St. Albert the Great and Robert Grosseteste on the nature and causes of comets

Introduction

In his commentary on St. Matthew’s Gospel, the German Dominican bishop, St. Albert the Great (c. 1190–1280), makes the following remark: ‘the whole world is theology for us, because the heavens proclaim (enarrant) the glory of the Lord’. ¹ Albert’s interest in the nature of the heavens – that is to say, the nature of the stars, the moon and the planets, and their respective celestial spheres – is well attested and has been studied in relative depth by noted historians of science, including P. Duhem and E. Grant. ² One important aspect of Albert’s thought about the heavens has, however, been largely overlooked. This is his thinking on the nature of one of the most spectacular events within the night sky, comets. Perhaps a partial explanation for this is the fact that Albert’s thought on comets is not to be found in his principal work on the heavens – namely in his commentary on Aristotle’s De caelo. Instead, it occurs in his commentary on the latter’s Meteora, written sometime 1254–1257. ³ As we will see, for Albert – just like Aristotle, and indeed most medievals – comets are not celestial phenomena; rather, they are meteorological ones. Comets occur, so the thesis goes, when earthly ‘vapours’ ascend through the atmosphere and congregate beneath the vault of the lowest celestial sphere, namely that of the moon, whereupon they ignite and become visible. This article looks at what Albert has to say on the nature and causes of comets. In particular, it explores how his thinking relates to that of his English contemporary and fellow bishop, Robert Grosseteste (1168–1253), and how there are some subtle, though nonetheless important, differences between their views. ⁴ Surprisingly, very little has been written on Albert’s thinking on comets and no extended comparison of his thought with Grosseteste’s has been produced. Indeed, little has been written on the medieval theory of comets in general.

Composed during the 1220s, Grosseteste’s little treatise on comets, De cometis, postulates that comets consist of ‘sublimated’ (sublimata) earthly matter which, upon reaching the celestial barrier, is ‘inflamed’. ⁵ As for Albert, so for Grosseteste comets are caused by the ascension and ignition of terrestrial vapours. What is notable, however, is that the two bishops have subtly differing interpretations of the precise causal role which the planets and fixed stars play in the generation of comets. For Grosseteste, the sublimation of earthly vapours occurs solely as a result of the direct causal agency of the celestial bodies, with the latter acting like magnets drawing up these combustible fumes. For Albert, by contrast, a more complex picture is the case. While he affirms that the celestial bodies do indeed stir up different types of vapours within the atmosphere and do, under certain circumstances, contribute to the ascension of these to the celestial vault, thereby causing a comet’s generation, he nonetheless follows Aristotle in noting that comets often arise independent of any direct celestial agency. The celestial bodies, so he tells us, play an integral role in producing the vapours needed for a comet’s generation. However, it is not always the case that the ascension of these vapours occurs as the result of the direct causal agency of a celestial power. To this extent, for Albert, there are times when the stars and planets function at the level of dispositive causes, merely creating the material conditions needed for the creation of a comet, but yet are not directly involved in the latter’s generation itself.

While Albert’s discussion of comets in his De meteora post-dates Grosseteste’s De cometis by several decades it is important to note that there is little to suggest that Albert had any direct knowledge of Grosseteste’s treatise or the general contours of its content. Having said this, some of the positions which Albert critiques concerning the causes involved in a comet’s production do share certain traits with the ideas found in Grosseteste’s De cometis and helpfully serve to situate the latter. For purposes of clarity I shall thus discuss Albert’s thinking first and then address Grosseteste’s material on the subject.

