10 June 2021
Lewis of Caerleon’s eclipse tables in the spotlight
On the morning of 10 June, large parts of the Northern Hemisphere will see a total or partial solar eclipse, when the shadow of the Moon obscures the Sun. Nowadays we can easily find details about eclipses online, but medieval astronomers had to make complicated calculations. An impressive gathering of eclipse calculations survives in the most complete collection of the works of Lewis of Caerleon (d. in or after 1495), a Welsh physician and astronomer. The manuscript, made under his close supervision between 1485 and 1495, is dedicated largely to predicting future lunar and solar eclipses. The British Library acquired the manuscript for the nation in 2020, and has since digitised it in full (Add MS 89442).
The opening of Lewis of Caerleon’s work on eclipses (England, c. 1485–95): Add MS 89442, p. 39
Lewis of Caerleon made his eclipse tables in the 1480s. They are based on complicated mathematical exercises and provide exact details on the magnitude and duration of eclipses.
Lewis of Caerleon’s calculations for solar eclipses made in 1482: Add MS 89442, p. 41
What makes Lewis’s eclipse calculations remarkable is that he made some while incarcerated in the Tower of London. He had been personal physician to Elizabeth Woodville (c. 1437–1492), Queen of England, and to Lady Margaret Beaufort (1443–1509) and her son, the future King Henry VII (r. 1485–1509). In 1484, his loyalty to the Tudors caused King Richard III (r. 1483–1485) to order his arrest. Lewis remained imprisoned at the Tower until 1485. In his volume of collected works, he explains that his original eclipse calculations were lost when he was in prison, but that he composed new tables at the Tower. His new computations resulted in new values for his eclipse tables.
In a note at the foot of this page, Lewis Caerleon refers to eclipse tables that were taken from him at his arrest in 1484, and new ones that he made while he was incarcerated at the Tower of London: Add MS 89442, p. 65
At the Tower, Lewis probably had access to scientific books, enabling him to consult pre-existing eclipse tables and to expand upon these with his own computations. He was also able to measure the accuracy of his calculations. According to another manuscript of his works (Royal MS 12 G I), he observed an eclipse from the Tower of London on the afternoon of 16 March 1485. His collected works (Add MS 89442) contain detailed calculations for this solar eclipse. In observing it, Lewis may have been able to measure its different phases by using a simple stick, if the weather was not too cloudy.
Lewis of Caerleon’s calculations for a solar eclipse on 16 March 1485: Add MS 89442, p. 71
Solar and lunar eclipses were considered important events in the Middle Ages. They were commonly believed to have an effect on people’s health and fortune, and to anticipate major world events. However, Lewis of Caerleon did not attach any astrological significance to eclipses. His main interest, it seems, was the study and computation of the ‘mechanics’ of the universe. He donated other copies of his eclipse tables to the universities of Oxford and Cambridge, and he provided detailed instructions to his readers on how they could calculate eclipses for themselves.
Lewis of Caerleon explains how to make eclipse tables using the diameter of the Sun and Moon: Add MS 89442, p. 42
Lewis of Caerleon was not the first British astronomer to calculate future eclipses. In 1380, John Somer, a Franciscan friar of Bridgwater, made predictions about the times of future eclipses in his Kalendarium. He produced this work for his Provincial Minister, Thomas Kingsbury, at the request in turn of Joan of Kent (1326/1327–1385), mother of King Richard II.
Predictions for solar eclipses in the years 1431, 1433 and 1436 in a physician's folding almanac based on John Somer's Kalendarium (England, c. 1430–1431): Harley MS 937, f. 8r
In 1386, Nicholas of Lynn, a Carmelite friar, composed his own Kalendarium, at the request of John of Gaunt (1340–1399), Duke of Lancaster.
Figures of solar eclipses from Nicholas of Lynn’s Kalendarium (England, 1387–15th century): Harley MS 1811, f. 30v
Similarly, another English astronomer known for his eclipse calculations is Richard of Thorpe, an Augustinian friar of York. Richard used the works of John Somer and Nicholas of Lynn for making a calendar for the Use of York for the years 1387–1462.
Calendar, Use of York, with astronomical tables by Richard of Thorpe (England, c. 1430): Add MS 82946, f. 28v
Lewis of Caerleon was clearly familiar with the works of previous astronomers. He made clear that his calculations were based on those of leading English astronomers such as Simon Bredon (d. 1372), fellow of Merton College, Oxford, Richard of Wallingford (d. 1336), Abbot of St Albans, and John Holbroke (d. 1437), proctor and chancellor of the University of Cambridge. He also drew upon the works of Arabic astronomers such as Al-Battānī (c. 858–929), and Jabir ibn Aflah (c. 1100–c. 1160).
Tables for solar eclipses by Richard of Wallingford and expanded by Lewis of Caerleon: Add MS 89442, p. 67
Add MS 89442 is witness to the importance attributed to eclipses in the Middle Ages, as well as to the great skill of Lewis of Caerleon in making precise predictions of these events. The manuscript was acquired with the generous support of the Shaw Fund, the T. S. Blakeney Fund, the Bernard H. Breslauer Fund of the American Trust for the British Library, the British Library Collections Trust, the Friends of the National Libraries, and those who wish to remain anonymous.
We would like to thank Dr Laure Miolo, Munby Fellow in Bibliography, Cambridge University Library, for discussing her research with us. You can read more about Lewis of Caerleon’s manuscripts in her two-part blogpost (Part 1 and Part 2).