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 Mecca for prayer. It is believed that 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. 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 Umayyad brass astrolabe below is currently on display at the British Museum (London) and forms part of the Albukhary Foundation Gallery of the Islamic world on Level 3 (rooms 42 and 43) where a total of nine astrolabes are on display. The astrolabe's manufacture is credited to the al-Andalusi astrolabist Muhammad ibn al-Saffar of Cordoba, Spain, and represents the oldest known astrolabe from Muslim Spain. Made in 1020-21 AD, the astrolabe is one of three astrolabes credited to Muhammad ibn al-Saffar (the other two reside at the Royal Scottish Museum in Edinburgh and the Deutsche Staatsbibliothek in Berlin) and is considered a historically significant work. The astrolabe measures 12.5 cm in height, is accompanied by three tympanums serving latitudes 30° to 40° with plates specific for Cairo (Egypt); Almeria, Cordona, Ibiza, Toledo, Valencia, Denia and Badajoz (Spain); Harran and Tarsus (Turkey); and Samarqand (Uzbekistan) and has a rete with twenty-seven star pointers and Latin inscriptions for the constellations which suggests a later modification believed to have occurred during the 13th to 14th centuries AD. As noted, the astrolabe is accompanied by a total of three latitude plates, thus correcting for differences due to latitude and making the astrolabe planespheric in nature, for it projects the three-dimensional celestial sphere onto a two-dimensional plane comprised of disks with the latter correcting for differences due to latitude. 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). For further information on Muhammad ibn al-Saffar, see here.

The British Museum in central London is widely regarded as one of the finest museums in the world. Founded in 1753 AD, the British Museum boasts the largest permanent collection in the world with over eight million works, is comprised of ninety-four galleries and is rewarded with about six million visitors annually. Admission to the British Museum is free but an online ticket reservation is highly recommended for quick entry with visitation hours being between 10:00 and 17:00 daily with Friday's closing hour being later (20:30). Public transportation includes four stations all around the museum (Tottenham Court Road, Holborn, Russell Square and Goodge Street).

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