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 quadrant below is one of approximately six astrolabe quadrants in the collection of the British Museum in central London and certainly represents one of the most photogenic quadrants one will ever encounter. Astrolabe quadrants were simplified versions of the more complicated standard astrolabe and were usually made either of wood or brass with a very small number made of other materials including iron and ivory. Constructed in 1399 AD, the quadrant below was made for Richard II (1367-1400 AD) who reigned over England between 1377 and 1399 AD and whose emblem is depicted in the circle just below the apex where a tree with a rabbit immediately below are depicted separated by a scroll with the text "Tabula bisexti" and which refers to the leap-years depicted by the small ring. The quadrant measures 92 mm in diameter and is 6 mm thick, weighs 157 grams and is attributed to the well-known scientific instrument maker of the time Charles Whitwell. The quadrant is calibrated for a latitude of 52° north and which matches that of London. The front face of the quadrant includes the month of the year on the lower left edge whereas the day of the month is depicted on the ring at the bottom with demarcations ranging from 1 to 31 and using 1-day intervals with the tabular data at the intersection of these two items providing the altitude of the Sun when reaching the meridian. The apex of the quadrant normally holds a plumb bob which swings depending on the angle produced when aligning the quadrant's sighting scope with the celestial object of interest, thus immediately providing the altitude of the object such as the daytime Sun using graduated arcs. The determination of the altitude of a celestial object immediately allows one to identify the local time and, hence, astrolabes and quadrants were vital timing devices and clocks.

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 metro 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.