Albert, Grosseteste and the medieval debate on comets

It is important to note that Albert and Grosseteste were not unique in discussing the nature and causes of comets during the early-to-mid 13th century. The questions ‘how are comets generated?’, ‘how do they move?’ and ‘what do they signify?’ were discussed widely during this period. Thus, besides Albert’s and Grosseteste’s contributions, several other treatises on comets have come down to us from this period. These include a lengthy anonymous text composed in Spain around 1238 and Giles of Lessines’ important De essentia, motu, et significatione cometarum – the latter was composed following the appearance of the comet of 1264 and, as even a cursory glance reveals, demonstrates a strong familiarity with Albert’s and Grosseteste’s writings. ⁶ Also notable is the fact that the subject of how comets were produced and what they signified often found its way into theological literature of the time. It is thus not uncommon to find some mention of comets in commentaries on the second book of Peter Lombard’s Sententiae, which discusses – amongst other things – God’s creation of the celestial bodies and how they serve to govern the pattern of terrestrial life. ⁷ Indeed, the subject of comets and their generation was even discussed in the quaestiones disputatae conducted by theology masters in the Papal court itself. ⁸ It is, however, in the various commentaries and glosses from this period on Aristotle’s Meterora that the most in-depth study of comets is to be found. For example, St. Thomas Aquinas offers a lengthy exposition of Aristotle’s discussion of comets in his Sententia super meterora, while Roger Bacon and Adam of Buckfield both discuss comets in their respective Meterora commentaries. ⁹

Aside from the fact that Albert and Grosseteste constitute two of the most well-known scientific writers of the 13th-century – and were both recognized as such during their own day – these two thinkers are worthy of particular consideration because their differing interpretations of how comets are generated helped to shape the debate which was to run throughout much of the later middle ages on this subject. The question of how the celestial bodies draw up the flammable vapours needed for a comet’s generation, and indeed that of other atmospheric phenomena visible in the night sky, – that is, whether it is by means of their motion or by means of some attractive agency – was one which can be detected throughout the literature of the following centuries. For example, in his Quaestio de aqua et terra, Dante Alighieri, the famous 14th-century Venetian poet and natural philosopher, states that this question remained unresolved and much-disputed even in his own day. ¹⁰ In turn, as already indicated, Albert and Grosseteste are also worthy of our attention because they are the thinkers most frequently referenced, alongside Aristotle himself, by later medieval writers as the leading auctores on the subject of cometary generation. Thus, both are frequently quoted in Giles of Lessines’ extensive De essentia, motu, et significatione cometarum and Gerard of Silteo’s Summa de astris, both of whom, like Albert himself, observed the great comet of 1264. Similarly, a much neglected late-13th century Meterora commentary, once thought to be by the celebrated Scottish Franciscan friar, Bl. John Duns Scotus, repeatedly quotes Albert concerning the nature and signification of comets. ¹¹

Aristotle, comets and the celestial vault

In order to understand Albert’s and Grosseteste’s thinking on comets properly, it is necessary to offer a few brief remarks on the medieval understanding of the structure of the cosmos itself. Following the cosmologies articulated by the Ptolemaic and Aristotelian schools, the medieval mind maintained that the earth stood at the centre of the universe and was, as Grosseteste’s De sphera explains, surrounded by a series of transparent concentric spheres, each which was constantly in motion. ¹² To these were attached one or more of the visible celestial bodies – that is, the stars, planets, sun and moon. Made of what Aristotle called the unchanging and eternal ‘quintessence’, each of these spheres, and the celestial bodies which they contained, was hierarchically organized according to their degree of rarefication. ¹³ Thus, as John Sacrobosco affirms in his De sphera, the moon was situated within the lowest celestial sphere – that is, the sphere immediately adjacent to the earth – while the fixed stars, as the most rarefied celestial bodies, were attached to the outermost sphere. ¹⁴ Possessed of perfect actuality and motion, it was this outermost sphere – on account of its proximity to God – which provided the motor which kept the lower spheres in motion and thus governed the behaviour of the planets within them. It is for this reason that St. Bonaventure of Bagnoregio, echoing Aristotle and the Islamicate scholar Averroes, tells us that the motion of the planets, and their respective spheres, was to be understood according to the manner in which a key in a lock operates: as the key – in this case the outermost celestial sphere – turns, so it causes all the lower spheres, and their respective planets, to move and obey its direction. ¹⁵

