A sundial is a device that measures time by the position of the Sun. In common designs the sun casts a shadow onto a surface marked with lines indicating the hours of the day.
A sundial shows the current solar time during the day. It does this because the sun appears to move through the sky. At different times in the day when the sun is shining, a shadow is cast in different places on the dial. A person marks the dial with the time at a certain shadow. This lets users easily see the time. There are a few commonly seen designs, such as the 'ordinary' or standard garden sundial. However, sundials can be designed for any surface where a fixed object casts a predictable shadow.
Ancient Egypt had sundials. Other cultures developed them further, including the Greeks and Romans.
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A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style (a thin rod or a sharp, straight edge) onto a flat surface marked with lines indicating the hours of the day. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate. Such designs rely on the style being aligned with the axis of the Earth's rotation. Hence, if such a sundial is to tell the correct time, the style must point towards true North (not the north or south magnetic pole) and the style's angle with horizontal must equal the sundial's geographical latitude.
Sundials can be categorized in several ways: first, some sundials use a line of light to indicate the time, where others use the edge of a shadow. In the former case, the spot of light may be formed by allowing the sun's rays through a small hole or reflecting them from a small circular mirror; a line of light may be formed by allowing the rays through a thin slit or focusing them through a cylindrical lens. In the other case, the shadow-casting object — the sundial's gnomon — may be a thin rod, or any object with a sharp tip or a straight edge. Second, sundials employ many types of gnomon. The gnomon may be fixed or moved according to the season; it may be oriented vertically, horizontally, aligned with the Earth's axis, or oriented in an altogether different direction determined by mathematics. Third, sundials may use many types of surfaces to receive the spot or line of light, the shadow-tip or shadow-edge. Planes are the most common surface, but partial spheres, cylinders, cones and even more complicated shapes have been used for greater accuracy or intriguing aesthetics. Fourth, sundials differ in their portability and their need of orientation. The installation of many dials requires knowing the local latitude, the precise vertical direction (e.g., by a level or plumb-bob), and the direction to true North. In contrast, other dials are self-aligning; for example, two dials that operate on different principles, such as a horizontal and analemmatic dial, may be mounted together on one plate, such that their times agree only when the plate is aligned properly.
Sundials indicate the local solar time, unless otherwise corrected. To obtain the standard clock time, three types of corrections need to be made. First, the solar time needs to be corrected for the longitude of the sundial relative to the longitude at which the official time zone is defined. For example, a sundial located west of Greenwich, England but within the same time-zone, shows a later time than the official time; it will show "noon" after the official noon has passed, since the sun passes overhead later, since the sundial is further in the west. This correction is often made by rotating the hour-lines by an angle equaling the difference in longitudes. Second, the practice of daylight saving time shifts the official time away from solar time by an hour or, in rare cases, by another amount. This correction is usually made by numbering the hour-lines with two sets of numbers. Third, the orbit of the Earth is not perfectly circular and its rotational axis not perfectly perpendicular to its orbit, which together produce small variations in the sundial time throughout the year. This correction — which may be as great as 15 minutes — is described by the equation of time. A more sophisticated sundial design is required to incorporate this correction automatically; alternatively, a small plaque can be affixed to the sundial giving the offsets at various times of the year. The marker of the sundial,the gnomon is set parallel to the Earth's axis and pointing at the celestial pole.Its angle therefore depends on the latitude. The shadow of the gnomon falls on a dial, allowing the time to be read off.
A sundial at a particular latitude in one hemisphere must be reversed for use at the reciprocal latitude in the other hemisphere. A vertical direct south sundial in the Northern Hemisphere becomes a vertical direct north sundial in the Southern Hemisphere ; an opposite. To position a horizontal sundial correctly, one has to find true North or South. The same process can be used to do both. The gnomon, set to the correct latitude, has to point to the true South in the Southern hemisphere as in the Northern Hemisphere it has to point to the true North. Also the hour numbers on a horizontal dial run anti-clockwise rather than clockwise. Sundials are not as common in the Southern hemisphere as in the North. This is possibly because when Europeans arrived the mechanical clock was accurate enough for their purposes of time keeping and there was no need to erect sundials.
The most common reason for a sundial to differ from clock time is that the sundial has not been oriented correctly or its hour lines have not been drawn correctly. For example, most commercial sundials are designed as horizontal sundials as described above. To be accurate such sundials must have been designed for that latitude and their style must be parallel to the Earth's rotational axis; the style must be aligned with true North and its angle with the horizontal must equal the local geographical latitude. To align the style, the sundial can sometimes be tilted slightly on its north south axis.
Scottish sundials of the renaissance period are not just more numerous than in any other country, they are also stylistically unique. This is particularly notable when the size and wealth of Scotland at the time are taken into account. They are free standing stone sculptures of the 17th and 18th centuries with many small dials inscribed on them. Accurate timetelling is clearly not the aim. Precision is sacrificed for decorative effective. Unlike the sundials of similar date on the continent where baroque decoration may be added, the mathematical complexity of the Scottish dials is decoration enough. The ancient sundials of Scotland can be grouped into three main styles: lectern, obelisk and facet-head.
A nocturnal is an instrument used to determine the time based on the position of a certain star in the night sky. Sometimes called an "horologium nocturnum" (time instrument for night) or nocturlabe (in French and occasionally used by English writers), it is closely related to the sun dial. A nocturnal is typically a navigational instrument. Knowing the time is important in piloting for calculating tides and some nocturnals incorporate tide charts for important ports.
Moondials are time pieces similar to a sundial. The most basic moondial, which is identical to a sundial, is only accurate on the night of the full moon. Every night after it becomes an additional (on average)* 48 minutes slow, while every night preceding the full moon it is (again on average)* 48 minutes fast, assuming there is even enough light to take a reading by. Thus, one week to either side of the full moon the moondial will read 5 hours and 36 minutes before or after the proper time.
A digital sundial is a clock that indicates the current time with numerals formed by the sunlight striking it. Like a classical sundial, the device is purely passive and contains no moving parts. It uses no electricity nor other artificial sources of energy. The digital display changes as the sun advances in its daily course.
The Sundial Bridge is a cantilever spar cable-stayed bridge for bicycles and pedestrians that spans the Sacramento River in Redding, California, United States and forms a large sundial. It was designed by Santiago Calatrava and completed in 2004 at a cost of US$23,500,000.
The Carefree Sundial, in Carefree, Arizona, the "third largest sundial in the Western Hemisphere", designed by architect Joe Wong and solar engineer John I. Yellott (1908-1986), was erected in the Sundial Circle plaza in 1959. The sundial is made from a steel frame and covered in anodized copper. It measures 90 feet in diameter. The metal gnomon, the shadow-casting portion of the dial, stands 35 feet above the plaza and extends 72 feet.
The Saxon sundial at St Gregory's Minster, Kirkdale, near Kirkbymoorside, North Yorkshire, England dates to the mid 11th century. The panel containing the actual sundial above the church doors is flanked by two panels, bearing an inscription in Old English.
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