolemy, as by previous geographers, as the westernmost point of Europe). Hence all Ptolemy’s longitudes, reckoned eastwards, were about 7° less than they would have been if really measured from the meridian of Ferro, which continued so long in use. This error was the more unfortunate as the longitude was really calculated, not from this imaginary line, but from Alexandria, westwards as well as eastwards (as Ptolemy himself has done in his eighth book), and afterwards reversed, so as to suit the supposed method of computation.

*Prasum*(Delgado?), which he placed in 15½° S. At the same time he assumed the position in about the same parallel of a region called Agisymba, inhabited by Ethiopians and abounding in rhinoceroses, which was supposed to have been discovered by a Roman general, Julius Maternus, whose itinerary was employed by Marinus. Taking, therefore, this parallel as the limit of knowledge to the south, while he retained that of Thule to the north, Ptolemy assigned to the inhabited world a breadth of nearly 80°, instead of less than 60°, as in Eratosthenes and Strabo.

*Ivernia*) farther north than any part of Wales, and (2) twisted round the whole of Scotland, so as to make its length from west to east and to place the northern extremities of Britain and Ireland almost on the same parallel. These errors are probably connected and are naturally accompanied by the placing of Thule, the Orkneys (

*Orcades*) and the Hebrides (

*Ebudae*) indiscriminately on the left or north of Caledonia. Here he was perhaps embarrassed by adopting Marinus' conclusion that Thule lay in 63° N., while regarding it, like earlier geographers, as the northernmost of all lands. Ptolemy also supposed the northern coast of Germany, beyond the Cimbric Chersonese (Denmark), to be the southern shore of the Northern Ocean, with a general direction from west to east. Of the almost wholly landlocked Baltic he was entirely ignorant, as well as of the Scandinavian Peninsula; his Scandia is an island smaller than Corsica, lying in the true position of southern central Sweden. Some way east of the Vistula, Ptolemy, however, makes the Sarmatian coast trend north, to the parallel of Thule; nor did he conceive this as an actual limit, but believed the Unknown Land to extend indefinitely in this direction as also to the north of Asiatic Scythia.

*Periplus of the Erythraean Sea*, about A.D. 80, contains sailing directions for merchants from the Red Sea to the Indus and Malabar, and even indicates that the coast from Barygaza (Baroch) had a general southward direction down to and far beyond Cape Komari (Comorin), which, taken together with its account of the shore-line as far as the Ganges, affords some suggestions at least of a peninsular character for south India. But Ptolemy, following Marinus, not only gives to the Indian coasts, from Indus to Ganges, an undue extension in longitude, but practically denies anything of an Indian peninsula, placing capes Komaria and Kory (his southernmost points in India) only 4° S. of Barygaza, the real interval being over 800 geographical miles, or, according to Ptolemy’s system of graduation, 16° of latitude. This error, distorting the whole appearance of south Asia, is associated with another as great, but of opposite tendency, in regard to Taprobane (in which ancient ideas of Ceylon and Sumatra are confusedly mingled). The size of this was exaggerated by most earlier Greek geographers; but Ptolemy extended it through 15° of latitude and 12° of longitude, so as to make it about fourteen times as large as the reality, and bring down its southern extremity more than 2° south of the equator.

*Periplus*and that of Marinus it seems probable that Greek mariners had not only crossed the Gangetic gulf and visited the land on the opposite side, which they called the Golden Chersonese, but pushed considerably farther east, to Cattigara. But these commercial voyagers either brought back inaccurate notions, or Ptolemy’s preconceptions destroyed the value of the new information, for nowhere does he distort the truth more wildly. After passing the Great Gulf, beyond the Golden Chersonese, he makes the coast trend southward, and thus places Cattigara (perhaps one of the south China ports) 8½° south of the equator. In this he was perhaps influenced by his notion of a junction of Asia and Africa in a terra incognita, south of the Indian Ocean.

*Progress*

*of Geographical Knowledge in Certain Special*

*Regions*.—Ptolemy records, after Marinus, the penetration of Roman expeditions to the land of the Ethiopians and to Agisymba, clearly some region of the Sudan beyond the Sahara desert, perhaps the basin of Lake Chad. But while this name was the only recorded result of these expeditions, Ptolemy also gives much other information concerning the interior of North Africa (whence derived we know not) to which nothing similar is found in any earlier writer. Unfortunately this new information was of so crude a character, and is presented in so embarrassing a form, as to perplex rather than assist. Thus Ptolemy’s statements concerning the rivers Gir and Nigir, and the lakes and mountains with which they were connected, have baffled successive generations of interpreters. It may safely be said that they present no resemblance to the real features of the country as now known, and cannot be reconciled with them except by arbitrary conjecture.

*fl. c.*260 b.c.), the admiral of Ptolemy Philadelphus, as to coasts and maritime distances. Claudius Ptolemy, however, introduced many changes in Marinus' results, some of which he has pointed out though there are doubtless many others which we have no means of detecting. For the interior of the different countries Roman roads and itineraries must have furnished both Marinus and Ptolemy with a mass of valuable materials. But neither seems to have taken full advantage of these; and the tables of the Alexandrian geographer abound with mistakes—even in countries so well known as Gaul and Spain—which might easily have been obviated by a more judicious use of such Roman authorities.