As already indicated, demarking the boundary between the celestial and the terrestrial realms, it is the interface between the underside of the convex surface of the lunar sphere and the upper reaches of the terrestrial air which constitutes the space within which a comet is generated. According to Aristotle and his medieval interpreters, immediately beneath the concave surface of the lunar sphere are to be found several spheres of highly rarefied versions of the lower earthly elements – the most important of these, for our purposes at least, being that of fire. ¹⁶ Caused by the friction of the lunar sphere’s quintessence rubbing against the body of air beneath it, the fire in this space is of a diaphanous nature – hence it does not prohibit our ability to see the higher celestial bodies, as would be the case were it the same as the coloured fire found here on earth. ¹⁷ As mentioned above, for Grosseteste and Albert – like Aristotle – it is when combustible vapours from the lower atmosphere ascend, in enough quantity, up through the air and enter into the sphere of fire that the heat of the latter causes them to ignite and produce a comet on the underside of the celestial vault. It is the same process, so the medievals note, which give rise to other, more short lived, luminescent phenomena in the night sky, including shooting stars and what Aristotle calls ‘torches’ and ‘goats’. ¹⁸ It is this location of comets within the boundary between the lowest celestial sphere and the terrestrial realm which explains why for the medievals they are to be classified as meteorological, as opposed to celestial, phenomena.

Before proceeding to explore Albert’s and Grosseteste’s thought on the role which the celestial bodies play in a comet’s production it is worth noting what Aristotle himself says on the subject. For the Stagirite, it is the motion of the celestial bodies – as opposed to any attractive or quasi-magnetic agency – which is the force responsible for stirring up the vapours from which comets and the like are birthed: ‘so whenever the circular motion stirs this stuff up in any way, it catches fire at the point at which it is most inflammable’. ¹⁹ In turn, he adds: ‘we may say then, that a comet is formed when the upper motion introduces into a gathering of this kind a fiery principle’. ²⁰ At this level, Aristotle’s thought, as we will come to see, has more in common with Albert’s position than it does with Grosseteste’s – something which is hardly surprising given that Albert’s account of comets is to be found in his commentary on the Stagirite’s Meteroa. This, in turn, is coupled with the fact that it is not overly clear, in the opinion of some at least, that Grosseteste had any direct knowledge of the Meterora at the time he composed the De cometis. ²¹ Importantly, as John North has noted, the notion that the stars and planets stir up the vapours by means of attraction, as opposed to doing so simply by through their motion, is something which Grosseteste shares with the Islamicate tradition, in particular Albumasar’s Introductorium in astronomiam. ²² Interestingly, however, Grosseteste – unlike Albert – does not reference Albumasar in his discussion of comets. ²³ Furthermore, as North points out, Albumasar’s account of how the celestial bodies govern the metrological events, echoes the ideas found in early Indian astronomy in that it accords a much greater causal agency to the stars and planets in governing terrestrial and meteorological changes than Aristotle himself permitted. ²⁴

Albert’s critique of the ‘modern doctors’

Albert begins his discussion by outlining the erroneous opinions which Aristotle himself successfully refuted – namely, the argument that comets arise from a conjunction of two or more planets or the blurring of a planet’s light through a distortion of the air. ²⁵ These authors, so he tells us, erred through weak reasoning and ignorance concerning the nature of vision and how light interacts with the transparent atmosphere. ²⁶ Similarly, Albert rejects the views advanced by later thinkers, such as the Roman philosopher Seneca and the Eastern Orthodox theologian John Damascene. ²⁷ For the latter, comets are not meteorological events, but rather genuine new occurrences within the celestial spheres themselves. While these thinkers were unknown to Aristotle, Albert insists that their arguments are nonetheless proved wrong by his logic. This is so because, as the Stagirite’s De caelo clearly shows, nothing new is ever generated within the celestial region itself. The unchanging and sempiternal nature of the heavenly quintessence prohibits any change within it. ²⁸ Comets, however, so Albert observes, are clearly sporadic phenomena. Not only is their appearance unpredictable, but so also is their movement within the night sky. Moreover, they themselves are subject to change and decay. The result, so Albert asserts, is that they cannot be part of the celestial realm but must instead be a purely sub-lunar reality. Particularly interesting, however, is the fact that Albert tells us that aside from these mistaken ancient authors there are also ‘certain learned modern doctors’ (Quidam modernum doctorum) who, due to their erroneous reasoning and inadequate observations, offer up what Albert regards as false opinions concerning comets. ²⁹ Who exactly these ‘modern doctors’ are, however, is hard to discern. ³⁰

