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 brass astrolabe below is one of four astrolabes currently on display at the Victoria and Albert Museum in London and is signed by Abd al-Aimmah of Isfahan, Iran. The astrolabe is 19.6 cm high (with crown) and 12.5 cm wide and is dated 1715 AD as per the inscription accompanying the craftsman's name. Although there are five tympanums which accompany the astrolabe, there is no information with respect to the latitudes they serve but one would expect to be in the general vicinity of 32° 40' and which is the latitude of Isfahan Iran. The image below illustrates the various components and layers which make up a functional astrolabe comprised of the throne (with or without inscription; decorated with floral patterns in this unit) 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) as seen in the image below. The mater acts as a receptor for the tympanums (five 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 in the middle of 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 richly engraved with a gazetteer of cities significant to the Islamic World, a feature characteristic of only a small number of astrolabes, where supplemental data for each city may include longitude and latitude, the azimuth of the Qibla and the distance to Mecca. The availability of five tympanums which accompany the astrolabe make 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).

The Victoria and Albert Museum in London is one of the premier museums in the city with over 2.8 million works in its collection comprised of five thousand years worth of human creativity and inspiration. The foundation stone was laid by Queen Victoria in 1899 with the museum having expanded dramatically to now include five venues across London with the South Kensington branch having four astrolabes currently on display (room 42: Islamic Middle East section; room 99: Photography; and room 116: Metalware). The museum is open daily between 10:00 and 17:45 (10:00 to 22:00 on Fridays) and admission is free with no advance booking required. Access to the museum is trivial using London's undergound tube and where one exits at the "South Kensington" station (easily reachable by the District, Circle and Piccadilly lines) and follows an underground tunnel approximately 750 meters in total length. A brief walk leads to the proper underground entrance to the V&A Museum approximately halfway through the tunnel. Walking the full length of the tunnel leads to an exit above ground and which immediately leads one to the Science Museum no more than 20 meters away to the right as they exit (the Science Museum is physically across the street from the V&A Museum).

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