Alfonso López Borgoñoz
(To read in Antwerp, Belgium, in the Eclipse Conference, 15 october 2000)
Saint Isidore of Seville (Cartagene, 560-Seville, 636), except for his theological and philosophical studies, was also keen on writing about nature and its phenomenons, protected his knowledge emanating from the Greek Latin tradition, and thus he wrote a treatise in prose titled De rerum natura (About Nature), in Latin, at the beginning of the VII century, on request of King Sisebute, who reigned in the visigothic Hispania between the years 612 and 621. This book was soon known all over Europe, reaching a large distribution, also named Liber Rotarum (Book of the Wheels), because of the different illustrations that accompanied the text, and which had this shape when treating, normally, astronomic phenomenons. In this writing he tried to give a clear and synthetic vision about the state of the question of the scientific knowledge in that time.
Of course Sisebute did not have to insist too much to the Sevillian bishop in order to write this treatise, as Isidore never turned up his nose at the study of some scientific questions, moreover, in some of his former books he had already mentioned some notions that indicated his knowledge about the natural world. As is normal, and different from the classics such as Lucrece (of whom he copied the title from his most famous work), the work of Isidore is full of Christian references, and is adapted to the theological conception of the author.
But what we are interested in today is not so much the study of the astronomical concepts of Isidore, but the epistle/treatise (Epistula metrica ad Isidorum de libro rotarum o Epistula Sisebuti) that, in verse and also in latin, the very king Sisebute answered to Saint Isidore, after he received the book of which he had asked the editing. In this letter the monarch tried to give a rational and precise explanation, without giving in to superstition or to histories of witches, of lunar eclipses, in the first place, and of solar eclipses. Since then, the book of Isidore and the letter of Sisebute were known as a whole.
From the historical facts that we know and from those that are mentioned in the text -as well as from some events that are not mentioned in it-, it could be supposed that the exact date of editing was the year 612, this means, in the same year as the start of the mandate of Sisebute. Probably, at the end of August or in September of that year. But under which circumstances were both these treatises written ?
Sisebute and his time
The Gothic king Sisebute got the above-mentioned function after the short, but intensive mandate of two years of the king Gundemare. His reign signified a certain consolidation of the power of the Visigothic monarchy and of the Romanized culture, as well as a protection of the Catholic religion (that included persecution of heretics - especially arians - and of rebellious nobles, taking furthermore a series of tough measures during his mandate to control the Jewish population). He enlarged the possessions of his crown in the peninsula, to the detriment of the Byzantines, and tried to calm down a revolt of Basques and Cantabrians.
Though as a monarch he was a man of his time, in a stirring period, we have indisputable facts about his notable predilection for science and his knowledge of the celestial mechanism, according to the then overpowering model. It is very possible that his taste for the astronomical science emanates from his learning and education period in the monastery of Agali, near Toledo, where the study of the celestial bodies had a great authority among a certain number of monks of that time, who knew to cultivate this science saving the knowledge of the “old”. His work has been studied and debated by numerous specialists in history of science such as the Spaniard Julio Samsó, the Frenchman Jacques Fontaine or the North American Stevens, among many others.
Origin of De rerum natura and of la Epistula Sisebuti
Although it is difficult to determine which are the direct causes that motivate the realization of a scientific work, according to Fontaine, the motive of the request, and of the interest of the monarch in the astronomy in this time, is due to a series of astronomical events which took place in the course of one and a half year on the soil of the Iberian Peninsula, and which were visible for the whole population.
Thus, in the year 611 a partial lunar eclipse took place and one total (on 4th Mars and 29th August, respectively). The first of them passed in the afternoon, and it was not possible to see its partiality as the end of it coincided with the rise of our satellite, situated in Leo, in the east, at 18h 15m. It was penumbral, so that we doubt that almost nobody could observe it. Undoubtedly the second, total, was more seen, although the hours do not seem very indicated, as the beginning of partiality was around 3h 15 m, and totality did not begin until 4h 20m (the moon set at 5h 45m, when it was still eclipsed in its totality). The Moon was then located in Aquarius, very close by Saturn.
