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,[1] 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.
[citation needed] The only work of Geminus to survive is his Introduction to the Phenomena (Greek: Εἰσαγωγὴ εἰς τὰ Φαινόμενα), often just called the Isagoge.
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.
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.