Astronomy of Medieval Europe

Astronomical work in the Middle Ages centered mainly on its utility in setting the feast days of the Church, that is, the construction of church calenders, but the continuing interest in astrology also caused a demand for improved astronomical accuracy for horoscopes and astrological forecasts of a wider nature; such practices were officially condemned but frequently and sometimes widely practiced. In fact, Tertullian [~150—220 a.d.], one of the early Fathers of the Church, made use of an astrological argument, among other tactics, to attempt to alleviate persecutions of Christians. His letter to Scapula, the Roman governor, refers to the portents of the solar eclipse of August 14, 212:

That sun, too, in the metropolis of Utica, with light all but extinguished, was a portent which could not have occurred from an ordinary eclipse, situated as the lord of day was in his height and house. You have the astrologers, consult them about it. (Roberts and Don-aldson/Coxe 1869, Ante-Nicene Fathers III, Tertullian (repr. 1994), "Letter to Scapulla," p. 106; quoted by Schove (1984, p. 34)).

Augustine, Bishop of Hippo [354-430], distinguished between astrology and astronomy, noting that the latter was useful but the former pernicious. Thomas Aquinas [and other influential theologians (but not all)] took a similar position. The struggle over astrology continued throughout the Dark and Middle Ages; it was resolved in academia first, but more generally only after the start of the scientific era.

The Middle Ages, loosely defined as the millennium between the 5th and 15th centuries, are sometimes divided into three sections because of the outbreaks of waves of plague in the 6th and again in the 14th centuries. The later outbreaks were known as the Black Death. The onset of epidemics was sometimes blamed on planetary conjunctions, comets, or aurorae. Conjunctions imply an astrological—and therefore presumably predictable—origin, but the essentially unpredictable character of comets as a class (as opposed to individual, short-period comets) suggests the opposite. Curiously, the waves of epidemics had an 11-year cyclicity—just as one would expect of aurorae, which are strongly affected by the 11-year sunspot cycle (see §5.3.1). Dendrochronology (see §4.3) suggests that the 540s and 1340s, when the plagues began, were years of cold summers in Northern Europe. Meteorological conditions, also linked to the solar cycle, may have affected the vectors of the disease (the rat and the black flea).

The calendrical uses for astronomy, which were rooted for millennia in Europe (refer to §6.2 for the Neolithic evidence), were pursued throughout the Middle Ages. The Ecumenical Council of Nicaea in 325 a.d. adopted a formula for the date of Easter.48 It was noted that the Vernal equinox had shifted from March 25 to March 21 (in the prevailing Julian calendar). Dionysius Exiguus proposed a 19-year cycle of fixing the dates of Easter in 532 a.d. that gradually came to be accepted in the West (cf. Percival 1899, pp. 54-56). In the eastern churches, the date of Easter was further constrained to occur after Passover ("the Zonaras

Proviso") (Patrinacos 1992, pp. 125-127). The Venerable Bede (English monk and theologian, 672/673-735), who was interested in ecclesiastical history and chronology, adopted the Dionysian cycle. Among Bede's works are an essay on the calendar (De Temporis) and a major work, The Ecclesiastical History of the English Nation. Bede wrote also about a comet of 678 and popularized the use of the backcalcu-lated birth of Christ to specify the year. Church scholars had previously favored the date of the crucifixion as the era base.

Astronomy became increasingly important starting with the reign of Charlemagne,49 (768-814) who brought the English monk Alcuin [732-804] to his palace at Aix-la-Chapelle (Aachen) to reform education, and who proclaimed that each school should have a scriptorum. Because "calculation" was one of the subjects to be taught, and the seven liberal arts, the Carolingian renaisssance benefited astronomy too. Dall'olmo's (1978) compilation of medieval European meteor sightings includes at least one seen by Charlemagne, a bright fax ("torch") seen before dawn sometime in 810, during a campaign against the Danish king Godofridus. An earlier possible sighting by him, in 776, may have been the aurora.

