Poseidonius of Rhodes was a scholar of many disciplines including mathematics, astronomy, mechanics and meteorology. As is the
case for most of the ancient Greek astronomers, the works of Poseidonius of Rhodes have not survived the ravages of
time but, fortunately, have been described in detail by later generation scholars including Cleomedes.
His greatest contributions to astronomy are in relation to his attempts to estimate the size and distance of both the moon and sun as well as the circumference of the earth. For the latter, he noted that the star Canopus is on the horizon when observed from Rhodes whereas it sits at an altitude of 7° 30' when observed from Alexandria and whose distance from Rhodes is 5000 stadia. Using simple arithmetic, he concluded from these observations that the circumference of the earth must be 240,000 stadia (5000 x 360 / 7.5) which is very accurate but contains two off-setting errors in relation to the distance between Rhodes and Alexandria as well as the altitude of Canopus when observed from Alexandria. For the former, a distance of 3750 stadia in lieu of his initial estimate of 5000 stadia is the more accurate figure; for the latter, the altitude used should have been 5° 15' and not 7° 30'. When the correct figures for these two key parameters are employed, the circumference of the earth is computed to be 257,142 stadia which is very close to his original estimate of 240,000 stadia and virtually identical to the 252,000 stadia computed approximately 150 years earlier by Eratosthenes.
With respect to the distance of the moon from earth, he computed a distance equal to 58 earth radii which is short of the modern-day estimate by less than 4%. In his book "Περί του Ηλίου μεγέθους", he estimates the distance of the sun from earth to be only 2.5 times that of the distance of the moon from earth (a gross underestimate). Other works include "Φυσικοί Λόγοι", "Περί Κόσμου" as well as "Περί Μετεώρων" where he discusses physics, the universe and various celestial phenomenon observable with the naked eye, respectively, as well as the altitude of the atmosphere (40 stadia according to his calculations).
Finally, Poseidonius is credited with attempting to use scientific principles to study meteorology and proposed various theories in relation to the formation of clouds, rain, wind, mist, frost, hail and rainbows. Furthermore, he suggested on the possible relationship between the moon and tides and on earth!
Poseidonius of Rhodes has been honored with the naming of a formation on the northeast periphery of Mare Serenitatis (see Rukl: 14) after him. The circular and walled plain formation Poseidonius, measuring 95 km in diameter and located at (31.8° N, 29.9° E), is characterized with high walls and multiple craterlets and is best observed just before first or third quarters. A 70-km long rille on the eastern half of this formation is also named in his honor (Rimae Poseidonius). A wide-field view of Mare Serenitatis including the formation named after Poseidonius is available elsewhere on this site (see here) with additional high-power images of this formation available shortly.
For a thorough discussion on the life and creative work of Poseidonius of Rhodes, the reader is referred to: