| From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 415-417 |
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Courtesy of 
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Gersonides: Levi ben Gerson
Tamar M. Rudavsky
Born Bagnols, (Gard),
France, 1288
Died probably Provence,
France, 20 April 1344
Gersonides
left few letters and does not talk about himself in his writings; nor is his
life discussed at great length by his contemporaries. He may have lived for
a time in Bagnol sur‐Ceze. It is probable that his father was Gershom
ben Salomon de Beziers, a notable mentioned in medieval histories. Though
Gersonides made several trips to Avignon, he most likely spent his entire
life in Orange. There is some evidence that he may have followed the traditional
occupation of his family, moneylending. With the decline of Spanish Judaism
in the 13th century, Provence quickly became the cultural center for Jewish
intellectual activity. The popes in Avignon had a lenient policy toward the
Jews, whose creative life flourished, particularly in philosophy and theology.
Although Gersonides spoke Provençal, his works are all written in Hebrew,
and all of his quotations from Ibn Rushd,
Aristotle,
and Maimonides are in Hebrew as well.
He may have had a reading knowledge of Latin; he appears to manifest an awareness
of contemporary scholastic discussions. Gersonides might, however, have learned
of such discussions in oral conversations with his Christian contemporaries.
Although
Gersonides wrote no scientific works as such, scientific discussions were
included in his philosophical works. Gersonides' major scientific contributions
were in the area of astronomy; his works were known by his contemporaries,
both Jewish and Christian. One of the craters of the Moon, Rabbi Levi, is
named after him. Gersonides' astronomical writings are contained primarily
in Book 5, part 1 of The Wars of the Lord (Milḥamot
ha‐Shem), his major philosophical opus, which was completed in
1329. The astronomical parts of The Wars were translated into Latin
during Gersonides' lifetime. Although the astronomy chapters were conceived
as an integral part of the work, they were omitted in the first printed edition
of The Wars but have survived in four manuscripts. In the 136 chapters
of Book 5, part 1 of The Wars, Gersonides reviews and criticizes astronomical
theories of the day, compiles astronomical tables, and describes one of his
astronomical inventions.
With
respect to his astronomical observations, what distinguished Gersonides from
his Jewish philosophical predecessors was his reliance upon and consummate
knowledge of mathematics, coupled with his belief in the accuracy of observations
achieved by the use of good instruments. Because of this rootedness in empirical
observation, which was bolstered by mathematics, Gersonides believed that
he had the tools to succeed where others had failed, particularly in the area
of astronomy.
That
Gersonides clearly considered his own observations to be the ultimate test
of his system is explicit from his attitude toward Ptolemy.
“We did not find among our predecessors from Ptolemy to the present day observations
that are helpful for this investigation except our own” (Wars V.1.3,
p. 27), he says, in describing his method of collecting astronomical data.
Often, his observations do not agree with those of Ptolemy, and in those cases
he tells us explicitly that he prefers his own. Gersonides lists the many
inaccuracies he has found trying to follow Ptolemy's calculations. Having
investigated the positions of the planets, for example, Gersonides encountered
“confusion and disorder,” which led him to deny several of Ptolemy's planetary
principles (Goldstein, 1988, p. 386). He does warn his colleagues, however,
to dissent from Ptolemy only after great diligence and scrutiny. It is interesting
to note that Gersonides briefly discusses, and then dismisses, the heliocentric
model of the Universe before rejecting it in favor of geocentrism (Wars,
Chapter 51; also Commentary on Deuteronomy, 213c).
Gersonides
is perhaps best known for his invention of the Jacob's Staff. This instrument,
which he called Megalle ‘amuqqot (Revealer of profundities) and which
was called Bacullus Jacobi (Jacob's staff) by his Christian contemporaries,
is described in detail in Chapters 4–11 of Wars 5.1. The material contained
in these chapters was translated into Latin in 1342 at the request of Pope
Clement VI and survives in a number of manuscripts. Gersonides' instrument
was used to measure the heights of stars above the horizon. It consisted of
a long rod, along which a plate slides, that could be used to find the distance
between stars.
Gersonides
was interested in other instruments as well, including the astrolabe for which
he suggested several refinements, and the camera obscura. The latter instrument
was used by him for making observations of eclipses. Gersonides also applied
the principle of the camera obscura to make a large room into an observing
chamber, taking advantage of the image cast by a window on the opposite wall.
Chapter
99 of Book 5, part 1, contains astronomical tables commissioned by several
Christian clerics. Besides containing a general explanation of the tables,
Chapter 99 contains instructions on how to compute the mean conjunction and
opposition of the Moon and Sun; a method for deriving the true conjunction
or opposition of the Moon and Sun; a computation of solar time; and a discussion
of eclipses, with tables for positions of the Moon for each day.