Bibliography.—Ptolemy’s *Geographia* was printed for the first time in a Latin translation, accompanied with maps, in 1462(?), and numerous other editions followed in the latter part of the 15th and earlier half of the 16th centuries, but the Greek text did not make its appearance till 1533, when it was published at Basel in quarto, edited by Erasmus. All these early editions, however, swarm with textual errors, and are critically worthless. The same may be said of the edition of (Gr. and Lat., Leiden, 1618, typ. Elzevir), which was long the standard library edition. It contains a new set of maps drawn by Mercator, as well as a fresh series (not intended to illustrate Ptolemy) by Ortelius, the Roman Itineraries, including the *Tabula peutingeriana*, and much other miscellaneous matter. The first attempt at a really critical edition was made by F. G. Wilberg, and C. H. F. Grashof (4to, Essen, 1838–1845), but this only covered the first six books of the entire eight. The edition of C. F. A. Nobbe (3 vols., 18mo., Leipzig, 1843), presents the best Greek text of the whole work, and has a useful index. The best edition, so far as completed, is that published in A. F. Didot’s *Bibliotheca graecorum scriptorum* (*Claudii Ptolemaei geographia*; 2 vols., Paris, 1883 and 1901), originally edited by Carl Müller and continued by C. T. Fischer, with a Latin translation and a copious commentary, geographical as well as critical. See also. F. C. L. Sickler, *Claudii Ptolemaei Germania*, (Hesse Cassel, 1833); W. D. Cooley, *Claudius Ptolemy and the Nile* (London, 1854); J. W. McCrindle, *Ancient India described by Ptolemy* (Bombay, 1885), reprinted from *Indian Antiquary* (1884); Henry Bradley, “Ptolemy’s Geography of the British Isles,” in *Archaeologia*, vol. xlviii. (1885); T. G. Rylands, *Geography of Ptolemy Elucidated* (Dublin, 1893); and a Polish study of Ptolemy’s Germany and Sarmatia, in the Historical-Philosophical Series (2) of the Cracow University (1902), vol. xvi. (E. H. B.; C. R. B.)

- ↑ Weidler and Halma give the ninth year; in the account of the eclipse of the moon in that year Ptolemy, however, does not say, as in other similar cases,
*he*had observed, but it had been observed (*Almagest*, iv. 9). - ↑ Ideler has examined the degree of accuracy of the numbers in these tables and finds that they are correct to five places of decimals.
- ↑ On the theorem of Menelaus and the rule of six quantities, see Chasles,
*Aperçu historique sur l’origine et développement des méthodes en géométrie*, note vi. p. 291. - ↑ Comte,
*Système de politique positive*, iii. 324. - ↑ Cantor,
*Vorlesungen über Geschichte der Mathematik*, p. 356. - ↑
- ↑ Καιρικαί, temporal or variable. These hours varied in length with the seasons; they were used in ancient times and arose from the division of the natural day (from sunrise to sunset) into twelve parts.
- ↑
*Alm*. ed. Halma, i. 159. - ↑
*Systéme de politique positive*, iv. 17. - ↑ This Sosigenes, as Th. H. Martin has shown, was not the astronomer of that name who was a contemporary of Julius Caesar, but a Peripatetic philosopher who lived at the end of the 2nd century.
- ↑
- ↑ Είσαγωγὴ είς τὰ φαινόμενα, c. 1. in Halma’s edition of the works of Ptolemy, vol. iii. (“ Introduction aux phénomènes célestes, traduite du grec de Géminus, ” p. 9), Paris, 1819.
- ↑ This has been noticed by Pliny, who says, “ Multiformi haec (luna) ambage torsit ingenia contemplantium, et proximum ignorari maxime sidus indignantium ” (
*N.H.*, ii. 9). - ↑ Delambre,
*Histoire de l’astronomie ancienne*, xi. 264. - ↑ This is true of their mean distances; but we know that Mars at opposition is nearer to us than the sun.
- ↑
- ↑ Transits of Mercury and Venus over the sun’s disk, therefore, had not been observed.
- ↑ This was known to Eudoxus. Sir George Cornewall Lewis (
*An Historical Survey of the Astronomy of the Ancients*, p. 155), confusing the geocentric revolutions assigned by Eudoxus to these two planets with the heliocentric revolutions in the Copernican system, which are of course quite different, says that “ the error with respect to Mercury and Venus is considerable ”; this, however, is an error not of Eudoxus but of Cornewall Lewis, as Schiaparelli has remarked. - ↑
- ↑
- ↑ Theon (Smyrnaeus Platonicus),
*Liber de astronomia*, ed. Th. H. Martin (Paris, 1849), pp. 174, 294, 296. Martin thinks that Theon, the mathematician, four of whose observations are used by Ptolemy (*Alm.*ii. 176, 193, 194, 195, 196 ed. Halma) is not the same as Theon of Smyrna on the ground chiefly that the latter was not an observer. - ↑ Delambre compares these mean motions with those of our modern tables and finds them tolerably correct. By “ motion in longitude” must be understood the motion of the centre of the eplcycle about the eccentric, and by “ anomaly” the motion of the star on its epicycle.
- ↑ Hipparchus pointed out the mode of determining longitudes by observations of eclipses, but the instance to which he referred (of the celebrated eclipse before the battle of Arbela, which was also seen at Carthage) was a mere matter of popular observation, of no scientific value. Yet Ptolemy seems to have known of no other.