Two sciences, one route of transmission
Alchemy and astronomy had different subjects, but in medieval Europe they traveled by much the same road. Both drew heavily on Greek ideas that had been preserved, translated and extended in the Islamic world, and both reached Latin readers mainly through translations made in the twelfth and thirteenth centuries. The same cities, especially Toledo in Spain and centers in Sicily and southern Italy, produced translations in both fields. Alchemy is covered more fully elsewhere on this site, so this article concentrates on the sky and on how the two traditions relate.
Looking up: why people watched the stars
Observing the heavens had practical and religious reasons. The calendar depended on it, and the date of Easter was calculated from the cycles of the sun and moon, a task that kept monks and clergy working with astronomical tables. Travelers and sailors used the stars for direction. Farmers used seasonal markers such as the rising of certain stars or the position of the sun.
There was also a belief that heavenly bodies influenced earthly events. Medieval astrology was closely tied to astronomy, and many people who calculated planetary positions did so partly to cast horoscopes or choose a good time for medicine or a journey. Learned writers debated how far such influence went, since a strong claim seemed to threaten free will, but the connection between the two subjects was firm.
The inherited model
The standard picture came from Ptolemy, a Greek writer of the second century whose major work, known by its Arabic-derived title the Almagest, described the motions of the sun, moon and planets in a system centered on the earth. It used circles within circles to match observed movements. The system predicted positions with reasonable accuracy for its time, which helps explain its long life.
A widely used introductory textbook in medieval Europe was a short work on the sphere by Sacrobosco, written in the thirteenth century, which explained the basic structure of the heavens and the earth. It was commonly taught in universities. The idea that educated medieval people believed the earth was flat does not match these sources, which describe a round earth.
Instruments
The best-known instrument is the astrolabe. It is a flat metal disc with a rotating map of the stars that can be set for a given latitude. Users could measure the height of the sun or a star above the horizon, then use the instrument to find the time, the position of stars, or the direction of sunrise. Astrolabes were refined by astronomers in the Islamic world and reached Europe through Spain. In the fourteenth century Geoffrey Chaucer wrote an instructional treatise on the astrolabe for a young relative, which shows that it was understood outside the strictly scholarly world.
Other instruments included:
- Quadrants, quarter-circle devices for measuring the angle of a star or the sun.
- Armillary spheres, models made of rings that represent the circles of the heavens.
- Sundials and nocturnals, used to tell time by the sun or by the position of stars around the pole.
- Equatoria, devices for calculating planetary positions without lengthy arithmetic.
None of these involved telescopes, which came later. Observation depended on the naked eye, careful sighting and long records.
What the Islamic world added
Astronomers working under the Abbasid caliphate and in later centuries across the Islamic world translated Greek works, corrected some of their values, built larger instruments and compiled tables of planetary positions. Major observatories were established, such as the one at Maragha in the thirteenth century. Some Islamic astronomers suggested changes to Ptolemy’s models, and historians continue to study how far these ideas traveled west.
In Spain, scholars in Muslim lands and then in Christian kingdoms handled the same material. The Cordoba libraries were part of a wider culture of collecting and copying scientific books. In the thirteenth century, a set of planetary tables was prepared at the court of King Alfonso X of Castile, known as the Alfonsine Tables, and they remained in use across Europe for a long time. The spread of Arabic numerals also made calculating with such tables easier.
Where alchemy fits
Alchemists and astronomers often shared a view of the universe in which the heavens and earthly materials were connected. In many alchemical systems, each metal was linked to a planet, so that gold corresponded to the sun, silver to the moon, and so on. Some alchemical texts recommend working at particular times of the heavens. In this sense, the two fields shared a symbolic language.
They differed in method. Astronomy relied on observation and calculation, and its predictions could be checked against the sky. Alchemy worked with materials in the laboratory, where claimed outcomes were often impossible. Astronomy therefore gained a more secure reputation, while alchemy was admired by some and doubted by others.
Teaching and spread
Astronomy was part of the quadrivium, the group of four mathematical arts taught in the universities, alongside arithmetic, geometry and music. That gave it a regular place in the curriculum of the medieval universities, with set textbooks and trained teachers. Alchemy had no comparable place in formal teaching, and was passed on through manuscripts, patrons and personal contact.
Limits and legacy
Medieval astronomy was good at tracking and predicting but did not question the earth-centered picture in a way that overturned it. A few scholars discussed whether the earth might rotate, though they generally did not adopt the idea. Instruments were useful but limited by what the eye could see. Even so, the accumulated tables, instruments and techniques, together with the translation of Arabic works, gave later astronomers a rich inheritance. Historians generally regard the transmission of this knowledge, and the habit of recording and calculating, as the main achievement of the period.