One of Europe's scourges during Carolingian times was the fierce onslaught of the Vikings. Indeed, the major sacks of Rome indicate the difficulty the Roman empire and its successors faced in attempting the preservation, let alone advancement, of learning in the Middle Ages: 410 (Goths), 455 (Vandals), 846 (Saracens), and 1084 (Vikings). The Vikings were intrepid warriors and sailors, mastering seafaring navigation in the West just as islanders were doing so in the Pacific. They had tables of the altitude of the noon Sun, with which the latitude could be determined and maintained. Moreover, even on cloudy days, they had the use of calcite crystal (Icelandic spar), which they called solarsteinn or sunstone, which had the property of analyzing the direction of maximum polarization of sunlight by scattering in the atmosphere (Jones 1964/1984, pp. 5-14; but see Roesdahl 1987, p. 92, for a more skeptical view). Like the Pacific islanders, however, they also made use of dead reckoning, wave swells, and birds (see §11). In any case, the Vikings or Norsemen raided and later settled in parts of what would become the British Isles (from the early 9th century), Russia (9th century), France (9th-10th centuries; Normandy still bears their name), and from Normandy, Sicily (early 11th century), and, again, from Normandy, England (1066). They raided the Mediterranean as well as Eastern Europe, and even attacked Byzantium during the absence of the emperor. Their attacks on monasteries such as Lindisfarne at the beginning of the expansion, in 793, shocked Alcuin and the Christian world. By 1000 a.d., they had colonized Iceland, Greenland, and gone on to Vineland in North America. The Vikings eventually came to adopt Christianity and other aspects of European community. Islam was another source of conflict with Christianity in southern Europe and the Near East. The history of that conflict is lengthy and complex, and it will not be discussed here.

48 The first Sunday after the first spring full moon (i.e., the full moon following the date of vernal equinox).

49 Also known as Karl der Grosse, who lived from 742 to 814, reunited Europe, and restored learning.

Instead, we concentrate on the transmission of classical knowledge through the Arab and, more broadly, the subsequent Moslem world. We have already mentioned (§5.6) the effect of the intense and widespread April 1095 meteor shower on helping to create the groundswell for the First Crusade. Meteors, along with comets, and other transient phenomena, were considered to be omens for most of the Middle Ages.

Like most of the classical learning, many Greek astronomical writings came to Europe through Islamic sources (cf. §7.4), such as Al-Battani (Islamic astronomer; b. <858), Avicenna (physician, ca. 980-1037), and Averroes (Ibn Rushd, Islamic philosopher, 1126-1198) late in the Middle Ages.

The great Jewish philosopher, Moses Maimonides [Cordoba; 1135-1204], wrote on astronomical matters as well as the Torah. Adelard of Bath [fl. 12th century] translated Euclid's Elements from Arabic into Latin for use in schools, and he wrote on astronomy and the abacus. Gerard of Cremona (~1114-1187) translated the Almagest from Arabic into Latin in 1175; he headed a school of translation of scientific and mathematical treatises in Toledo. The treatises and tables of Al-Battani were first translated by Robertus Retinensis [d > 1143] and, independently, by Plato Tibatinus [1st half of 12th century]; still later, Alfonso the Learned [or Alfonso X, king of Castile and Leon, 1252-1282] had them translated into Spanish. A Latin translation of one of Al-Battani's books, De Motu Stellarum, was published in Nuremburg in 1537. His spherical trigonometry solutions were used by Johannes Müller, better known as Regiomontanus [1436-1476], the leading European astronomer of his day.

After the translation of the works of Al-Bitruji into Latin in the 13th century, controversy arose between adherents of the epicyclical Ptolemaic system and those of the Eudoxian homocentric system. Roger Bacon (1214—1292), the doctor admirabilis of the Middle Ages, participated in the debates, but accepted neither system. Bacon was a Franciscan monk and was among the most learned scholars of his time; he studied and wrote both philosophy and science. Among his writings is a book on optics, in which he discussed experiments with concave mirrors. He said that the Milky Way should be examined cum specilis in order to understand its nature; although the meaning of the phrase is unclear, it suggests the use of a speculum concave mirror as a primitive telescope. Along with one of his teachers, Robert Grosseteste [Franciscan monk, then Bishop of Lincoln; fl. 1175-1253],50 he recognized the inadequacy of the Julian calender and attempted to reform it, but without success. A similar attempt by John de Meurs in the 14th century also failed. Calendar reform succeeded only through the forceful advocacy of Pope Gregory in the 16th century.

Others who were interested in optics included Dietrich of Freiberg [Dominican monk; ~1250—1311], who formulated a theory for the rainbow involving the action of sunlight within water droplets and advocated the use of experimentation in science, and the mathematician and philosopher

50 Who wrote on optics and on the theory of refraction.

Witelo [Silesia; b. 1225], who studied the propagation and transmission of light through material.

Although there was a clear increased interest in general empirical science in the 13th century, time, calendrics, and position determination were recognized as appropriate applications of astronomy throughout the Middle Ages. One of the great promoters of astronomy (and astrology) in the Middle Ages was King Alfonso X, the Scholar (El Sabio), who ascended the throne of Castile and Leon in 1252. He commissioned translations of many Arabic astronomical treatises and created a major compendium of Arabic astronomy; when astronomical topics were missing, additional material was written by Alfonso's scholarly teams. Besides descriptions of planetary motions and constellations, there is an important section on astronomical instruments. The translation of the Al-Battani tables was produced by a team headed by the Jewish astronomer Isaac ibn Sid, around 1270. Alfonso also commissioned the Alfonsine Tables, which began to appear elsewhere in Europe (in slightly different forms) in the 1320s and were issued multiple times over the next 120 years (Gingerich 1993, pp. 115-128).

