The Hellenistic astronomers Hipparchus ( c. The Egyptians subdivided daytime and nighttime into twelve hours each since at least 2000 BC, hence their hours varied seasonally. 1/31,557,600 Julian year (IAU system of units).Free space (which, like absolute zero, is a theoretical reference state that cannot be attained in practice, with exact values for its electromagnetic properties: c 0, μ 0, ε 0, and Z 0) is the reference state for the SI units for the metre and ampere. Consequently, the transition in quantum vacuum may not have the same frequency as in free space. In practice, the transition is measured in the quantum vacuum where vacuum fluctuations can lead to shifts in atomic energy levels in vacuum relative to their values in free space, for example, to a Lamb shift. The realization of the standard second is described briefly in a special publication from the National Institute of Science and Technology, and in detail by the National Research Council of Canada. (See History below.) The international standard symbol for a second is s (see ISO 31-1). The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements. The ground state is defined at zero magnetic field. This definition refers to a cesium atom at rest at a temperature of 0 K (absolute zero), and with appropriate corrections for gravitational time dilation. The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
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