Although time and its measurement is a fundamental component of human life, a special type of clock which takes matters to the next level is the so-called astronomical clock and whose purpose is not to measure time per se but to convey astronomical information and, in particular, the relative position of the Sun and Moon as well as the zodiacal constellations and, in some cases, the position of the planets all as a function of time.

The oldest such effort to effectively emulate the overhead celestial sphere (ie a mini-planetarium) is the Antikythera Mechanism which was discovered in 1901 totally by accident by sponge divers off the coast of the Greek island of Antikythera. It has been dated to approximately 80 BC and it is believed to be the work of Poseidonius of Rhodes. The Antikythera Mechanism is currently on display at the National Archaeological Museum in Athens, Greece.

Note: The astrolabe is yet another instrument constructed to describe the overhead sky and monitor time. Some versions also have religious implications such as the Muslim Qibla where a magnetic compass on the back side of the astrolabe helps the user identify the proper orientation so as to point at the holy city of Mecca for prayer. Astrolabes were first developed by the Greeks around the second century BC and were slowly adopted throughout the known world of the time by numerous cultures. Astrolabes describe the position of the planets and prominent stars visible at a particular latitude (see curved markings on each dial in the photo below) and, by extension, act as time-keeping devices. Similarly, they assist in the time of expected sunrise which is vital for the Muslim religion and the timing of morning prayers at dawn ("fajr") and thereafter at noon ("dhuhr"), afternoon ("asr"), sunset ("maghrib") and nightfall ("isha"). As a result, it is no surprise that the overwhelming number of astrolabes found in museums around the world are islamic in origin given the adoption of these instruments from around 700 AD and thereafter by the Islamic World. Today's planispheres can be said to be a direct extension of the ancient astrolabe, for they function and inform the user of precisely the same information sought-after by users many centuries ago.

The brass astrolabe below is one of two astrolabes available at the Societa Ligure di Storia Patria. The astrolabe is 178 mm in diameter, 8 mm thick and is both unsigned and undated. It has been studied intensely and has led to controversy surrounding its precise date and provenance. Perhaps the most detailed analysis was by Remondini in 1878 who described the character set of the inscriptions as Maghreb (ie North African and which had a rich history of astrolabe construction) and suggested a date of manufacture of 1632 to 1637 AD using the position of the most accurate stars denoted on the rete but did note that the date of vernal equinox cited on the back side, namely March 10th, suggests 1708 AD and concluded that a date around 1650 AD is perhaps the best estimate. Others have suggested the astrolabe to be the work of Ibn Baso (from 1304 AD) whereas King (1996) notes that the rete is in the tradition of al-Khama'iri (from Seville, Spain) and has nothing to do with Ibn Baso. The 29 star-pointers are quite obvious by virtue of the silver rounded bolt which is in place at the base for each star (three stars have the silver bolt missing). The astrolabe is accompanied by seven latitude plates (or tympanums) servicing latitudes 21° 40' to 45°, representing the region from Mecca to Istanbul, and is thus planespheric in nature, for it projects the three-dimensional celestial sphere onto a two-dimensional plane comprise of a single disk. During the Middle Ages, three-dimensional astrolabes were first described but only one such example seems to have survived to present day and which can be found at the History of Science Museum in Oxford, England (travel and photography forthcoming).

The city of Genoa is host to two museums which have at least one astrolabe in their collections, namely the Galata Museo del Mare (Galata Sea Museum) with one astrolabe and the Societa Ligure di Storia Patria (Ligurian Society of Patriotic History) where two astrolabes are available. The Societa Ligure di Storia Patria is a museum/library rich in manuscripts and other printed media which document the rich history of Genoa. It is open daily from 09:30-12:30 and 15:00-19:00 and admission is free. The Galata Museo del Mare is approximately a 15-min walk from the Societa Ligure di Storia Patria and at the heart of the city's historical center.

Note: For additional results involving astronomical clocks and astrolabes (including astrolabe quadrants) from around the world, please click here.