For example, six fractional sexagesimal places of a day was used in their specification of the length of the year, although they were unable to measure such a small fraction of a day in real time. The day was subdivided sexagesimally, that is by 1 ⁄ 60, by 1 ⁄ 60 of that, by 1 ⁄ 60 of that, etc., to at least six places after the sexagesimal point by the Babylonians after 300 BC, but they did not sexagesimally subdivide smaller units of time. Instead they used simple fractions of an hour. AD 150) subdivided the day sexagesimally and also used a mean hour ( 1 ⁄ 24 day), but did not use distinctly named smaller units of time. The Hellenistic astronomers Hipparchus ( c. The Egyptians had 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).The international standard symbol for a second is s (see ISO 31-1) The second thus defined is equivalent to the ephemeris second, which was based on astronomical measurements. The ground state is defined at zero magnetic field. This definition refers to a caesium atom at rest at a temperature of 0 K ( absolute zero). Under the International System of Units, the second is currently defined as 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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