Rejecting the scholarly consensus that comets arise from inflamed earthly vapours, these ‘moderns’ argue that a comet is in fact an ‘impression’ (impressio) of one or more of the five planets caused at the boundary separating the sphere of fire from that of air. ³¹ The planets, so their thesis maintains, illumine certain ‘lines’ (lineae) of air and fire in the upper atmosphere, and, through doing so, give the illusion of a tail being attached to one or more of the celestial bodies. ³² So as to support this highly novel position, Albert notes how adherents of this theory highlight what they see as several weaknesses in the Aristotelian theory of comets. They argue, for example, that if comets arise from a collection of ignited vapours condensing beneath the heavenly vault, then surely the same process of ascending condensation and ignition should be seen within the celestial spheres themselves. ³³ After all, the principle of universal concordance dictates that what occurs in the earthly realm should also occur, or at least be mirrored within, its celestial counterpart. Yet clearly this is not the case. Furthermore, these ‘modern doctors’ argue that the theory that comets arise from ignited terrestrial vapours fails to convince on account of the weak and highly transient nature of these vapours themselves. Either they would be immediately consumed by the sphere of fire on account of its great heat, thereby preventing the accumulation of sufficient fuel to generate a comet; or they would be confined to the lower regions of the atmosphere which are ‘cold and depressed (deprimitur) down to earth’, thus meaning they would never be ignited and rendered luminous. ³⁴

Albert’s refutation of the ‘modern doctors’

For Albert, the arguments put forward by the ‘modern doctors’ are entirely unacceptable. While firmly rejecting the theory that comets arise from illuminated lines of air and fire at the celestial barrier – a position which he rejects as both nonsensical and contrary to reason – Albert takes particular exception to their claim that comets arise solely as a result of the direct causality of the planets and stars. The principal difficulty which he sees with this claim is that it does not fit the evidence. If the celestial bodies are the primary forces responsible for the generation of comets, that is to say the illumination of the lines of fire and air, then why are comets not always visible in the night-sky? After all, the planets and stars are permanent fixtures within the celestial realm, and yet comets appear only rarely. ³⁵ Moreover, how can these ‘modern doctors’ account for the wide diversity of places in which comets appear? If comets are caused by the planets, then surely comets ‘should never be seen outside the path (extra viam) of the planets’. ³⁶ This, however, is manifestly not the case. Comets often appear far outside the zodiac, even coming as close as the horizon of the North Pole itself. Indeed, Albert himself remarks that he himself had once witnessed a comet of this sort: ‘I with many others in Saxony in the year 1240 from the Incarnation of the Lord saw a comet close (iuxta) to the North Pole’. ³⁷ In short, for Albert the principal error of the ‘modern doctors’ is that they attribute too close a causal link between the celestial bodies and the generation of comets. Moreover, by doing so, they fail to appreciate that there are other causes involved in a comet’s creation.

Albert’s theory of comets

What then does Albert say that comets are? A comet, so he tells us, is a ‘sort of flame’ (flamma quaedam) arising from ‘enkindled fumes’ (fummus ascensus). ³⁸ It is generated from ‘coarse terrestrial vapour (vapor terrestris grossus), whose parts lie very close together’. ³⁹ These ascend through the atmosphere until they reach the ‘concave surface of the sphere of fire’ whereupon they are ‘diffused and inflamed’ (diffusus et inflammatus). ⁴⁰ Albert explains that the reason why the ascending vapour must be of a coarse nature is due to the fact that if it were of a subtle, wisp-like quality then ‘it would quickly evaporate (cito evaporatur)’, thereby meaning that a comet would not persist but would burn up quickly, which, as experience reveals, is clearly not the rule of nature. Comets, after all, often last for days, even months. ⁴¹ Here we thus see Albert offering a very succinct solution to the principal objection of the ‘modern doctors’ against the vapour theory of cometary generation. In turn, Albert notes that the reason why the ‘parts’ of this vapour are said to lie close together and are ‘well-mixed’ (bene comixtus) is because if the consistency of the vapour were of an irregular nature then it would be insufficient to generate a uniformly inflamed body. ⁴² In Albert’s opinion it is thus the coarseness and the viscosity of the ascending vapour which serves to guarantee both a comet’s longevity and the regularity of its luminosity.

