The astrolabe of Ptolemy, or spherical astrolabe, was an ingenious astronomical instrument of antiquity. It simulated the celestial sphere and its rotation and is also known as an armillary sphere because, instead of having a solid spherical surface depicting the most important stars and constellations, it was constructed solely of rings representing the principal celestial circles, such as the celestial equator and the ecliptic with the band of the zodiac constellations. Adjustment for the observer’s meridian and geographical latitude was made on the outer rings, while at the center there were graduated rings with a sighting system. Thanks to this system, Ptolemy’s astrolabe served not only as a common teaching instrument but also as an observational device for measuring and recording the coordinates of stars or planets. Unfortunately, none has survived, but Claudius Ptolemy (100–170 AD) describes it in detail in his treatise Mathematical Syntaxis or Megiste, which became known in the West as the Almagest. The depiction of Ptolemy’s astrolabe on the obverse of the coin is based on a reconstruction by the Kostas Kotsanas Museum of Ancient Greek Technology.