Isis Volume 81 issue 4 1990 [doi 10.2307%2F233811] Edgar S. Laird -- Robert Grosseteste, Albumasar,...

Robert Grosseteste, Albumasar, and Medieval Tidal TheoryAuthor(s): Edgar S. LairdSource: Isis, Vol. 81, No. 4 (Dec., 1990), pp. 684-694Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/233811 .Accessed: 07/04/2014 05:36

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Robert Grosseteste, Albumasar, and Medieval Tidal Theory

By Edgar S. Laird*

A SHORT TREATISE on tides attributed to Robert Grosseteste, Questio de fluxu et refluxu maris, has received considerable attention since its discov-

ery in 1926 by Franz Pelster, and especially since its publication in the pages of Isis by Richard C. Dales in 1966.1 Part of the attention is due to its intrinsic interest as a statement of medieval tidal theory and part to its problematic status in the canon and chronology of Grosseteste's work.2 If it is by Grosseteste, it represents an interesting development in his thinking; if it is by another, it never- theless constitutes an interesting development in medieval thought. In either case, it is the purpose of the present essay to point to the large part played in that development by the Latin versions of the work of the famous Arabic astrologer Albumasar (AbM Macshar).

It may be said at the outset that generally those parts of De fluxu which do not appear to be especially characteristic of Grosseteste come from Albumasar, and those parts which do not come from Albumasar appear very characteristically Grossetestian. That such is the case will emerge gradually from the following comparison of De fluxu with Albumasar's work. The comparison reinforces my belief that Grosseteste wrote De fluxu, and in the remainder of this essay I shall designate him as its author. On that assumption it is possible to say that the dependence of De fluxu on Albumasar is very likely direct rather than transmit- ted through some intermediary, for Grosseteste begins early in his career to cite Albumasar and gives every evidence of familiarity with his work. I shall therefore be speaking of what Grosseteste does with the materials he finds in Albuma- sar, and of how Albumasar influenced Grosseteste.

The work on which De fluxu is based is Albumasar's Introductorium maius, written in Arabic in the ninth century and translated into Latin twice, once by John of Seville, in 1133, and once by Hermann of Carinthia, in 1140.3 Book 3 of

* Department of English, Southwest Texas State University, San Marcos, Texas 78666. l Richard C. Dales, "The Text of Robert Grosseteste's Questio de fluxu et refluxu maris with an

English Translation," Isis, 1966, 52:455-472 (hereafter cited as De fluxu, ed. or trans. Dales). Refer- ences to Grosseteste's other works are, unless otherwise specified, to Die philosophischen Werke des Robert Grosseteste, Bischofs von Lincoln, ed. Ludwig Baur (Beitrage zur Geschichte der Philosophie des Mittelalters, 9) (Miinster: Aschendorff, 1912) (hereafter Werke).

2 Discussions of the question before 1962 are summarized in Richard C. Dales, "The Authorship of the Questio de fluxu et refluxu maris attributed to Robert Grosseteste," Speculum, 1962, 37:582-588. Subsequent developments are noted in R. W. Southern, Robert Grosseteste: The Growth of an En- glish Mind in Medieval Europe (Oxford: Oxford Univ. Press, 1986), p. 123. Dales dates the work "within a year or so of 1227" (De fluxu, ed. Dale, p. 458) and is seconded by James McEvoy, The Philosophy of Robert Grosseteste (Oxford: Oxford Univ. Press, 1982), p. 510.

3 On Albumasar's place in Arabic astrology, see David Pingree, "Abu Macshar," in Dictionary of Scientific Biography, ed. C. C. Gillispie, 16 vols. (New York: Scribner's, 1970-1980), Vol. I, pp.

ISIS, 1990, 81: 684-694 684


MEDIEVAL TIDAL THEORY 685

the Introductorium describes the influence on the sublunary world of the fixed stars and the planets, and especially the "luminaries"-which is to say, the sun and moon. Since Albumasar's best examples of the moon's influence are its effects on the tides, he writes, in Chapters 4 through 8 of Book 3, what amounts to a treatise on tides. De fluxu takes its account of tides almost entirely from those chapters; it is divided into three parts, each corresponding with a chapter in Albumasar. De fluxu 1 is an account of the causes of tides, and its gist comes from Introductorium 3.4, "De proprietate ducatus lune in marium accessu & recessu." De fluxu 2 is an account of eight causes of increase and decrease of tides; they are the same eight causes, and in the same order, as those that make up Introductorium 3.6, "De augmento & decremento aquarum." De fluxu 3 is a reworking of Introductorium 3.8, on the three kinds of sea and the varying effects of the moon upon them. Occasionally, especially in De fluxu 1, Grosseteste im- ports material from other chapters in Albumasar, and he introduces here and there throughout the work small but highly interesting additions and modifica- tions of his own.

