Astronomy in the time of D. Dinis

One of the astronomical events that took place between D. Afonso Henriques and D. Dinis was not recorded in Western Europe, but in China and Japan

When King D. Dinis (1261-1325, king in 1279) created, on March 1, 1290, the “Estudo Geral” in Lisbon, starting what is now the University of Coimbra, the Portuguese kingdom already took 147 years. What has happened most importantly in astronomy over the past century and a half?

Contrary to what is common, the Middle Ages is far from having been a «night of ten centuries». Astronomical science was always advancing, while maintaining the Aristotelian cosmovision, adopted by Ptolemy, in which the Earth was at the center of the world and the five other planets then known – Mercury, Venus, Mars, Jupiter and Saturn – circulated around the Earth.

The divine Comedy, by the Italian Dante Alighieri, written between 1308 and 1320 and published in 1472, presents this Aristotelian-Ptolemaic model, with the planets in concentric circles.

To explain the setbacks of the planets in the sky, it was necessary to create, even in Ancient Greece, a set of epicycles, which Ptolemy incorporated into his model. Epicycles are small circles traveled by a planet around a point, which, in turn, travels a large circle around the Earth.

King Alfonso X of Castile (1221-1284, king in 1252), nicknamed the “Sage” or the “Astrologer”, who was the grandfather of D. Dinis, knew this geocentric system well with the epicycles.

The “Sage” supported the school of translators in Toledo, a group of Christians, Jews and Muslims who ensured the transmission of many works from Classical Antiquity, and ordered the installation of an astronomical observatory in that city as well.

It was also the compiler of the calls Afonsine Tables, completed in 1270, which describe the positions of the planets in 1252, the year of their coronation, and the main driver of the Books of Astronomy Knowledge, prepared between 1276 and 1279, therefore shortly before the foundation of the first Portuguese university. Both works would have wide circulation until the Renaissance.

A well-known but apocryphal quote from Alfonso X is famous: "If Almighty God had consulted me before creation, I would have recommended something simpler."

The fact that the powerful king of Castile was interested in astronomy reveals the high status of this science in the XNUMXth century. Certainly, in Portugal, in the time of D. Dinis, the works of Afonso X were known.

According to some historians, D. Dinis would have received the Books of Knowledge. Later, D. Duarte (1391-1438, king in 1433) would call, in the Loyal Counselor (1438), to Alfonso X "that honorable astrologer king how many multitudes did readings".


At that time, Astronomy and Astrology got mixed up, because the positions of the planets were used to make horoscopes. D. Afonso IV (1291-1357, king since 1325), son of D. Dinis, had in his court some astrologers, such as Afonso Dinis, bishop of Guarda and Évora.

In the Avis dynasty, King Duarte had in his court the astrologer Mestre Guedelha, whose instructions he did not always follow. Several decades later, Friar António de Beja (1493-1517), of the Order of S. Jerónimo, would publish his book Against the Judgments of Astrologers (1523), where he disputes the prophecy that the end of the world would be in the following year.

The mathematician Pedro Nunes (1502-1578), professor at the University of Coimbra, refers to astrology only once, in his book of twilight (1542). He says that his disciple D. Henrique, the future Cardinal King, «is admirably pleased with the theory of Astronomy, that is, with the science that deals with the course of the stars and the universal composition of the sky, rather than vain belief and already almost rejected that it issues judgments on life and fortune.” This, the Portuguese sage insists on separating astronomy, the science of the stars, and astrology, predictions based on the stars.

The first almanac in Portuguese, translated from Latin, Lasting Almanac to Find Planet Places in Signs (c. 1321), which came out, therefore in the reign of D. Dinis, served as the basis for astrological predictions.

However, the first Portuguese almanacs in printed form only appeared in the second half of the XNUMXth century, with the Almanach Perpetuum (1496), by the Jewish astronomer Abraão Zacuto (c. 1450 – c. 1522), which would be used during the travels of Vasco da Gama and Pedro Álvares Cabral.

One of the astronomical events that took place between D. Afonso Henriques and D. Dinis was not recorded in Western Europe, but in China and Japan: It was the 1181 supernova, which remained visible in the Cassiopeia constellation for 183 days. A supernova is a large star that explodes violently because it cannot maintain the balance of matter inside. There have only been nine supernovas observed with the naked eye in our galaxy throughout historical times.

In the Middle East, astronomy was, in the Middle Ages, cultivated to the highest level by Muslims. The Persian polymath Naceradim de Tus or Nasir al-Din al-Tusi (1201-1274) built, from 1259, an astronomical observatory at Maragha (in present-day Azerbaijan), which made it possible to complete tables of the planets in 1272. These tables would be translated. into Greek in the Byzantine Empire, but did not reach the West.

The time for the globalization of knowledge was yet to come.