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 supplements 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 named after Sir Hans Sloane whose collection included this enigmatic piece and which was eventually bequeathed to the British Museum in 1753 as part of the opening/initial collection. Although the craftsman behind this impressive piece is not known, various details (see later) strongly suggest the work to be that of a very skilled English craftsman. Measuring 462 mm in diameter and 12 mm in thickness, this very large astrolabe represents one of the oldest and largest examples of an English astrolabe to have survived the Middle Ages given its dating (1290-1300 AD). The application of laquer in the past has helped darken some of the astrolabe's components and in spite of the fact the basic material is brass. Three accompanying tympanums serving latitudes 48° 30' to 54° include specific references to London, Rome and Paris. Furthermore, the inscription of three saints of particular English interest (Dunstan, Augustine of Carterbury and Edmund) at the back of the plate for London further reinforce the belief this astrolabe to be of English origin. The rete is gothic in design and includes three quatrefoils as well as two trefoils in the inner circle and a further three semi-quatrefoils in the outer circle. Also present on the rete are fourty star-pointers. The perimeter of the mater has a graduated scale ranging from 0 to 360 degrees (five-degree increments) and the absence of a rule is very much worthy of note whereas the throne is blank with no inscribed text. Given the presence of latitude plates serving specific latitudes, the astrolabe can be considered to be 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.