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"second; s"

From Nick Greene,
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Definition: second; s: The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. A nanosecond is equal to one-billionth (10-9) of a second.
    rp = a(1-e) and ra = a(1+e).
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