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 wooden astrolabe quadrant below is one of approximately twenty-five astrolabe quadrants on display at Museo Galileo in central Florence. 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 quadrant below measures 16.7 cm in diameter and is dated from 1556 AD whereas it is calibrated for Florence at 43° 40' latitude. As a result, it is almost certain that the craftsman behind this work is Giovanni Battista Giusti who maintained his own workshop in Florence during the latter half of the 16th century AD. Giusti was known for making scientific instruments with a number of his works on display at Museo Galileo including two other very similar signed quadrants by him being available (ex. see here and here). A graduated arc along the quadrant's curved periphery at the bottom representing 90 degrees starts from the bottom left corner and continues onto the right corner with demarcations and text for every five degrees. 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 the aforementioned 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 quadrant 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 slanted arcs seen in the astrolabe below and which dominate its front side. Also present is a shadow square in the upper corner near the apex with the typical references to "Umbra Versa" and "Umbra Recta" which provide the respective vertical and horizontal scales.

The historic center of Florence is characterized with a number of epic museums (with very impressive collections) which are a must-see while visting the city. One such museum is the impressive Museo Galileo (Galileo Museum) and which was originally named Museo di Storia della Scienza (History of Science Museum) owing to its rich collection of scientific instruments including many astrolabes, horary quadrants, nocturnals, sundials, armillary spheres as well as Galileo's instruments including his telescopes. The museum's collections are displayed across two floors and eighteen thematic rooms including "Astronomy and Time" (Room II), "The Science of Navigation" (Room V), "The Science of Warfare" (Room VI), "Galileo's New World" (Room VII), "After Galileo: Exploring The Physical and Biological World" (Room IX), "The Spectacle of Science" (Room XI), "Measuring Natural Phenomena" (Rooms XV and XVI) and "Science at Home" (Room XVIII). Museo Galileo is open daily between 09:30 and 18:00 except for Tuesdays when it is open 09:30 to 13:00. Admission costs 13 euro and one can easily be engaged for a number of hours. The Museo Nazionale del Bargello with its magnificent collection of art and sculpture masterpieces is less than 400 meters to the north and only a few minutes away by foot with two astrolabes and an astrolabe quadrant in its collection and on display whereas the Fondazione Scienza e Tecnica with its impressive collection of scientific instruments from the nineteenth and twentieth centuries is 1200 meters further north and boasts a Galleria dell'Astronomia with a rich suite of astronomical instruments including an astrolabe.

Note: For additional results involving astronomical clocks and astrolabes from around the world, please click here.