It is likely that Grosseteste first became familiar with the Introductorium dur- ing his period of employment, near the end of the twelfth century, in the diocese of Hereford. In recommending him to that employment Gerald of Wales praised him very fully for his learning but did not mention any learning in astrology or astronomy, although Grosseteste was later to demonstrate considerable knowl- edge of both. Arguing from Gerald's silence one may suppose that Grosseteste's serious study of these topics began when he arrived in Hereford, for the intellec- tual atmosphere he encountered there was permeated by the Arabic sciences of the stars.4 One of the leading figures in the Hereford scientific movement was Daniel of Morley, on whom Albumasar was a pervasive influence and who had in his Liber de naturis inferiorum et superiorum made much use of the Introducto- rium and indeed closely copied large sections of it-including 3.4, on the moon's influence on the sea. So extensive and so close are Daniel's paraphrases and direct appropriations that they could not have been done from memory; and since the Liber de naturis was not written until after his return to England from Spain, sometime before 1189, the Introductorium was very likely among the "preciosam multitudinem librorum" that, he says, he brought back with him.S

The translation used by Daniel is that of Hermann of Carinthia rather than that of John of Seville,6 but Grosseteste gives evidence of having at one time or

32-39. On the Latin translations and their influence in the west, see F. J. Carmody, Arabic Astro- nomical and Astrological Sciences in Latin Trcanslation (Berkeley/Los Angeles: Univ. California Press, 1956), pp. 88-91; and Richard Lemay, Abu Macshar and Latin Aristotelianism in the Twelfth Century (Beirut: American University of Beirut, 1962). In this essay I quote chiefly from Hermann's translation of Albumasar, Introductorium maius (hereafter Introductorium), from the edition of Er- hard Ratdolt (Venice, 1489), occasionally from John's translation, Trinity College, Cambridge, MS 0.8.34 (early 13th century). For Albumasar on tides see Pierre Duhem, Le systeme du monde, 5 vols. (Paris: Hermann et Fils, 1913-1917), Vol. II, pp. 369-386.

4 Giraldus Cambrensis, Opera, ed. J. S. Brewer (Rolls Series, 1861), Vol. I, p. 249, quoted in Southern, Robert Grosseteste (cit. n. 2), p. 65. On the Arabic sciences at Hereford, see J. C. Russell, "Hereford and Arabic Science in England c. 1175-1200," Isis, 1932, 18:14-25. On Grosseteste's involvement in them, see McEvoy, Philosophy (cit. n. 2), p. 6; and Southern, Robert Grosseteste, pp. 82-96, 130, 149.

5 Lemay, Abui Macshar (cit. n. 3), p. 332; and Theodore Silverstein, "Daniel of Morley, English Cosmogonist and Student of Arabic Science," Medieval Studies, 1948, 10:179-196, on p. 179.

6 Silverstein, "Daniel of Morley," p. 186.


686 EDGAR S. LAIRD

another seen both. His earliest reference to the Introductorium is in De generatione stellarum, written in about 1220; there he follows Hermann's practice of calling each major division of the work "liber," whereas John calls each "trac- tatus." By the end of the decade, when Grosseteste has occasion in De natura locorum to quote a reference by Albumasar to a work of Hippocrates, he gives the title of Hippocrates' work in Arabic ("Alaceb"), as John does, rather than in Latin ("Liber climatum"), as Hermann does.7 It seems reasonable to suppose that Grosseteste came to know Hermann's version at Hereford and John's later on, perhaps at Oxford.

In De fluxu 1, Grosseteste takes from Albumasar the main features of his discussion: that every motion of a generable or corruptible thing takes place in water or in air; that (leaving air aside) motion in water is caused by a power of the sky or of a star in the sky; and that the moon in particular is the cause of the tides, as is shown by the proportion or congruence between its motion and that of the tides.8 Grosseteste's own additions, in the discussion of these features, are the concept of "rarefaction and condensation" as a mechanism for the distribution of matter; a summary and refutation of the views of Alpetragius (al-Bitrtiji) on tides; and the explanation that it is the moon's "luminous rays" that cause the motion of the tides to follow and be congruent with that of the moon. All these additions belong to a coherent cosmology and in their present application offer a coherent view of the causes of tidal motion as described by Albumasar.

