Portal:Time
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 Time is a fundamental component of our measuring system and has long been a major subject of art, philosophy, and science. In physics and other sciences, time is considered a fundamental quantity; part of the basic structure of the universe, a dimension in which events occur in sequence. The unit of time in the International System of Units (SI) is the second...
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An atomic clock is a type of clock that uses an atomic resonance frequency standard to feed its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on absorption spectroscopy of cold atoms in atomic fountains. National standards agencies maintain an accuracy of 10-9 seconds per day (approximately 1 part in 1014), and a precision set by the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). 
A sundial is a device that measures time by the position of the Sun. The most commonly seen designs, such as the 'ordinary' or standard garden sundial, cast a shadow on a flat surface marked with the hours of the day. As the position of the sun changes, the time indicated by the shadow changes. However, sundials can be designed for any surface where a fixed object casts a predictable shadow.  John Harrison (24 March 1693 – 24 March 1776) was an English clockmaker who revolutionised and extended the possibility of safe long distance sea travel in the Age of Sail by inventing a long-sought and critically-needed key piece in the problem of accurately establishing the East-West position, or longitude, of a ship at sea. The problem was so intractable that the British Parliament offered a huge fortune for the day (£20,000, roughly £6 million in 2007 terms), for a solution. ...that the second was known as a "second minute", the second small division of an hour. ...that the second is defined as the duration of 9,192,631,770 oscillations between the two hyperfine levels of the ground state in the Cesium-133 atom. ...that the smallest unit of time that could ever be measured is the Planck time (~ 5.4 × 10−44 seconds).
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