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ephemeris second

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ephemeris second [ə′fem·ə·rəs ′sek·ənd]
(astronomy)
The fundamental unit of time of the International System of Units from 1960 until 1968, equal to 1/31556925.9747 of the tropical year defined by the mean motion of the sun in longitude at the epoch 1900 January 0 day 12 hours.
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This lead to the present definition of the ephemeris second based on the oscillation of the Cesium 133 atom in 1967.
Another positive Leap Second was introduced on 31 December 2008 by Monthly Notes of the Astronomical Society of Southern Africa
 
 
 
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