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second |
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second, abbr. sec or s, fundamental unit of time time, sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological. ..... Click the link for more information. in all systems of measurement. In practical terms, the second is 1/60 of a minute, 1/3,600 of an hour, or 1/86,400 of a day. Since the length of the day day, period of time for the earth to rotate once on its axis. The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian . ..... Click the link for more information. varies, however, the second must be defined in more precise terms. For many years it was defined as 1/86,400 of the mean solar day (see solar time solar time, time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial . ..... Click the link for more information. ), thus eliminating seasonal variations. Because the rotation of the earth itself is not constant, the second was redefined (1956) in terms of ephemeris time (ET), which is calculated from the motions of celestial bodies in accordance with the laws of motion; 1 sec is 1/31,556,925.9747 of the length of the tropical year for 1900. In 1967 the second was redefined to be 9,192,631,770 periods of vibration of the radiation emitted at a specific wavelength by an atom of cesium-133. |
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| These include pointing at 80 to 250 celestial targets with an accuracy of 5 to 10 arc seconds. We drew on our experience integrating more than 550 payloads to achieve alignments on the spacecraft platform within arc seconds (one arc second equals 1/3,600 degree). 2 arc seconds apart, registers in radio wavelengths, while both can be seen in the visible range. |
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