**Geminus** ( ) 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.

## 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, 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.
## 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

constellations; 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.

He also wrote a commentary on Posidonius' work

*On
Meteorology*. Fragments of this commentary are preserved by

Simplicius in his commentary
on

Aristotle's *Physics*.

## Mathematics

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* ( )
and

*Observable* ( ), (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 Elements.

## Notes

- Dicks, D.,
*Dictionary of Scientific Biography.* New
York. (1970).
- 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).

## Bibliography

## External links