0 at 12 hours ephemeris time[6] However, astronomical observations of the 19th and 20th centuries revealed that the mean solar day is slowly but measurably lengthening, and the length of a tropical year is not entirely predictable either. Thus the sunâ€"earth motion was not considered a suitable basis for the definition. With the advent of atomic clocks, it became feasible to define the second based on a fundamental property of nature. Thus, a mere seven years later in 1967, the International Committee for Weights and Measures (abbreviated CIPM from the French Comité international des poids et mesures) changed the definition to "the duration of 9192631770 The second (symbol: s) (abbreviated s or sec) is the base unit of time in the International System of Units (SI).[1][2] It is qualitatively defined as the second division of the hour by sixty, the first division by sixty being the minute.[3] It is quantitatively defined in terms of a certain number of periods â€" about 9 billion â€" of a certain frequency of radiation from the caesium atom: a so-called atomic clock. Seconds may be measured using a mechanical, electric or atomic clock.
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