This coarse vapour, Albert argues, arises from ‘ignited terrestrial parts’ which are found within rain-producing vapours in the atmosphere. ⁴³ Some of these fall with the rain itself but then re-ascend through a process of evaporation. Eventually these ignited parts, both those that remained in the sky and those that re-ascend to it, ascend above the cold air from which they are birthed into the higher, warmer parts of the atmosphere. ⁴⁴ Here they gradually multiply and begin to converge, eventually forming a coherent mass, albeit one that is not entirely uniform or spherical. The reason for the irregular shape of this vaporous mass, so Albert tells us, is that as it comes into contact with the convex surface of the sphere of fire and is inflamed, parts of it are diffused and begin to streak across the sky. ⁴⁵ However, the middle part of the ascending vapour cloud remains dense. This is because it is continuously supplied by the reservoir of ignited vapours existing below it in the lower atmosphere. ⁴⁶ This condensed middle, Albert reasons, forms the body of the comet for it burns the brightest. By contrast, the vapours which are diffused from the vaporous body form a flame-like white cloud, and it is this which gives the comet its tail: ‘et haec vocatur coma’. ⁴⁷ Albert claims that a comet will persist in the sky as long as the supply of vapours from the lower atmosphere beneath it lasts. Once these vapours begin to exhaust, the comet begins to fade as the intensity of its ignition starts to weaken. ⁴⁸

Albert postulates that the diversity of colour, shape and brightness found within comets is derived from the quality and coarseness of the vapours which produce them. Following Algazel, he argues that there are three different types of inflamed vapours. First, there are those vapours whose rarity is such that they are enflamed entirely and burn brightly as pure flame. Second, there are those vapours whose coarseness is denser and is of a black appearance. This type of vapour functions like coal so that when it is enflamed through contact with the sphere of fire it gives the comet a glowing red colour. ⁴⁹ Finally, there are those vapours whose coarseness is so great that when enflamed they produce comets which are ‘black and extinct’ (niger et extinctus). ⁵⁰ Albert notes, however, that there are two further types of comet. The reason for this is that between these three different degrees of condensed vapour there are two intermediary states; namely, a mean between the first and the second, and one between the second and the third. ⁵¹ Thus, on this fivefold spectrum, the coarser and more ‘sticky’ (coherens) the vapour, the darker and less luminous a comet will be. ⁵² In turn, the more rarefied – and thus more combustible – the vapours, the brighter and more iridescent its colour will be. ⁵³

As noted earlier, Albert does not deny that the stars and the planets have a causal role in the generation of comets. Rather, he merely maintains that for the most part this role is not that of immediate efficient cause. Instead, the stars and planets function more akin to what may be termed ‘dispositive’ causes. ⁵⁴ It is beyond question, so Albert tells us, that the celestial bodies do indeed play an active role in shaping the pattern of terrestrial life. After all, he observes, Albmuasar convincingly shows that some of the planets stir up ‘humid’ and ‘aqueous’ vapours which precipitate storms. ⁵⁵ Conversely, others – in particular Mars – stir up dry and highly flammable vapours. ⁵⁶ These cause certain ‘coruscations’ or ‘scintillations’ in the air. ⁵⁷ The latter, however, are of a highly transitory nature, producing short-lived ‘running fires’ in the atmosphere. ⁵⁸ In Albert’s thinking, therefore, it is clear that, through their stirring up of different kinds of vapours within the atmosphere, the stars and planets serve to provide the material resources needed for many of the different meteorological phenomena observed within the night sky, including comets. Crucially, however, in the case of the latter, it depends more often than not on the atmospheric conditions, and indeed the predominance of the required type of flammable vapours, as to whether these then ascend to the underside of the heavenly vault and proceed to be ignited. What exactly these conditions are, however, Albert does not spell out for us. What is clear nonetheless is that for Albert a comet’s generation may occur independently of any direct involvement by a celestial power.

