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Geminus (Greek: Γεμῖνος) of Rhodes, was a Greek 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.
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
calendar of 120 years before his own time, has been used to
imply a date of c. 70 BC for the
time of writing, 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.
The crater Geminus on the Moon is named after him.
The only work of Geminus to survive is his Introduction to
the Phenomena, (Greek: Εἰσαγωγὴ εἰς τὰ Φαινόμενα), 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 constellations; the
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.
He also wrote a commentary on Posidonius' work On
Meteorology. Fragments of this commentary are preserved by Simplicius in his commentary on
Geminus also wrote extensively on mathematics, including a comprehensive
Doctrine, (or Theory) of Mathematics.
Although this work has not survived, many extracts are preserved by
Proclus, Eutocius, and others. He
divided mathematics into two parts Mental (Greek: νοητά) and Observable (Greek: αἴσθητα), (or in other words, Pure 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 (musical harmony), and logistics. Long extracts of his work are also
preserved by Al-Nayrizi in his commentary on Euclid's
Dicks, D., Dictionary of Scientific Biography. New York.
Neugebauer, O., A History of Ancient Mathematical
Astronomy. New York. (1975).
Evans, J., The History and Practice of Ancient Astronomy, page 91.
Oxford University Press. (1998).
- ^ Heath, T., A Manual of
Greek Mathematics, Dover Publications. (2003).