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 seven astrolabes on display at the Chester Beatty Library in central Dublin comprised of one standard (circular) astrolabe and six (wooden) quadrant astrolabes. Its construction is credited to Abd al-Husayn ibn Abd al-A'imma from Isfahan, Iran and who was recognized as an accomplished astrolabe craftman with approximately 50 such instruments bearing his name. The unit below was constructed between 1708 and 1709 AD and dedicated to the Safavid shah of Iran Sultan Husayn (1668-1727). The astrolabe is quite large measuring 197 mm in height and 229 mm in width and is accompanied by six plates representing eleven latitudes whereas two quadrants, one sinical and one zodiacal, are inscribed on the backside and which include shadow squares. The astrolabe is planespheric in nature, for it projects the three-dimensional celestial sphere onto a two-dimensional plane comprised of the six aforementioned metal plates which correct for differences due to latitude, and is inscribed in arabic. 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.

The Chester Beatty Library and Museum in central Dublin is regarded as one of the top must-see attractions for any visitor to the Irish capital. The museum is named after the mining magnate Alfred Chester Beatty (1875-1968) who, along with his wife, was a philanthropist and avid collector of north African, Asian, European and Middle Eastern manuscripts and rare printed books which were eventually bequeathed to the Irish government and its people along with the museum he had established near Dublin Castle in the early 1950's to house his prized collection. The number of catalogued items exceeds 25,000 in number and includes particularly rare early papyri of christian and islamic religious manuscripts and printed books across two galleries, namely "Sacred Traditions" and "Arts of the Book" with the latter also having a section on astronomy books as well as the 18th century brass astrolabe below and six Turkish wooden astrolabe quadrants. The Chester Beatty Library and Museum is considered one of the premier museums in the world and won the European Museum of the Year award in 2002. The museum is open daily with free admission (Tuesday to Saturday: 09:45-17:30 except Wednesday (09:45-20:00) and Sunday (12:00-17:30)).

Note: For additional results involving astronomical clocks and astrolabes from around the world, please click here.