"सेकेंड": अवतरणों में अंतर

In 1956 the second was defined in terms of the period of revolution of the [[Earth]] around the Sun for a particular [[epoch (astronomy)|epoch]], because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in [[Newcomb's Tables of the Sun]], which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made between 1750 and 1892.<ref name="USNO"/> The second thus defined is

 

 

This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. The ''tropical year'' in the definition was not measured, but calculated from a formula describing a mean tropical year which decreased linearly over time, hence the curious reference to a specific ''instantaneous'' tropical year. Because this second was the independent variable of time used in [[ephemeris|ephemerides]] of the Sun and Moon during most of the twentieth century (Newcomb's Tables of the Sun were used from 1900 through 1983, and [[Ernest William Brown|Brown]]'s Tables of the Moon were used from 1920 through 1983), it was called the ''ephemeris second''.<ref name="USNO"/>