As Aristotle teaches, however, there are certain occasions when comets do appear to arise under the immediate direction of one or more of the celestial bodies. This occurs, for example, when the planets move in an unusual way or when two or more of them are in conjunction. Here, so the Stagirite affirms, the celestial bodies do not simply stir up vapours in the atmosphere, but facilitate their ascension and ignition. In light of this Albert notes that there are a number of ‘commentatores’ – including Haly, Abraham and Bulgafarus – who, taking their lead from Pseudo Ptolemy’s Centoquilium, accept that comets are ‘the effects of stars upon vapours elevated and ignited’. ⁵⁹ Similarly, Albert observes that Albumasar affirms that there are times when Mars does not just stir up the dry fumes needed to form a comet, but directly facilitates the latter’s ascension to the heavenly vault. This occurs primarily when Mars is in conjunction with Jupiter. What is interesting, however, is that despite conceding that occasionally the celestial bodies do facilitate the ascension and ignition of the fumes which they regularly stir up, it is not overly clear that Albert sees this causal agency as functioning at the level of attraction or direct efficient causality. Rather, he seems to view their causal agency at this level as functioning akin to a secondary effect of their motion and, in particular, the warming effect which their ‘rays’ (radii) of light have as they traverse across the atmosphere. ⁶⁰ Thus, in the same way that a stick upon being moved through a pond will stir up particles of dirt hidden at the bottom and cause them to ascend to the surface, so the same appears to be the case with those vapours whose ascension to the heavenly vault is caused by the motion of the celestial bodies and their radii.

Robert Grosseteste: Comets as ‘sublimated fire’

While Robert Grosseteste’s theory of cometary generation may not fit the profile of the opinions of Albert’s ‘modern doctors’, what is clear, even from a cursory reading of the De cometis, is that the Bishop of Lincoln nonetheless shares their assumption that the celestial bodies are the direct causes of comets. ⁶¹ Written early in his career, Grosseteste’s De cometis argues that comets arise from fire ‘sublimated’ (sublimatus) from the earthly realm by the agency of one or more of the planets and stars. ⁶² A comet, so he tells us, is ‘nothing else than fire’ (nihil aliud quam ignis) generated from rarefied matter arising from below (i.e. the earth) which is then ‘assimilated to the celestial nature’ (assimilatus nature celesti) by means of the agency of one or more of the celestial bodies. ⁶³ Grosseteste notes, however, that fire is of two sorts. First, there is that fire which is associated with the combustion of earthly matter; second, there is that hyper-rarefied fire which is found just below the lunar boundary. ⁶⁴ Sense experience reveals, however, that earthly fire cannot be responsible for cometary generation. This is on account of the fact that it does not persist long enough in the air to form a stable body. ⁶⁵ Conversely, the hyper-rarefied ignition found within the upper atmosphere does not descend from its proper place and has a purely diaphanous nature, hence it also cannot constitute the fire generating comets. ⁶⁶

The result, so Grosseteste argues, is that the fire responsible for the generation of comets must be unique. It is a fire which arises from the ascension of earthly matter – by this Grosseteste means, like Albert, earthly ‘vapours’ – which, upon ascending to the heavenly vault, is enflamed and ‘assimilated (assimilatus) to the celestial nature’ itself. ⁶⁷ This ‘assimilation’ to the celestial nature explains why, unlike ordinary material fire, the fire generated by the ascending vapours which gather beneath the celestial vault is capable of persisting in the air for a long-time. By being ‘separated from [its] earthly nature’ (seperata a natura terrestri) it comes to possess something of the stability and perfect motion of the celestial spheres themselves. ⁶⁸ The cause of this assimilation, however, Grosseteste asserts, cannot arise from the earthly vapours themselves, but must instead possess a celestial origin. The reason for this is that only a celestial agent, being the superior and more potent nature, has the capacity to act upon, and thereby assimilate to itself, inferior earthly matter. Consideration reveals, however, that the only celestial forces capable of such agency are the planets and fixed stars. ⁶⁹

What is notable, however, is that for Grosseteste – unlike Albert – this causal agency exercised by the celestial bodies in drawing up earthly matter occurs not simply at the level of motion, and the subsequent stirring up of vapours within the atmosphere, but also, and most properly, at the level of attraction. The sublimated matter from which comets are birthed, so Grosseteste tells us, ascends to the underside of the heavenly vault due to the fact that the stars and planets actively draw up this matter to themselves. Thus, in the same way that a magnet attracts iron filings to itself, so the celestial bodies draw up rarefied earthly matter. ⁷⁰ For Grosseteste there is, therefore, very much a direct causal link between the stars and planets and a comet’s epiphany. It is thus the agency of the celestial bodies, and nothing else, which underpins each stage of a comet’s production. Not only are the latter responsible for producing the material conditions necessary for a comet’s generation, but they also actively facilitate the ascension and ignition of the vapours from which a comet derives. Here we can thus detect a very sharp point of opposition to the line of reasoning which Albert was later to adopt in his Meteora.