The idea of rarefaction and condensation that Grosseteste brings to bear on Albumasar's account of the tides may be derived, at least in part, from Albuma- sar as well-from the statement in Introductorium 3.3, on the sun's effects on air, that if the stars' rays did not rarefy the air at night the air would be so condensed as to make human and animal life impossible. This is the statement that, as mentioned above, Grosseteste quotes in De natura locorum, citing Albumasar by name. The yield from the idea in Grosseteste's work is large. It becomes, as James McEvoy shows, a "cosmogonic principle" in De luce, an exhibition of the mathematical foundation of reality, a sign of the organic quality of nature as Grosseteste conceives it, surviving as late as his Hexaemeron.9 In De fluxu it is used to argue as follows: condensation is motion toward the center of the universe, and rarefaction motion away from it; earth, at the center, cannot be further condensed, nor fire, at the periphery of the elementary spheres, further rarefied; but water and air are capable of both condensation and rarefaction. Since Grosseteste is writing about tides, he says, "Let us omit the motions of air and speak of those of water"-saying, in effect, let us omit Albumasar's 3.3, on the sun and air, and pursue his 3.4, on the moon and water.'0 Thus rarefaction and condensation, which in De luce explain the distribution of matter throughout

7 Robert Grosseteste, De generatione stellarum, in Werke, p. 33; Grosseteste, De natura locorum, ibid., p. 72. In dating Grosseteste's works I follow James McEvoy, "The Chronology of Robert Grosseteste's Writings on Nature and Natural Philosophy," Speculum, 1983, 68:614-655. On the differing practices of Hermann and John, see Lemay, Abu Macshar (cit. n. 5), Ch. 1 and App. 3; on the naming of sections of the work, see p. 41.

8 Introductorium, 3.4. 9 McEvoy, Philosophy (cit. n. 2), pp. 154, 174, 181. 10 Defluxu, trans. Dales, p. 468. Toward the end of De luce (Werke, pp. 57-58) Grosseteste alludes

obliquely to Alpetragius's theory of tides by saying that some think ("putant tamen aliqui") that the circular motion of the celestial spheres extends to water and thus causes the tides. But he rejects the theory according to the same mechanics of rarefaction and condensation that he employs in Defluxu.



the cosmos, furnish in De fluxu the material cause ("causa materialis") of the tides.

That the material cause of the tides should be rectilinear motion toward or away from the center of the universe is essential in Grosseteste's view, and hence his refutation of Alpetragius's theory-which of course could not have been contemplated by Albumasar, who died some three centuries before Alpe- tragius wrote. Alpetragius's theory is that the circular motion of the primum mobile, imparted with ever-decreasing force from sphere to sphere until it reaches the sea, is the cause of the tides. Against this account Grosseteste says that we perceive by experience ("percipimus experimento") that the rise and fall of the tides are the result of rarefaction and condensation; for, among other evidence, water is found to be hotter at high tide, and this can only be because of the greater subtlety of its parts.1' In fact, the "experience" Grosseteste here cites also comes from Albumasar (Introductorium 3.5). But Albumasar explains the greater heat as a boiling up at high tide of water from the ocean floor, where it was believed to be hotter than at the surface; whereas Grosseteste attributes both the high tide and the heat to rarefaction-thus making use of a theory of heat, apparently original with him, that he would later develop more fully in his De calore solis.'2

Having disposed of the circular motion of the primum mobile as the cause of the tides, Grosseteste is ready to assert that the moon alone is the cause, and to offer reasons for this assertion, all deriving from Albumasar. First, there are the arguments of astrologers ("rationes astronomie") that the sun governs motion in air and the moon governs motion in water. The arguments can be found in the writings of any number of astrologers, although they are in fact the substance of Albumasar's Introductorium 3.3 and 3.4, respectively. Then Grosseteste adds that we also know by experience ("experimento scimus") that the moon exer- cises greatest control over moist and cold bodies, and hence certain people are called "lunatics" because when the moon wanes they suffer a diminution of the cerebrum, which is cold and moist. This "experience" too is reported from Albu- masar, on the moon's effects on animals and plants.13 Grosseteste's marshalling of evidence here, as elsewhere in De fluxu, is important in the light of modern interest in his role in the development of experimental and observational science -that is, the extent to which he was willing to undertake original investigation by new methods, as opposed to relying on existing traditions. Here he is taking both his rationes and his experimenta from Albumasar.

He is likewise following Albumasar when he reports the central empirical data around which De fluxu 1 is organized: the correspondence of the sea's motion with the moon's. Albumasar says that as soon as the moon arises over the horizon, the sea begins to rise, and it continues to do so until the moon reaches the meridian; then the sea decreases until the moon sets, at which time the sea begins to rise again and increases until the moon reaches the meridian in the

1" Here I follow Dales's translation, although his Latin text reads "minorem . . .subtilitatem" (De fluxu 1.69, ed. Dales p. 460; trans. p. 469).

12 See A. C. Crombie, Robert Grosseteste and the Origins of Experimental Science (Oxford: Ox- ford Univ. Press, 1953), p. 87.

13 De fluxu, 1.71-88, ed. Dales, pp. 460-461 (quoting lines 75, 82, 84). Cf. Introductorium 3.9: "Quod quoque etiam animalibus frigidum & humidum est ut lac cerebrum medulla crescente luna habundat decrescente attenuatur."


688 EDGAR S. LAIRD

opposite hemisphere; then the sea decreases until the moon begins again to rise above the horizon. To this account Grosseteste has but one thing to add: that the moon's "luminous rays" are the cause of the sea's rising. The addition, however, transforms the argument. Grosseteste is introducing into tidal theory one of his most characteristic principles: the principle of light as the source of motion.

