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 very impressive brass astrolabe below is one of three astrolabes on display at the Science Museum in London. The astrolabe is the work of Muhammad ibn Abi Bakr from Isfahan, Iran, measures 185mm in diameter and is dated 1221-1222 AD. It is very unique due to the presence of a gear system embedded into the back side of the mater and which is responsible for the physical display of the Moon's phase and age as well as its position relative to the Sun (and zodiac) with all of this information presented via three small windows thanks to the interaction of five interacting gears. The crown has been described as Indo-Persian whereas the mater has the outer ring inscribed in Arabic with the womb being devoid of any text or graphics. The astrolabe is accompanied by two tympanums which serve latitudes 30° 0', 32° 0' (ex Lahore), 36° 0' and 40° 0'. The rete includes a quatrefoil in the inner ring along with zoomorphic star-pointers as well as two sighting vanes. The two latitude-specific plates characterize the 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 "Exploring Space" exhibit is located on the ground floor of The Science Museum of London and hosts the astrolabe below. Admission is free but an online reservation for a (free) ticket is not only highly adviseable but ensures immediate entrance and without any need to stand in queue. The hours of operation of the museum are 10:00 to 18:00 and a visitor will easily be kept highly engaged for a few hours. The metro station "South Kensington" has an underground tunnel approximately 750 meters in length and which leads to the proper entrance above ground and about 20 meters to one's right as they exit the tunnel.

Note: The astrolabe below is a replica and faithful copy of the original astrolabe currently in the collection of the Oxford Museum of the History of Science (MHS Inventory No 48213).

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