Geoffrey Chaucer [ca. 1340-1400] wrote A Tretise on the Astrolabe in 1391, which indicated its use in calculations of declinations and latitudes as well as for observations (see §3.3). John D. North (1988) has shown that many of Chaucer's tales are anthropormorphized descriptions of astronomical events (comparable in many ways to ancient myths). The descriptions contain enough detail that North was able to determine the precise dates of a number of the phenomena that Chaucer described. Richard of Wallingford (fl. 1320) designed observational instruments and an astronomical clock for the abbey of St. Albans, where he was abbot.

As noted in §7.2.1, a number of individuals believed with Heracleides of Pontus that a rotating Earth made the rotation of the sphere of fixed stars unnecessary. Among them was Nicholas Oresme [scientist and economist, later Bishop of Lisieux; 1320-1382]. Like Jean Gerson [Theologian at Paris, pietist and mystic, 1363-1429], and St. Thomas Aquinas [1225-1274], he attacked astrology. Oresme also used three-dimensional rectangular coordinates to represent geometric figures and wrote an astronomical Treatise on the Sphere (1377). Nicholas of Cusa [1401-1464; Cardinal] championed the movement of the Earth around the Sun and the existence of other worlds (Avey 1954/1958, p. 112; Jaki 1972/1975, p. 48). Giovanni Pico de la Mirandola [1463-1494] opposed astrology on the grounds that it degraded free will, which he believed was not constrained from above. The lapsed priest, Giordano Bruno [1548-1600; Italy] was influenced by Nicholas of Cusa and Copernican cosmology, and he believed that the stars were distant suns circled by planets (cf. §15.1).

One of the principal philosophic movements of the later Middle Ages was realism. Stemming ultimately from Plato and transmitted through the Neo-Platonism of Plotinus and the Greco-Roman world, realism encompassed the idea that essences underlay everything conveyed to us by the senses, and they were in a more fundamental way more real than were the things we perceive through the five senses. Moreover, it was possible for the mind of a person to comprehend and know these essences. Philosophers and theologians of the Middle Ages generally agreed on these principles. One of the best known medieval philosophers was Anselm, Bishop of Canterbury [1033-1109]. In his Proslogium, he produced a proof for the existence of God on the basis of reasoning alone. The syllogism, known ever since as the ontological proof, is as follows:

(1) God is that greater than which nothing can be conceived.

(2) To exist in reality is greater than to exist in the imagination alone.

(3) Therefore, God exists.

The major premise of this syllogism, "God is that greater than which nothing can be conceived," is a statement about a principal property of God, universally accepted in Anselm's time. The minor premise, "To exist in reality is greater than to exist in the imagination alone," would seem to be unassailable. The conclusion then follows. Nevertheless, the proof was criticized by a contemporary monk named Gaunilon, and later by Thomas Aquinas and Immanuel Kant among many theologians and philosophers. Gaunilon argued that thinking about money in one's pocket did not perforce create any there, but Anselm's response rested on the idea that God alone is superlative so that parallel arguments regarding the merely finite are not relevant. One can argue that the proof has meaning only in a context in which ideas have real substance, and in particular, in which the idea of God has the force of reality. Modern readers who are not rooted in a tradition of faith or of Pla-tonism may regard this as strange, but they should consider that no reasoned discourse is without its philosophical preconceptions, even if they remain unrecognized by either writer or reader.

In contrast to the ontological argument for the existence of God, Thomas Aquinas [1225-1274] in his Summa Theo-logica asserted that the existence of God could be proved from evidence of the world of experience, in five different ways:

(1) From motion in the world, which requires a first mover

(2) From the necessity of a first efficient cause (of things in the world)

(3) From the existence of things, which requires the existence of a necessary preexistent necessity in order to come into being

(4) From the gradations of heat, goodness, and so on, which, from an Aristotelian argument that the source of a gradation is its maximum form, implies a fire for heat, or God for goodness

(5) From the governance of the world, because things (even things lacking intelligence) act always, or nearly always, in such a way as to bring about the best result. From this, Aquinas concluded that they act not fortuitously, but by design.

Aquinas's arguments involve evidence from the created universe and are known as the cosmological proofs of the existence of God. The existence of a universe perceived to be designed is the essence of these proofs, and although again the eye of faith is required to accept the underlying premises, the latter have modern echoes, such as the anthropic principle, which holds that even a modestly changed universe would be unable to support the development and existence of human life.

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