According to Grosseteste, the attractive agency of the celestial bodies is confirmed by the fact that comets not only follow the rotation of the night sky, and thereby the orbit of the stars and the planets themselves, but the tail of the comet nearly always points towards one or more of the celestial bodies; and this, in turn, so Grosseteste notes, is often the body responsible for creating the comet. ⁷¹ The fact that the fixed stars differ in nature and correspond in quality to at least one of the seven planets also explains why comets differ in appearance, duration and timing. Some of the planets and fixed stars, for example, assimilate only certain types of matter to themselves – for instance, dry and hot – while others, by contrast, assimilate matter that is of a different nature. ⁷² The result of course, so Grosseteste tells us, is that different types of comet will signify a change in specific earthly realities. For example, if the comet arises from the agency of the sun, which on account of the perfection of its luminosity, attracts ‘well-complexioned’ (complexionata) matter, then there will result a ‘weakening’ in human, animal and vegetative life forms. This is so because as the comet is formed it will abstract matter from these beings and they thus will lose something of their vital being. ⁷³ A similar corruption, Grosseteste argues, would arise if Mars were the planet responsible for the comet. ⁷⁴

Conclusion

In light of all this, it is clear that there is an interesting dialogue to be had between the cometary theories of Albert the Great and Robert Grosseteste. While both affirm the traditional peripatetic claim that comets are meteorological, as opposed to celestial, phenomena, careful inspection reveals that the two bishops nonetheless diverge on the question of the causal agency which the celestial bodies play in a comet’s generation. Broadly speaking, these differences concern two key areas. On the one hand, Albert distances himself from Grosseteste’s insistence that the celestial bodies are always the direct causes of comets, arguing instead that more often than not their agency is restricted to merely creating the material conditions necessary for a comet’s generation – that is, the stirring up of combustible vapours. On the other hand, Albert asserts that when the planets and stars do act as the immediate efficient cause of a comet’s generation they do so not by means of attraction, as Grosseteste supposes; instead, their causal agency appears to be simply an extension of their pattern of motion or conjunction with one another.

By way of conclusion, it can be noted that one possible reason for these differences is that there may be something of a divergence between the way Albert and Grosseteste understand the nature of the celestial bodies themselves. Where Albert repeatedly, and indeed unhesitatingly, affirms the Aristotelian doctrine that the planets and stars are made of the same celestial quintessence as the ethereal spheres in which they are nested, and thus are materially identical with them, Grosseteste’s position, by contrast, is by no means as clear cut. Indeed, it is decidedly ambivalent. Not only does the English bishop confess himself unsure of Aristotle’s claim that the celestial bodies are made of the quintessence – he is particularly explicit on this front in his magisterial Hexaemeron – but in the influential little treatise entitled De generatione stellarum, the Grossetestian heritage of which it should be noted is a matter of much dispute, ⁷⁵ it is claimed that the stars and planets are in fact made up of the same four elements found here in the sub-lunar realm: earth, air, fire and water. ⁷⁶

If such an elemental reading of the celestial bodies is indeed to be attributed to Grosseteste then this may, perhaps, offer some insight as to why, in contrast to his German counterpart, he views the planets and stars as attractive agents drawing up vapours to the celestial vault. ⁷⁷ The reason for this is that in the De generatione stellarum the argument is advanced that it is only because of the elemental – as opposed to ethereal – nature of the celestial bodies that they are able to influence sub-lunar realities. ⁷⁸ Indeed, it is this agreement in substance, so the De generatione stellarum implies, which explains why the celestial bodies are able to affect the earthly elements in such a potent way, one which would not be possible were they materially different from the earthly recipients of their agency. Admittedly this view is not advanced, as far as this author can see, in the Hexaemeron, the De cometis or indeed in any of Grosseteste’s other undisputed works; and whether one accepts it as offering a possible avenue for explaining Grosseteste’s departure from Albert’s position on the causal role of the celestial bodies in the generation of comets depends, of course, on whether one is willing to attribute the De generatione stellarum to him or not. ⁷⁹ Questions of authorship, however, are beyond the scope of this article. What is clear, however, is that during the early- to mid-13th century the study of comets burned brightest in the hands of these two great bishops and scientists.