In the year 612, there were moreover two other partial eclipses of the Moon visible in Hispania. The first took place on 22nd February, with our satellite in Leo, near Regulus, and was covering the shadow of the Earth, at the moment of the maximum of the eclipse, at about 6h 35m, 59% of the Moon (which set around 7 h, with half of the satellite still eclipsed), and the other on 17th August, with the Moon in Aquarius, of which the maximum of 40% was seen in Toledo at about 5h 35m, little time before our satellite was occulted.
To all this we have to add that at noon on 2nd August of the year 612, there was, furthermore, a partial eclipse of the Sun, in which 87% of the surface of the celestial body Rey was occulted when it passed over Toledo at 16h 15m. The central line went from the zone of Moscow, passed the south of Italy and Tunis to the Sahara. As is obvious, it was partial in the whole of the peninsula.
All these facts, in a period full of superstition (you only have to look at the inexhaustible fight of the Christian bishops and prelates in the beginning of the early middle ages against the heretics and still profane practices which were being held in the different states that then composed Western Europe), produced a certain alarm and a certain nervousness in the Hispanic population of that time, as the accumulation of astronomical events in such a short time, associated with the arrival of a new king, could not be left to be seen as something alarming, especially by the hostile nobles or by the communities of followers of practices which were considered to be heresy. Moreover, certain authors gathered from the lecture of the book of the Apocalypse the possibility of a near end of the world, which many expected already, and which would be preceded by big signals in the sky, according to what the same Gregore the Great had written in a letter to the English king Edilberte in the year 601.
For Fontaine, Sisebute undoubtedly understood that maybe the mentioned social alarm could cost him his head. In those times it was easy to change kings in the most violent way, and every excuse was good for it (remind that it was supposed that the same Sisebute died poisoned -according to Saint Isidore-, and his successor, his young son, Recarede II reigned only for two days). For this reason was decided to ask the man most respected and erudite of his court, like Isidore of Seville, to write a text with an explication as rational as possible of this series of phenomenons. The book, consequently, could have been written against the fears of the superstitious, proud from certain nobility, and tried, searching in classical texts, to give an explanation of what had been seen.
Another alternative explanation
Although this is a beautiful story, it does not seem excessively credible. We have to think that since the year 601 till 610 numerous partial eclipses of the Sun and of the Moon could be seen from Toledo. Thus, a total of six partial eclipses of the Sun could be witnessed in the capital of the Visigothic reign between those years (almost one per year). Although that of 26th October 607 was semi-imperceptible and only covered 3% of the surface of the Sun, that of 12th August of the year 603 occulted 97% of the same, passing the central line near Barcelone and the north of the Iberian Peninsula. Another very striking one must have been the partial of 26th December 604, which got to cover 77% of the Sun, and which was an annular eclipse only visible from the north of Africa. Also, as is normal, some thirteen lunar eclipses more could be seen from Toledo, between the years 601 and 610 (a half more than one per year), all penumbral or partial, such like those of the years 611 and 612. And there had even been a total one on 16th July of the year 604, around 21h. We don't think, consequently, that there was something special in the astronomical events of the years 611 and 612 that motivated the editing of those works about natural science. As we have seen, the occurrence of the phenomenons was not superior to the normal.
We think that the reason of the editing of these two treatises was only the liking of a period in which there was a certain revival of the knowledge of the Greek Latin tradition, and a king and a court rather erudite, who liked the reading of these works in their not too many hours of peace, such as the same Sisebute indicates in the beginning of his poem, where no latent fears can be observed. Maybe in their time there was some alarm, it would not surprise us, but we doubt that the alarmed got to read the treatise of Isidore or that of Sisebute, or that which was expressed in it was understandable for a population mostly analphabetic
The explanation of the eclipses of the Moon and the Sun by Sisebute
His explanation is based on the aristotelic conception of the cosmos. The universe of Sisebute is geocentric with an immobile Earth around which turn the Moon, the Sun and the other celestial bodies. Our satellite would be the most close corpse, and served as a border between two different parts of the cosmos, the sublunar (which would go from our planet to the Moon), where all should be perishable, and the supralunar, where there would be a non-perishable and eternal sky. All movements would be uniform and in perfect circles.