In so doing he has created for himself a problem Albumasar did not face. Not having insisted on the role of light in the motion of the sea, Albumasar did not have to explain why the moon produces high tides both when it is approaching the meridian over any sea and when it is approaching the meridian in the hemisphere opposite that sea.15 But Grosseteste has said that the moon's light produces high tides. He repeats from the Introductorium the statement that the moon produces two high tides each day, but, inconveniently for his light theory, one of the high tides occurs when the moon is beneath the horizon. "Since," he says, "heavenly bodies can act on lower bodies only by their light, it is doubtful how the moon can cause the motion of the sea." He rejects what he calls the astrologers' explanation-in fact, it is Albumasar's-that opposite quarters of the sky have similar effects, and says that the problem needs further investigation.16 Grosseteste's solution, which is to claim that the moon's rays are reflected from the sphere of fixed stars upon the sea in the hemisphere opposite the moon, comes in De natura locorum, constituting one of several developments in his thinking which may have been stimulated by a momentarily awkward confronta- tion with Albumasar.17 At the time of writing De fluxu he felt strongly the appeal both of his own theory of light and of Albumasar's correlation of the motions of the moon and the tides, and so he simply declined to let either go.

Near the end of Introductorium 3.5, "De causa accessus et recessus," Albu- masar describes a method for calculating the times of daily tides, and in Defluxu 1 Grosseteste too describes such a method. Although the two descriptions are not closely parallel in details, they both depend on the assumption that the two rising tides begin with the rising and the setting of the moon and the two ebbings begin with the moon's passing the meridians. Grosseteste supplements his method with a rule-of-thumb version of the same method, which he says is "what sailors say.'"18

The chief business of Introductorium 3.5 is to describe the three conditions necessary for producing tides. They involve the location of water, the state of the water, and the motion of the moon. Grosseteste blends these conditions into a

14 Defluxu 1.91, ed. Dales, p. 461. 15 Albumasar credits the moon's influence on water to a kinship between the two: "Motus autem

lune desuper orientis atque occidentis sepius repetitus cognata virtute eiusmodi aquas trahit." (Intro- ductorium 3.5). In De impressione aeris Grosseteste had said that the water of the sea is raised toward the moon "quasi versus suam causam et originem" (Werke, p. 48).

16 De fluxu 1.102-114, ed. Dales, p. 462 (trans. p. 470). Grosseteste is rejecting Albumasar's explanation in Introductorium 3.5, which reads, in part: "Causa vero ab occasu usque ad terre cardinem sicut ab oriente ad summum id est cardinem lunam accessus sequatur triplex. Primo quod orientis & occidentis line sunt quantumcunque ab ortu ad summum sumitur tam ab occidente ad terre cardinem ille equedistans est: sicque totus orientalis quandrans toti cedit occidentali equedistans: estque uter- que accedens."

17 Dales, "Authorship" (cit. n. 2), pp. 587-588, describes the posing of the problem in Defluxu and its solution in De natura locorum as "the most compelling bit of evidence" for Grosseteste's authorship of De fluxu.

18 De fluxu 1.133, ed. Dales, p. 462. Grosseteste's calculation, incidentally, contains the phrase "in one third of this time" ("in tertio huius temporis," 1.124), where the calculation requires it should be "in thrice this time."



I t Figure 1. Diagram of the motions of the moon, depicting the center of the world, the center of the deferent, and the center of mean motion. From an anonymous Theorica Planetarum, Royal Library of Copenhagen, MS Add. 447, folio 49r. Courtesy of the Royal Library of Copenhagen.

summary statement of the specific material cause of tides: the place of water must be a deep and broad place; the state of the water must be that it is dense and filled with vapors; and the moon, in a position to exercise its force, must agitate those vapors. Albumasar adds that because of their subtlety neither rivers nor streams nor fountains have tides, or at least not appreciable ones. On that basis Grosseteste writes, "But since fresh water is subtle and its parts are pene- trable, when any vapor is generated in it, it is expelled at once; hence it does not, strictly speaking, have tides."'19

To this last statement Grosseteste is able to attach three of his most characteristic themes: the example, "one of Grosseteste's favorite observations," that snow remains on mountain tops longer than in valleys; the idea of light rays intersecting themselves as described in Euclid's Catoptrica (here, as elsewhere in Grosseteste, called De speculis); and the "particularly Grossetestian concept" of the incorporation of light rays in a dense medium and the consequent produc- tion of heat by the scattering of its parts.20 Heavenly bodies, Grosseteste argues, act only by light rays incorporated with the elements, and when they are reflected they intersect themselves, as described in Euclid, and produce heat by the scattering of parts. The subtler the matter the less heat will be produced, as is shown by the fact that snow remains longer on mountain tops, where the air is subtler. In the same way, in fresh water, which is subtler than sea water, lunar rays are less incorporated, less heat is generated, and effects are less.

