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Second World War Europe 1941
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as ​1--->⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Analog clocks and watches often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have a two-digit seconds counter
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Second (other)
A second is a unit of time. Second may also refer to:

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William IV, Landgrave Of Hesse-Kassel
William IV of Hesse-Kassel (24 June 1532 – 25 August 1592), also called William the Wise, was the first Landgrave of the Landgraviate of Hesse-Kassel (or Hesse-Cassel)
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Coordinated Universal Time
Coordinated Universal Time (or UTC) is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries, the term Greenwich Mean Time is used. The co-ordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967. The system has been adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments
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Universal Time
Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, London, UK. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see below). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time
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Strontium
Strontium is the chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly reactive chemically. The metal forms a dark oxide layer when it is exposed to air. Strontium has physical and chemical properties similar to those of its two vertical neighbors in the periodic table, calcium and barium. It occurs naturally mainly in the minerals celestine, strontianite and is mined mostly from the first two of these
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Terahertz (unit)
The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. Hertz are commonly expressed in multiples: kilohertz (103---> Hz, kHz), megahertz (106---> Hz, MHz), gigahertz (109---> Hz, GHz), and terahertz (1012---> Hz, THz). Some of the unit's most common uses are in the description of sine waves and musical tones, particularly those used in radio- and audio-related applications
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Gravitational Time Dilation
Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass. The higher the gravitational potential (the farther the clock is from the source of gravitation), the faster time passes. Albert Einstein originally predicted this effect in his theory of relativity and it has since been confirmed by tests of general relativity. This has been demonstrated by noting that atomic clocks at differing altitudes (and thus different gravitational potential) will eventually show different times. The effects detected in such Earth-bound experiments are extremely small, with differences being measured in nanoseconds. Relative to Earth's age in billions of years, Earth's core is effectively 2.5 years younger than its surface
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History Of Timekeeping
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. The Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae. The Zhou dynasty is believed to have used the outflow water clock around the same time, devices which were introduced from Mesopotamia as early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in ancient China, ancient Japan, England and Mesopotamia; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock
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Caesium
Caesium (IUPAC spelling) (also spelled cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium has physical and chemical properties similar to those of rubidium and potassium. The most reactive of all metals, it is pyrophoric and reacts with water even at −116 °C (−177 °F). It is the least electronegative element, with a value of 0.79 on the Pauling scale. It has only one stable isotope, caesium-133
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Orpheus
Orpheus (/ˈɔːrfiəs, ˈɔːrfjuːs/; Greek: Ὀρφεύς) is a legendary musician, poet, and prophet in ancient Greek religion and myth. The major stories about him are centered on his ability to charm all living things and even stones with his music, his attempt to retrieve his wife, Eurydice, from the underworld, and his death at the hands of those who could not hear his divine music. As an archetype of the inspired singer, Orpheus is one of the most significant figures in the reception of classical mythology in Western culture, portrayed or alluded to in countless forms of art and popular culture including poetry, film, opera, music, and painting. Orpheus was born as a son of the Muse Calliope and the Thracian king Oeagrus in a cave between Pimpleia and Leivithra. For the Greeks, Orpheus was a founder and prophet of the so-called "Orphic" mysteries
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Taqi Al-Din Muhammad Ibn Ma'ruf
Taqi ad-Din Muhammad ibn Ma'ruf ash-Shami al-Asadi (Arabic: تقي الدين محمد بن معروف الشامي, Turkish: Takiyüddin or Taki) (1526–1585) was a Ottoman era polymath active in Constantinople. He was the author of more than ninety books on a wide variety of subjects, including astronomy, clocks, engineering, mathematics, mechanics, optics and natural philosophy. In 1574 the Ottoman Sultan Murad III invited Taqī ad-Dīn to build the Constantinople observatory. Using his exceptional knowledge in the mechanical arts, Taqī ad-Dīn constructed instruments like huge armillary and mechanical clocks that he used in his observations of the Great Comet of 1577. He also used European celestial and terrestrial globes that were delivered to Istanbul in gift-exchange
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Jost Bürgi
Jost Bürgi (also Joost, Jobst; Latinized surname Burgius or Byrgius; 28 February 1552 – 31 January 1632), active primarily at the courts in Kassel and Prague, was a Swiss clockmaker, a maker of astronomical instruments and a mathematician.

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Tycho Brahe
Tycho Brahe (/ˌtk ˈbrɑːhi, ˈbrɑː, ˈbrɑːə/, born Tyge Ottesen Brahe (Danish: [ˈtyːə ˈʌdəsn̩ ˈbʁɑː]); 14 December 1546 – 24 October 1601) was a Danish nobleman, astronomer, and writer known for his accurate and comprehensive astronomical and planetary observations. He was born in the then Danish peninsula of Scania. Well known in his lifetime as an astronomer, astrologer and alchemist, he has been described as "the first competent mind in modern astronomy to feel ardently the passion for exact empirical facts." His observations were some five times more accurate than the best available observations at the time. An heir to several of Denmark's principal noble families, he received a comprehensive education. He took an interest in astronomy and in the creation of more accurate instruments of measurement
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Leap Year
A leap year (also known as an intercalary year or bissextile year) is a calendar year containing one additional day (or, in the case of lunisolar calendars, a month) added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting (also called intercalating) an additional day or month into the year, the drift can be corrected. A year that is not a leap year is called a common year. For example, in the Gregorian calendar, each leap year has 366 days instead of the usual 365, by extending February to 29 days rather than the common 28. These extra days occur in years which are multiples of four (with the exception of years divisible by 100 but not by 400)
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Carl Friedrich Gauss
Johann Carl Friedrich Gauss (/ɡs/; German: Gauß, pronounced [ɡaʊs] (About this sound
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