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 "exploding" Ayyubid 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 is unsigned and, as a result, the craftsman behind its manufacture is unknown. Although the astrolabe was found in Syria, it is considered to be Egyptian owing to the presence of tympanums with inscribed latitudes that serve Cairo and Alexandria. The astrolabe is dated from the 13th century AD and measures 25.50 cm in height (with the throne), 17.0 cm in diameter and is 3.50 cm thick. Its presentation in the image below illustrates the various components and layers which make up a functional astrolabe comprised of the throne (with or without inscription) used to secure the astrolabe by hand or some sort of suspended ring or hook and which is physically attached to the indented ringed plate (or mater) visible at the bottom of the image below. The mater acts as a receptor for the tympanums (middle three plates with visible curved azimuth lines in the image below) which are held in place using a pin which is passed through the rule at the front and secured on the back side of the astrolabe with the alidade (sighing scope) visible at the absolute bottom of the image below. Resting on the tympanum(s) is the rete (spider-like vane at the top in the image below) which not only identifies the overhead star patterns to be matched against the stars on a particular tympanum serving the user's latitude but also rotates so as to help in the precise alignment between the overhead sky and the plate's inscribed star patterns. During the day, the astrolabe is used in a slightly different manner and where the alidade is used to point to the celestial object of interest (ex. the Sun or Moon) and the front side is then consulted to extract the altitude of the object thanks to a graduated scale at the periphery which ranges from 0 to 360 degrees. The mater in the example below is totally blank and which is typical of most astrolabes whereas a select but small number of astrolabes are enriched with a gazetteer of cities, for example those in the Islamic World, with supplemental data. The availability of three tympanums clearly identfies this astrolabe as 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).

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.