Thus Grosseteste concludes the first part of Defluxu by subjoining to Albuma- sar some of his own characteristic thinking. It should be noted, moreover, that

19 De fluxu 1.141-150, ed. Dales, pp. 463-464 (trans. p. 471). Cf. Introductorium 3.5: ". . . nec in fluxu fluminum nec in collatione ex fontium augere vel minui sentiatur, . . . nec aquae discurrentes ut flumina & amnes propter subtilitatem minus densis vaporibus aptant huismodi conveniunt officio."

20 All three are listed in Dales, "Authorship" (cit. n. 2), pp. 584-585 (quoting p. 584). Cf De fluxu 1.149-163, ed. Dale, pp. 464-465 (trans. p. 471).


690 EDGAR S. LAIRD

the concept of the incorporation and scattering of light first appears in De fluxu; it may therefore be regarded as another important Grossetestian idea developed in the act of integrating Grosseteste's own principles with Albumasar's descrip- tion of tidal movements.

In De fluxu 2 Grosseteste follows Introductorium 3.6 very closely. His proce- dure mainly is to reproduce, through a combination of summary and paraphrase, the eight causes Albumasar describes of the augmentation and diminution of tides. But such variations as he does introduce are often complex and far- reaching in their ramifications.

The first cause to be discussed is the position of the moon relative to the sun. Grosseteste's treatment of this cause is remarkable both for what it retains and for what it omits from Albumasar. Both writers say that the tide-raising force of the moon is strong as the moon approaches conjunction with the sun (new moon), and that the force decreases until the moon is a quarter-circle away from the sun (first quarter), increases again until the moon is in opposition with the sun (full moon), decreases until the moon is a quarter-circle from the sun (last quarter), and increases again until the moon again reaches conjunction. Both writers remark that in the course of a month there are thus two tidal augmentations and two diminutions, just as in a day there are two high and two low tides. Then Albumasar adds that the moon is more efficacious in approaching opposition than in approaching conjunction because of its greater light as it approaches full moon. Grosseteste retains the attribution of the full moon's force to its light, but he omits Albumasar's contrasting assertion that its force at conjunction is less than at opposition.21 The reason for the omission is no doubt that Grosseteste regarded the proposition as erroneous, as he makes clear in his Commentarius in posteriorum analyticorum libros.22 Neither in that work nor in De fluxu does he speculate on how, lacking light, the new moon exerts increased force on the sea.

More consequential than what is omitted is what Grosseteste retains from Al- bumasar. By accepting the first of the causes Albumasar gives for augmentation and diminution, Grosseteste has forced himself to revise completely not only his tidal theory but also his view of how force is exercised in the cosmos. For earlier, in De impressionibus aeris, he had described the effect on the tides of the moon's position in its deferent, saying that when the moon is at aux of its deferent (i.e., when the center of its epicycle is at the point farthest from Earth), its force is increased, and when it is at the point opposite aux (i.e., when its epicycle's center is nearest Earth), its force is decreases.23 He then threw in as an apparent

21 De fluxu 2.5-14, ed. Dales, p. 465 (trans. pp. 471-472). Both translations of Albumasar state that the power of the moon is less at conjunction than at opposition. John's makes explicit that the cause is the diminution of the light: "quia luna lumen minuitur hoc tempore."

22 "In medio namque mensis plenitudo luminis eius quod recipit a sole confortat virtutem eius, licit hic confortatio et vigoratio non sit tanta quanta est per coniunctionem corporalem": Robert Grosse- teste, Commentarius in posteriorum analyticorum libros, ed. Pietro Rossi, Testi e Studi per il Corpus Philosophorum Medii Aevi, 2) (Florence: Olschki, 1981), p. 388. Dales notes that this work was written "about the same time as or shortly after De fluxa et refluxu maris": Richard C. Dales, "Robert Grosseteste's Scientific Works," Isis, 1963, 52:381-402, on p. 396.

23 Robert Grosseteste, De impressionibus aeris, in Werke, p. 48. In this he is followed by Roger Bacon and Richard of Wallingford. Bacon writes: "Et quando Lune est in augibus suorum circu- lorum, ut in novilunio et plenilunio, tunc sunt fortiores operationes ejus, ut patet in fluxibus maris": Opus maius, ed. J. H. Bridges, 3 vols. (Oxford: Clarendon Press, 1897-1900), Vol. I, p. 388. Richard writes: "Quando eciam planeta est in auge sui deferentis fortior est quam quando est in opposito au- gis. . . Similiter de Luna, quando est in auge deferentis sunt flumina et fluctus maris multo maioris



inconsequence the information that, according to astronomers ("astrologos"), whenever the moon is at aux it is in conjunction or opposition with the sun. In De impressionibus aeris the moon's effect on the tides is merely an example at the service of a more general point that Grosseteste regarded as very important ("admirabilis et digna memoria")-namely, that all planetary effects are stronger when the planets are farther from Earth and weaker when they are nearer.24 Dales notes that this statement "definitely contradicts the mathematical rules laid down in De lineis and De natura locorum."25 It does indeed, and in order to arrive at those rules-one of which is that short lines of force are more effective than long ones-Grosseteste had to revise his thinking completely.

