Geminus of Rhodes (), was a Ancient Greece astronomer and mathematician, who flourished in the 1st century BC. An astronomy work of his, the Introduction to the Phenomena, still survives; it was intended as an introductory astronomy book for students. He also wrote a work on mathematics, of which only fragments quoted by later authors survive.
Life
Nothing is known about the life of Geminus. It is not even certain that he was born in Rhodes, but references to mountains on Rhodes in his astronomical works suggests that he worked there. His dates are not known with any certainty either. A passage in his works referring to the
Annus Vagus (Wandering Year) of the Egyptian calendar of 120 years before his own time, has been used to imply a date of c. 70 BC for the time of writing,
[Dicks, D., Dictionary of Scientific Biography. New York. (1970).] which would be consistent with the idea that he may have been a pupil of
Posidonius, but a date as late as 50 AD has also been suggested.
[Neugebauer, O., A History of Ancient Mathematical Astronomy. New York. (1975).]
The crater Geminus on the Moon is named after him.
Astronomy
The only work of Geminus to survive is his
Introduction to the Phenomena (), often just called the
Isagoge. This introductory astronomy book, based on the works of earlier astronomers such as
Hipparchus, was intended to teach astronomy for beginning students in the subject. In it, Geminus describes the
zodiac and the motion of the
Sun, the
, the
celestial sphere, days and nights, the risings and settings of the zodiacal signs, luni-solar periods and their application to
calendars, phases of the Moon,
eclipses, star phases, terrestrial zones and geographical places, and the foolishness of making weather predictions by the stars.
[Evans, J., The History and Practice of Ancient Astronomy, p. 91. Oxford University Press. (1998).]
He also wrote a commentary on Posidonius' work On Meteorology. Fragments of this commentary are preserved by Simplicius in his commentary on Aristotle Physics.
Mathematics
Geminus also wrote extensively on
mathematics, including a comprehensive
Doctrine, (or Theory) of Mathematics.[Heath, T., A Manual of Greek Mathematics, Dover Publications. (2003).] Although this work has not survived, many extracts are preserved by
Proclus,
Eutocius, and others. He divided mathematics into two parts:
Mental () and
Observable (), or in other words,
Pure mathematics and Applied. In the first category he placed
geometry and
arithmetic (including
number theory), and in the second category he placed
mechanics,
astronomy,
optics,
geodesy, canonics (
harmony), and
logistics. Long extracts of his work are also preserved by
Al-Nayrizi in his commentary on Euclid's
Elements.
Notes
Bibliography
External links