In
Book 5, part 2, of The Wars, which was included in most manuscripts,
Gersonides deals with technical, albeit nonmathematical, issues in astronomy,
such as the interspherical matter (Wars 5.1, Chapter 2); topics concerning
the diurnal sphere, the Milky Way, and the movements of the planets (Wars
5.1, Chapters 4–5, 7–9); and how the Sun heats the air (Wars 5.1, Chapter
6). In Book 5, part 3,
Gersonides examines a number of additional topics, such as the Aristotelian
question of how many celestial spheres are needed to explain the movements
of the heavenly bodies (Wars 5.3, Chapter 6), and
whether the velocities of the heavenly bodies are related by a commensurate
number (Wars 5.3, Chapter 10). In this context, Gersonides addresses
Ptolemy's theory of cosmic distances based on a system of nested spherical
planetary shells. He introduces a fluid layer (“the matter that does not keep
its shape”) between two successive planetary shells so that motion of one
planet would not affect the motion of the planet adjacent to it. Gersonides
then computes the planetary distances according to three separate theories
(Wars 5.3, Chapters 130–135).
Gersonides
was also an avid supporter of judicial astrology, which plays an important
role in his philosophical views on free will and providence. The treatise,
Pronosticon de conjunctione Saturni et Jovis et Martis, was started
by Gersonides (possibly at the request of Pope Clement VI) and completed by
his Latin translator, Peter of Alexander, and Levi's brother, Solomon. This
work is a prediction based on the conjunction of Saturn and Jupiter to take
place in March 1345. Gersonides himself died in 1344, a year before the event
in question. In his prognostication, Gersonides predicts a number of calamitous
events. The Black Death, which arrived in Europe in 1347, was thus provided
with numerous astrological credentials.
In short,
according to Gersonides the ultimate function of astronomy is to understand
God. Astronomy, he claims, can only be pursued as a science by “one who is
both a mathematician and a natural philosopher, for he can be aided by both
of these sciences and take from them whatever is needed to perfect his work”
(Wars V.1.1, p. 23). Astronomy, he tells us, is instructive not only
because of its exalted subject matter, but also because of its utility to
the other sciences. By studying the orbs and stars, we are led ineluctably
to a fuller knowledge and appreciation of God. Astronomy thus functions as
the underpinning of the rest of his work.
Selected References
Dahan, Gilbert (ed.) (1991). Gersonide en son temps.
Louvain:
E. Peeters.
Feldman, Seymour (1967).
“Gersonides' Proofs for the Creation of the Universe.” Proceedings of the
American Academy for Jewish Research 35: 113–137.
Freudenthal,
Gad (1987). “épistemologie, astronomie et astrologie chez Gersonide.” Revue
des études juives 146:
357–365.
——— (ed.) (1992).
Studies on Gersonides: A Fourteenth‐Century Jewish Philosopher‐Scientist.
Leiden: E. J. Brill.
Gersonides. Sefer ha‐Heqesh ha‐yashar (On valid
syllogisms, written 1319). (Translated into Latin as Liber Syllogismi Recti.
Recently translated by Charles H. Manekin as The Logic of Gersonides. Dordrecht:
Kluwer, 1992.)
——— Sefer Ma'aśeh
hoshev (The work of a counter, written 1321). (Edited and translated into
German by Gerson Lange. Frankfurt am Main: Golde, 1909.)
——— Sefer Milhamot
ha‐Shem (The wars of the Lord, written 1329). Riva di Trento,
1560; Leipzig, 1866; Berlin, 1923. (Recently translated into English as The
Wars of the Lord by Seymour Feldman. 3 vols. Philadelphia: Jewish Publication
Society, 1984–1999.)
——— Perush ʿal
Sefer ha‐Torah (Commentary on the Pentateuch, written 1329–1338).
Venice, 1547; Jerusalem, 1967.
Goldstein, Bernard
R. (1974). The Astronomical Tables of Rabbi Levi ben Gerson. Transactions
of the Connecticut Academy of Arts and Sciences, Vol. 45. New Haven, Connecticut:
Connecticut Academy of Arts and Sciences.
——— (1985). The Astronomy
of Levi ben Gerson (1288–1344): A Critical Edition of Chapters 1–20.
New York: Springer‐Verlag.
——— “Levi ben Gerson's
Astrology in Historical Perspective.” In Dahan, Gersonide en son temps,
pp. 287–300.
——— “Levi ben Gerson's
Contributions to Astronomy.” In Freudenthal, Studies on Gersonides,
pp. 3–20.
Goldstein, Bernard R. and David Pingree (1990). “Levi ben Gerson's
Prognostication for the Conjunction of 1345.” Transactions of the American
Philosophical Society 80, pt. 6: 1–60.
Langermann, Y. Tzvi (1989). “Science, Jewish.” In Dictionary
of the Middle Ages, edited by Joseph R. Strayer, pp. 89–94. New York:
Charles Scribner's Sons.
——— “Gersonides on
the Magnet and the Heat of the Sun.” In Freudenthal, Studies on Gersonides,
pp. 267–284.
——— (1999). “Gersonides
and Astrology.” In Levi ben Gershom: The Wars of the Lord, edited by Seymour
Feldman, Vol. 3, pp. 506–519. New York: Jewish Publication Society of
America.
Rudavsky, T. M. (2000). Time Matters: Time, Creation, and
Cosmology in Medieval Jewish Philosophy. Albany: State University of New
York Press.
Touati,
Charles (1973). La pensée philosophique et théologique de Gersonide.
Paris:
Les éditions de Minuit.