It is in De fluxu, with the help of Albumasar, that the revision is achieved. For at this point in De fluxu, Grosseteste does not mention the moon's position relative to aux, or whether as a consequence its force is exercised in long or short lines; he mentions only the concomitant phenomenon of the moon's being in conjunction or opposition with the sun. Thus, what was offered as incidental information in De impressionibus aeris has become the basis of explanation in De fluxu, and what in De impressionibus aeris conflicts with Albumasar and with Grosseteste's own later thinking has been dropped.

The second cause is the moon's unequal motion along the ecliptic. Grosseteste says that when the moon's "diverse motion" varies from its mean motion in one direction the tide is increased, and when it varies in the other the tide is lessened (De fluxu 2.15-20). He adds nothing to Albumasar on this point and omits nothing except Albumasar's remark that rivers, streams, and fountains are also af- fected by this cause.

The third cause, the moon's position in its eccentric circle, is more interest- ingly treated. Already, in accepting Albumasar's first cause, Grosseteste has abandoned his earlier view that celestial bodies act more strongly the farther they are from Earth. Now, in treating the third cause, he can accept Albumasar's statement that when the moon is near the aux of its eccentric, and hence farthest from Earth, its force is diminished; and when it is nearly opposite aux, and hence nearest Earth, its force is augmented. He is now in a position to generalize, as indeed he will in De lineis, that in the world of nature, the shorter the lines of force the greater their power.26

The fourth cause has to do with the moon's latitude from the ecliptic, what Grosseteste calls "declinatio lune secundum latitudinem a circulo signorum." Hermann's translation of Albumasar reads: ". . . quandiu luna in latitudine sua ascendit accessus vis augetur quandiu descendit recessus"; John's translation reads: ". . . si fuerit luna descendens erit accessio multa & fortis. Et si fuerit ascendens erit accessio modica & debilis."27 The terms ascend and descend are

inundacionis quam in opposito": Exafrenon pronosticacionum temporis, in Richard of Wallingford: An Edition of His Writings, ed. J. D. North, 3 vols. (Oxford: Oxford Univ. Press, 1976), Vol. I, p. 212; see also Vol. II, pp. 123-126, on Bacon's and Richard's use of Grosseteste and Albumasar. Richard, oddly, cites Albumasar for the un-Albumasarian idea that the moon and the other planets exert greater force because of greater distance from earth.

24 De impressionibus aeris, in Werke, pp. 48-49: "Omnes planetae, quanto a terra remotiores sunt, tanto fortiores, quanto propinquiores, tanto debiliores in istis inferioribus habent operationes."

25 Dales, "Authorship" (cit. n. 2), p. 586. 26 De fluxu 2.21-26, ed. Dales, p. 466; on De lineis see Dales, "Grosseteste's Scientific Works"

(cit. n. 20), pp. 394-395. 27 Defluxu 2.27-28, ed. Dales, p. 466; Introductorium 3.6, trans. Hermann, fol. 4v; trans. John, fol.


692 EDGAR S. LAIRD

reversed in the two translations, but Grosseteste resolves the resulting ambiguity by saying that when the moon declines southward the high tide is increased and when it declines northward the high tide is lower. What lies behind his decision is probably the world model he describes in De sphaera, in which the great sea Oceanus encircles the earth under the equinoctial circle. In De sphaera also Grosseteste notes that whereas the moon's course is under the zodiac it is not directly under the ecliptic, as is the sun's, but declines from it and crosses it at two points.28 Given Grosseteste's model, it would follow that when the moon declines southward it is approaching Oceanus-the middle ocean-where, as he says in De fluxu, strong tides begin.

On the fifth cause, the main point for both Grosseteste and Albumasar is that when the moon is in northern signs high tides are augmented in northern seas, and when it is in southern signs high tides are augmented in southern seas. For the moment, Grosseteste is content to leave it at that-omitting Albumasar's elaborations, such as his statement of which signs are northern and which southern, but omitting also a proposition that Grosseteste makes telling use of elsewhere. This is the generalization that not only in the matter of the moon's southern or northern longitude but in all cases, its effect on any sea acts most powerfully in short, straight lines where the moon is directly over the sea ("in directo eius"); all of which is fully in accord with the mathematics of force as expressed by Grosseteste in De lineis.29

With regard to the sixth cause there is some confusion (perhaps because Gros- seteste used a defective manuscript) between Grosseteste's "days which by the ancients are called Egyptian" ("dies qui ab antiquis vocantur Egyptii") and Al- bumasar's "days which by the Egyptians are called sea days" ("dies quos ma- rinos vocant egyptii").30 With the mysterious "Egyptian days" of ill luck, which a number of medieval writers list and describe, tidal theory has no apparent con- nection. Grosseteste simply declines to discuss them on the ground that they contain many hidden causes.31

The seventh cause of tidal augmentation, Albumasar says, is the help that the sun gives to the moon when the sun is at the solstices, so that there are two augmentations each year, just as there are two augmentations each month and two high tides each day. In all this Grosseteste follows Albumasar, though he omits Albumasar's explanation that high tides are greater in the daytime when the days are longer and greater at night when the nights are longer (De fluxu 2.41-47).

