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 large 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 Persian astrolabist Abd Ali ibn Muhammad Rafi al-Juzi and his brother (Muhammad Baqir) who was responsible for the the instrument's decoration. Made in 1712 AD, the astrolabe is quite large measuring approximately 40 cm in width and 3.40 cm in thickness. In fact, given its size and weight, it is believed that the astrolabe was perhaps intended for use in a mosque rather than for personal (portable) use. The astrolabe was made for the Shah Soltan Hoseyn (1668-1726 AD) and who was the last Safavid monarch of Iran (1694-1722 AD). The astrolabe is accompanied by seven tympanums serving latitudes 25° to 43° whereas the rete is particularly rich with sixty-three star pointers with the base of the mater having a very impressive engraved list of over 100 cities characteristic of the Islamic World. Finally, the triangular crown is characterized with arabic text inscribed across eight lines praising both God and the emperor. As noted, the astrolabe is accompanied by seven latitude plates, thus 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 a thorough analysis and description of this astrolabe, see the book by William Morley entitled "Description of a planispheric astrolabe constructed for Shah Sultan Husain Safawi, king of Persia, and now preserved in the British Museum" 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.