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 quadrant below is one of approximately six astrolabe quadrants in the collection of the British Museum. 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. The astrolabe is both signed and dated thanks to the initials on the back side and which identify Thomas Gemini as the craftsman whereas the text immediately below and to the left of the apex describe the year of manufacture, namely 1551 AD, and the latitude being served, namely 51° 34', and which corresponds to London. The fine engraving is quite notable on the front side of the quadrant thanks to the hour lines (both equal and unequal versions) as well as the graduated arc used to identify altitude. Thomas Gemini (1515-1562 AD) was a fairly productive scientific instrument maker with a number of his works having survived to modern day and including four confirmed astrolabes and two quadrants as well as another suspected astrolabe most probably being his own creation. The bottom of the quadrant below is characterized with three rings: the innermost ring, running from left to right, represents a graduated arc ranging from 0 to 90° with demarcations every 10°; immediately below is a supplementary arc with equally spaced squares with ten such squares for each marking in the arc immediately above and which effectively provides one-degree markings for each one-degree interval; a third and final graduated arc at the bottom of the quadrant provides yet further accuracy using eight smaller white and black squares now representing 0.125° intervals. 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. Of particular interest in the astrolabe below is the depiction of the unequal hour system (aka temporal or seasonal hours) and where each day is divided into 12 segments for daytime (ie. sunrise to sunset) and nighttime (ie. sunset to sunrise) irrespective of season. As a result, a segment for winter is much less than a segment during summer owing to the fact that days during winter are much shorter than days during summer. This concept and discrepancy between segments and seasons is reflected by the very slanted arcs seen in the astrolabe below and which dominate its front side. The second set of arcs which are only slightly slanted represent the modern-day concept of time and where hours are of equal duration but with day and night represented by a variable number of hours depending on seasonality (ignoring the equinoxes).

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).

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