Albumasar's eighth cause is the aid that the wind gives to tides. The other

17v. In the later thirteenth century Leopold of Austria (working from John's translation, according to Carmody, Arabic Astronomical and Astrological Sciences [cit. n. 3], p. 88) writes, ". . . cum luna descendit in latitudine crescit accessio" (Compilatio de astrorum scientia [Augsburg: Erhardt Rat- dolt, 1498], fol. c3), where "descendit" evidently means "decreases." Cf. the French translation of the early fourteenth century, Leopold, Li compilacions de la science des estoilles, Book I-Ill, ed. F. J. Carmody (Berkeley: Univ. California Press, 1947), p. 91: "Quant li lune descroist en se latitude li yauwe acroist."

28 Robert Grosseteste, De sphaera, in Werke, pp. 24, 27. 29 De fluxu 2.35-47, ed. Dales, p. 466; cf. Robert Grosseteste, De lineis, in Werke, pp. 59-65; and

see Dales, "Grosseteste's Scientific Works" (cit. n. 20), pp. 394-395. 30 De fluxu 2.38-40, ed. Dales, p. 466. John's translation reads "dies quos nominatur marinam." 31 See Robert Steele, "Dies Aegyptiaci," Proceedings of the Royal Society of Medicine, Section of

the History of Medicine, 1920, 12:108-121; and Lynn Thorndike, History of Magic and Experimental Science, 8 vols. (New York: Columbia Univ. Press, 1923-1958), Vol. I, pp. 685-688.



Figure 2. Model of the earth and sea surrounded by air, fire, and heavens. From Nicole Oresme, Traite de la sphere, St. John's College, Oxford, MS 164, folio 2v. Courtesy of the President and Fellows of St. John's College.

seven causes, he says, are proper ("propria"), whereas the eighth is extrinsic ("alienum"). He describes in some detail the ways in which the wind acts in concert with or in opposition to astronomical forces, but Grosseteste, possibly because this eighth cause is extrinsic and nonastronomical, finds it sufficient to say that when the wind blows in the direction of the tide the tide is increased, and when it blows in the opposite direction the tide is decreased (Defluxu 2.50-54).

Defluxu 3 begins with a brief statement (3.1-6) which, at a stroke, takes care of a number of Albumasar's scattered remarks concerning the question of tides in fresh waters. Albumasar sometimes, in statements Grosseteste has omitted, allows the moon's effects on rivers, streams, and fountains, but he also, in Intro- ductorium 3.5, says that some people are deceived into thinking that certain fresh waters near the coasts of Ethiopia, France, and Germany have tides, whereas actually these waters do not themselves have tides but are pushed back and raised by the tides of seas contiguous with them. Grosseteste takes contiguity with the sea, whether in underground or surface channels, to be the general cause of apparent tides in fresh waters.

This statement about waters that appear to have tides, but do not, is appro- priate at the beginning of De fluxu 3, for the main business of this section is to reproduce Albumasar's discussion in Introductorium 3.8 of the three kinds of sea: those which neither have tides nor appear to, those which have tides but do not appear to, and those which both have tides and appear to. For each of these kinds ("genera") Albumasar gives three specific instances ("species"). Grosse- teste follows him through two of the kinds, each with its three instances; but then, almost as if he has lost interest in this nonastronomical discussion, he concludes by saying that seas of the third kind "behave similarly to a greater or lesser degree."32

Thus ends Grosseteste's treatise. Albumasar, of course, continues beyond this

32 Defluxu 3.45-46, ed. Dales, p. 468, trans. p. 473.


694 EDGAR S. LAIRD

point. He goes on to describe the moon's effects on animals, plants, and metals -his purpose being, as indicated above, to write an astrologer's account of the moon's influence on the things of this world. But Grosseteste stops, having ex- tracted what he needs and written an account of tides, especially of the role of astronomy in the theory of tides. What he has achieved is rather remarkable. He has recognized in Albumasar's work the best available account of the tides and has supported and defended it. Among the various features of his source, he has attended chiefly to theoretically interesting celestial causes of the tides and has made some keen and careful discriminations in the murky area of astrology. And finally, he has applied cosmological principles to the special problem of the tides and, where the received data dictated, has modified and developed those principles in directions that point toward or are consistent with their more general formulations in De luce, De lineis, De natura locorum, and De calore solis. De fluxu is a small work, but it represents sophisticated thinking on an important and challenging subject, and it occupies, in my opinion, a key position in Grosse- teste's thought.


