A clock is a device used to measure, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.
Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the Elements. Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions (theorems) from these. Although many of Euclid's results had been stated by earlier mathematicians, Euclid was the first to show how these propositions could fit into a comprehensive deductive and logical system. The Elements begins with plane geometry, still taught in secondary school as the first axiomatic system and the first examples of formal proof. It goes on to the solid geometry of three dimensions. Much of the Elements states results of what are now called algebra and number theory, explained in geometrical language.
A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate and a gnomon, which casts a shadow onto the dial. As the Sun appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.
A diptych is any object with two flat plates which are a pair, these are often attached at a hinge. For example, the standard notebook and school exercise book of the ancient world was a diptych consisting of a pair of such plates that contained a recessed space filled with wax. Writing was accomplished by scratching the wax surface with a stylus. When the notes were no longer needed, the wax could be slightly heated and then smoothed to allow reuse. Ordinary versions had wooden frames, but more luxurious diptychs were crafted with more expensive materials.
A gnomon is the part of a sundial that casts a shadow. The term is used for a variety of purposes in mathematics and other fields.
The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of "reconcile a difference". The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion. Apparent solar time can be obtained by measurement of the current position of the Sun, as indicated by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero.
Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day. The more familiar 12-hour analog dial has an hour hand that makes two complete revolutions in a day.
Analemmatic sundials are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern. The gnomon is not fixed and must change position daily to accurately indicate time of day. Hence there are no hour lines on the dial and the time of day is read only on the ellipse. As with most sundials, analemmatic sundials mark solar time rather than clock time. An analemmatic sundial is completely defined by
- The size of its ellipse.
- The latitude of its location.
- The declination of the sun.
François Lamathe Bédos de Celles de Salelles, known as Dom Bédos de Celles, was a Benedictine monk best known for being a master pipe organ builder.
For thousands of years, devices have been used to measure and keep track of time. The current sexagesimal system of time measurement dates to approximately 2000 BC from the Sumerians.
Astronomical rings, also known as Gemma's rings, are an early astronomical instrument. The instrument consists of three rings, representing the celestial equator, declination, and the meridian.
A sundial is a device that indicates time by using a light spot or shadow cast by the position of the Sun on a reference scale. As the Earth turns on its polar axis, the sun appears to cross the sky from east to west, rising at sun-rise from beneath the horizon to a zenith at mid-day and falling again behind the horizon at sunset. Both the azimuth (direction) and the altitude (height) can be used to create time measuring devices. Sundials have been invented independently in every major culture and became more accurate and sophisticated as the culture developed.
The Whitehurst & Son sundial was produced in Derby in 1812 by the nephew of John Whitehurst. It is a fine example of a precision sundial telling local apparent time with a scale to convert this to local mean time, and is accurate to the nearest minute. The sundial is now housed in the Derby Museum and Art Gallery.
A London dial in the broadest sense can mean any sundial that is set for 51°30′ N, but more specifically refers to a engraved brass horizontal sundial with a distinctive design. London dials were originally engraved by scientific instrument makers. The trade was heavily protected by the system of craft guilds.
A schema for horizontal dials is a set of instructions used to construct horizontal sundials using compass and straightedge construction techniques, which were widely used in Europe from the late fifteenth century to the late nineteenth century. The common horizontal sundial is a geometric projection of an equatorial sundial onto a horizontal plane.
The Sunquest Sundial is a sundial designed by Richard L. Schmoyer in the 1950s. Adjustable for latitude and longitude, the Sunquest's gnomon automatically corrects for the equation of time allowing it to tell clock time.
Vertical declining dials are sundials that indicate local apparent time. Vertical south dials are a special case: as are vertical north, vertical east and vertical west dials. The word declining means that the wall is offset from one of these 4 cardinal points. There are dials that are not vertical, and these are called reclining dials.
A sundial cannon, sundial gun, noon cannon or meridian cannon, also noonday gun is a device consisting of a sundial incorporating a cannon with a fuse that is lit by an overhanging lens, concentrating the rays of the sun, and causing the cannon to fire at noon, when properly oriented along a north–south axis. The cannon sizes ranged from large to small depending on the location of their use. The household variety was used in estates to signal the time for the midday meal. Larger sizes were used in European parks to signal noon.
Albert Edmund Waugh was an American economist and long-time academic administrator at the University of Connecticut (UConn) from 1924 to 1965. He served as provost from 1950 to 1965. Waugh's journal, which he kept daily from 1941 to 1974, gave unique insight into the history, academics, and life of the university and was an important primary source for historians such as Bruce M. Stave.
