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 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 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 Brown's Tables of the Moon were used from 1920 through 1983), it was called the ephemeris second.