After a short introduction, Sisebute enters the matter and begins to explain why at occasions the Moon loses its brilliant countenance of snow. Obviously, he declares, it is not because of superstitious reasons like a lot of people think. According to him, the Moon, which has no proper light (which enables him to explain the eclipses of our satellite and also those of the Sun) evolves in its orbit, through the sky, but there is a specific moment in which the Earth deprives it of the rays of the Sun - which is situated in its apogee - its enormous mass being located in the middle. When the rays of sunlight, which are freely expanded all over the universe, meet our planet, a pyramid is formed (he uses the term pyramid and not cone) of shadow at the part diametrically opposed to where the celestial body Rey is situated.
When the Moon enters there at full moon, the light that it reflects fades and extinguishes, which lasts until it leaves the mentioned shadow pyramid (the other celestial bodies -stars and planets- if they have light of their own and moreover never are afflicted by the shadow pyramid of the Earth, as a result of what they do not fade nor eclipse). As according to this explanation each full moon would coincide with an eclipse of the Moon, Sisebute reminds us that the orbit of the Moon is oblique, and that only in occasions its full moon is interrupted by the orbit of the Earth. In order to produce the eclipses the position of the Sun is equal, not importing if it is situated on the vertical of our planet or from an oblique axis.
On the solar eclipses he hardly dedicates some lines to indicate that the coincidence between the course of the Sun and the oblique course of the Moon, makes that the second covers the first, extinguishing its light. A problem arises in his explanation. If the orbits are circular, all the eclipses would be annular and never total, given the fact that the size of the Moon could never cover the solar disc (if the Moon can be covered by the shadow pyramid of the Earth, its size could never cover the Sun if the orbits were perfectly circular).
Various things can be deduced from the text. On the one hand, the conviction, the same as of Isidore, of a bigger size of the Sun than the Earth (up to 18 times), as well as of a smaller size of the Moon. Also it reminds us of the enormousness of the distance which separates us from the celestial body Rey, which is necessary for the terrestrial shadow pyramid to get so long that it reaches the Moon. He does not speak of the planets as a separate thing from the stars. These would not be eclipsed, as they never fall in the shadow of the Earth.
Did Sisebute think that the Earth was round? Although the idea of a spherical Earth was common thought among the scientists of the classic antiquity, it received less approval in the patristic Christian period at the end of that age. It is possible that the same Isidore - it is subject of debate - believed in an Earth in the shape of a flat disc. Although there are also discussions, in the case of Sisebute his belief in a spherical Earth seems to emanate from reading his text, as he talks about umbra rotae (rounded shadow) and about globus. The process of an eclipse in its whole (a Sun which turning always causes a round shape equal in the shadow that is cut by the Moon) also implies an Earth in the shape of a sphere. And all this in spite of his indicating that the shadow of the Earth has the shape of a pyramid and not of a cone (let's think that the shadow of our planet had the shape of a pyramid, this implied a square or cube earth, but the fact that the eclipses can occur as well when the Sun is directly on the vertical of the Earth as when they are obliques, implies that the Earth must be spherical, as the shape of the shadow of a square or of a cube varies according to the angle of incidence of the sun rays on the mentioned corpse). The best explanation about why he uses the term pyramid and not cone is due to the not very correct use of that term in his age, such as the imprecise definition that Isidore gives in his Etymologies.
In spite of his admiration for the sevillian scientist, Sisebute did not follow his theories literally, and so his belief in the proper luminosity of the stars and planets contradicts San Isidore, who thought that these did not have own light and were lit up by the Sun, as is the Moon.
(text translated to the english by Geert Poitevin)(many, many thanks Geert!)
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