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.

One of the largest astrolabes one will ever encounter is the magnificent example below and which forms part of the impressive collection of over 20 astrolabes at the Museo Galileo in Florence. Measuring a whopping 840 mm in diameter, the brass astrolabe is embedded in an octagonal (oak?) casing and which can be tilted so as to accommodate its user. The astrolabe is accompanied by a single tympanum (latitude plate) and is suitable for someone at latitude 43°40' which, of course, equates to Florence. The astrolabe was originally credited to Egnazio Danti but later research suggests that the craftsman behind this work of art is Giovanni Battista Giusti who maintained a workshop in Florence and was an active scientific instrument manufacturer. The astrolabe was used at one time by Galileo Galilei (1564-1642 AD) and, as a result, is also referred to as "Galileo's Astrolabe". Although no specific manufacture date is available, it is obviously from around the 16th century AD. With only one latitude disk produced for this astrolabe, it can be safely categorized as planespheric in nature, for it projects the three-dimensional celestial sphere onto a two-dimensional plane comprised of the aforementioned plate. 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.

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: Due to the impressively large size of Galileo's Astrolabe, the resampling below leads to a loss of fine detail which is otherwise available when examining the original high-resolution image. For two sample segments at 100% resolution, please click here and here.

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