Article Contentsp. 684p. 685p. 686p. 687p. 688p. 689p. 690p. 691p. 692p. 693p. 694

Issue Table of ContentsIsis, Vol. 81, No. 4 (Dec., 1990), pp. 631-854Volume Information [pp. 829-853]Front Matter [pp. 631-637]The Ph.D. Machine: Building on the Collegiate Base [pp. 638-662]Miracles, Experiments, and the Ordinary Course of Nature [pp. 663-683]Robert Grosseteste, Albumasar, and Medieval Tidal Theory [pp. 684-694]Critiques & ContentionsGalileo's Method of Analysis and Synthesis [pp. 695-707]

News of the ProfessionEloge: Pierre Costabel, 24 October 1912-20 November 1989 [pp. 708-709]

Letters to the Editor [pp. 710-712]A Second LookMarshall Clagett's Greek Science in Antiquity: Thirty-Five Years Later [pp. 713-721]

Essay ReviewReview: Raising Questions [pp. 722-731]

Book ReviewsGeneral WorksReview: untitled [pp. 732-733]Review: untitled [p. 733]Review: untitled [pp. 733-734]Review: untitled [pp. 734-735]Review: untitled [pp. 735-736]Review: untitled [pp. 736-737]Review: untitled [pp. 737-738]Review: untitled [pp. 738-739]Review: untitled [pp. 739-740]Review: untitled [pp. 740-741]Review: untitled [pp. 742-743]Review: untitled [pp. 743-744]Review: untitled [p. 744]Review: untitled [pp. 744-745]Review: untitled [pp. 745-746]Review: untitled [pp. 746-748]Review: untitled [pp. 748-749]Review: untitled [pp. 749-750]Review: untitled [pp. 750-751]Review: untitled [pp. 751-752]Review: untitled [pp. 752-753]Review: untitled [pp. 753-754]Review: untitled [pp. 754-756]

AntiquityReview: untitled [pp. 756-757]Review: untitled [pp. 757-758]Review: untitled [pp. 758-759]

Middle AgesReview: untitled [p. 759]Review: untitled [pp. 759-760]Review: untitled [pp. 760-761]Review: untitled [pp. 761-762]Review: untitled [pp. 762-764]

RenaissanceReview: untitled [p. 764]Review: untitled [pp. 764-765]Review: untitled [pp. 765-766]Review: untitled [pp. 766-768]Review: untitled [pp. 768-769]

Early Modern PeriodReview: untitled [pp. 769-770]Review: untitled [pp. 770-771]Review: untitled [pp. 771-772]Review: untitled [pp. 772-773]Review: untitled [pp. 773-774]Review: untitled [p. 774]Review: untitled [pp. 775-776]Review: untitled [pp. 776-777]Review: untitled [pp. 777-778]

Eighteenth CenturyReview: untitled [p. 778]Review: untitled [p. 779]Review: untitled [pp. 780-781]

Nineteenth CenturyReview: untitled [p. 781]Review: untitled [pp. 781-783]Review: untitled [pp. 783-784]Review: untitled [pp. 784-785]Review: untitled [pp. 785-786]Review: untitled [pp. 786-787]Review: untitled [pp. 787-788]Review: untitled [pp. 788-790]Review: untitled [pp. 790-791]Review: untitled [p. 792]Review: untitled [pp. 792-793]Review: untitled [pp. 793-794]Review: untitled [pp. 794-795]

Twentieth CenturyReview: untitled [pp. 795-796]Review: untitled [pp. 796-797]Review: untitled [p. 797]Review: untitled [p. 798]Review: untitled [pp. 798-800]Review: untitled [pp. 800-801]Review: untitled [pp. 801-802]Review: untitled [pp. 802-803]Review: untitled [pp. 803-804]Review: untitled [pp. 804-805]Review: untitled [p. 805]

Philosophy & Sociology of ScienceReview: untitled [pp. 805-806]Review: untitled [pp. 806-808]Review: untitled [pp. 808-809]Review: untitled [pp. 809-810]Review: untitled [pp. 810-811]

Reference ToolsReview: untitled [pp. 811-812]Review: untitled [pp. 812-813]Review: untitled [pp. 813-814]Review: untitled [pp. 814-815]Review: untitled [pp. 815-816]

Collections [pp. 816-818]

Back Matter [pp. 819-854]

Isis Volume 81 issue 4 1990 [doi 10.2307%2F233811] Edgar S. Laird -- Robert Grosseteste, Albumasar,...

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Transcript of Isis Volume 81 issue 4 1990 [doi 10.2307%2F233811] Edgar S. Laird -- Robert Grosseteste, Albumasar,...