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A New Set of Fourteenth Century Planetary Observations

Illustration of an excerpt of Pliny's Natural History on the positions and movements of seven planets, with earth in the centre, and around it the moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn

A New Set of Fourteenth Century Planetary Observations

By Bernard R. Goldstein

Proceedings of the American Philosophical Society, Vol.132 (1988)

Illustration of an excerpt of Pliny's Natural History on the positions and movements of seven planets, with earth in the centre, and around it the moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn

Introduction: Ever since antiquity astronomy has consisted of both theory and observation, but these two components have often received different treatments in the original sources. In the medieval period we find many texts that present theories (even new theories) for the motions of the Sun, the Moon, and the planets; and other texts that describe instruments (some newly invented) for making observations. Moreover, medieval scholars carefully read various works that survived from antiquity, notably Ptolemy’s Almagest, and these treatises served as a guide for the scientific study of astronomy. In particular, Ptolemy described methods of determining the planetary models (or parts of them) from sets of dated observations, and he gave numerous examples (including many based on observations he himself made) which take up a major portion of his magnus opus. In this respect, however, the vast majority of his successors did not follow him, for we find surprisingly few planetary observations in the medieval astronomical corpus. (A similar paucity of observations of the Sun, the Moon, and eclipses has also been noted.) Indeed, in most astronomical tables compiled in the Middle Ages observations play no role, and it can be demonstrated that the tabular entries are largely based on earlier astronomical theories.

The Astronomy of Levi ben Gerson (d.1344) is, therefore, unusual for recording 45 observations he made of planetary longitudes and latitudes that are presented here for the first time. The original Hebrew text has 136 chapters and, for the four chapters containing these observations, only two Hebrew manuscript copies survive. In the fourteenth century Latin version only one of these chapters, chapter 109, is preserved. This study is based on an unpublished edition I made of the Hebrew text of these chapters: chapter 109 contains report of 9 observations of Saturn, chapter 113 contains reports of 17 observations of Jupiter, chapter 117 contains reports of 8 observations of Mars, and chapter 122 contains reports of 11 observations of planetary latitudes.

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