The astrolabe of Ptolemy (the... G.P.S. of the ancient Greeks)

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The astrolabe of Ptolemy
(the... G.P.S. of the ancient Greeks)


It was an exceptional astronomical instrument which depicted the celestial sphere and was used for the measurement of geographic length (longitude) and width (latitude) of the observed stars from any part of the Earth but also reversely as locator of place (GPS) and also for the measurement of the Sun – Moon distance.

It consisted of seven concentric interlocking rings. The 7th ring (exterior) was fixed in the level of the meridian and it had four marks that defined the horizontal and vertical. The 6th was graduated and it turned freely in the level of the meridian with points 0ο and 90ο to represent the equator and the pole respectively and it was placed in the direction of the earth axis.

The 5th was turned in the direction of the Sun. The 4th was articulated in the earth axis and observed the daily rotation of the astral sphere. The 3rd was graduated and was articulated to the precedent at a distance of approximately 66ο from the poles. It was placed in the ecliptic zodiac; it had the names of the star signs (zodiacs) and was used for the reading of geographic lengths of stars.

The 2nd was graduated, turned round a vertical axis in the level of the ecliptic and was used for the reading of geographic width of stars. Finally, the 1st ring (internal) had the aiming device.

SOURCES: "Ptolemy, Mathematical Syntaxis", "Pappus of Alexandria, Annotation in the Books 5 and 6 of the Mathematical Syntaxis".