Person: Gerson, Rabbi Levi Ben
Levi ben Gerson was a French scholar who wrote on trigonometry as well as astronomy and philosophy.
Mathematical Profile (Excerpt):
- Other than Levi ben Gerson, the commonest seem to be Gersonides and Levi ben Gershon.
- However, others refer to him as Levi ben Gershom, and less commonly as Gersoni, Leo de Bagnols, Leo de Balneolis, Leo Judaeus, Leo Hebraeus , or by the acronym Ralbag.
- Levi was also closely related to Nahmanides (1194-1270) who was a Spanish scholar, a rabbi and Jewish religious leader who is known for his work on philosophy, poetry, and medicine.
- Levi was a cousin of Judah Delesfils whose grandson was Simon ben Zemah Duran (1361-1444), a Spanish Jewish rabbi who wrote several famous works.
- We know that Levi ben Gerson lived in Orange, a town which today is in France but this was not so when he lived there.
- This is, in fact, quite significant, for the king of France at the time Levi was born was Philip IV, known as Philip the Fair.
- He expelled the Jews from France in 1306 seizing their property and confiscating all money owed to them, but Levi was not affected.
- Levi certainly had good relations with leading men of the district, both Jewish and Christian.
- As is already evident from what we have just written, what little information we have about Levi comes from his works.
- How did Levi ben Gerson become an expert in so many subjects?
- We now look at Levi's works.
- In fact, it is the Art of Calculation which allows us to give the year of Levi's birth, since he says he finished writing it in 1321, when he was thirty-three years old.
- Gino Loria suggested that the sine theorem be named after Levi but he was not the first to present the theorem, which was known to Jabir ibn Aflah in the 12th century, but he may have rediscovered it.
- It is interesting to note that Levi was interested in Euclid's parallel postulate and appears to have been part of a lively debate about whether it could be deduced from the other axioms.
- We now look at some of Levi's contributions to astronomy.
- We should note that the term 'Jacob's staff' was not used by Levi but rather by his Christian contemporaries; he used a Hebrew name which translates as 'Revealer of Profundities'.
- We note that while Levi's method for constructing the scale is theoretically correct, it requires making measurements that seem extremely difficult, so perhaps the theory was never put into practice.
- These instruments are described in the astronomical part of Levi's major work The Wars of the Lord which took him twelve years to compose beginning in 1317 and finishing on 24 November 1328.
- These first 20 sections contain a general philosophical introduction, Levi ben Gerson's trigonometry, a description of the construction and use of an instrument invented by him (the Jacob staff) for exact determination of angular differences, remarks about the exact construction of the astrolabe and improvements invented by him, followed by practical and theoretical methods for the determination of the meridian and the position of stars and, finally, a proof that Ptolemy's theory does not account for the facts - neither for the moon (which in an epicycle motion would have to be seen from both its sides) nor for the planets, and a preliminary exploration of alternative (excentric circle) theories.
- So Levi's passion for accurate astronomical instruments came from a desire to make sure that any proposed theoretical model was actually in accord with observation, and the more accurate the observation, the better a theory could be put to the test.
- It was a bold move by Levi to reject Ptolemy's system and certainly shows his strength of character that he was prepared to argue against such a deeply held standard model of the universe.
- For example, according to Ptolemy's model the size of Mars should vary by a factor of 6 but Levi observed only a factor of 2 in the size of the planet.
- Levi observed a solar eclipse in 1337.
- Another eclipse observed by Levi was the eclipse of the Moon on 3 October 1335.
- Having explained that Levi rejected Ptolemy's model, we must make it clear that the model which Levi gives is of a similar type consisting of 48 spheres.
- Levi considers that these solid spheres are separated by layers of fluid that lubricate their motions, each sphere being moved by an immaterial intelligence.
- Despite spending some time on considering Levi's mathematical and astronomical contributions, we have not yet mentioned the bulk of his work which was on philosophy and religious studies; he wrote many complex Biblical commentaries.
- It is important to realise that Levi's scientific work was often in conflict with Judaism.
- In his discussion of the immortality of the soul, Levi examines the material intellect, which he believes is born with man but only has potential existence, but acquired intellect reaches an effective existence by acquired ideas and conceptions.
- The acquired intellect, Levi argues, does not cease to exist with the death of the body, so achieving immortality.
- On prophecy, Levi considers whether it is possible and argues strongly that it is.
- For anyone like Levi who believes in astrology, such a conclusion is inevitable.
- On divine providence, Levi reaches the conclusion that some are protected and guided by general providence while others are under an individual providence.
- Gersonides reflects the following characteristics: first, his writings demonstrate a fundamental interplay and harmony between astrological and theological beliefs.
- Gersonides believed that life on earth had a meaning, and that terrestrial events had an order.
- Gersonides' views on prophecy, providence, free-will and evil reflected ingredients of this philosophical determinism.
- Only in recent years has Gersonides received his rightful place in the history of philosophy.
- As scholars have rediscovered his thought and have made his corpus available to a modern audience, Gersonides is once again appreciated as an insightful, ruthlessly consistent philosopher.
Born 1288, Bagnols now Bagnols sur Cèze, Provence, France. Died 20 April 1344, Avignon, France.
View full biography at MacTutor
Tags relevant for this person:
Astronomy, Geography
Thank you to the contributors under CC BY-SA 4.0! 
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- non-Github:
- @J-J-O'Connor
- @E-F-Robertson
References
Adapted from other CC BY-SA 4.0 Sources:
- O’Connor, John J; Robertson, Edmund F: MacTutor History of Mathematics Archive
