Ephemeris second

International System of Weights and Measures defined the second to be 1/ 31,556,925.9747 for the tropical year 1900 January 0 at 12 hours ephemeris time. 4 In 1960 this standard was accepted by the General Conference with the caveat that work continue toward development of an atomic clock for the accurate measurement of time. 

The definitions of the seven SI base units are subject to changes as experimental methods lead to an increase in precision and accuracy. This led in the past to a redefinition of the second by counting the periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. In 1967/68 it replaced the previously adopted definition of the second as the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 h ephemeris time. Today, for example, one wishes to redefine the kilogram (along with the ampere, the kelvin and the mole) in terms of fundamental physical... 

The standard to measure the base unit of time—the second—has evolved as much as the standard to measure distance. During the 17-19th centuries, the second was based on the Earth s rotation and was set equal to 1 /86 400 of a mean solar day. In 1956, recognizing that the rotation of the earth slows with time as the Moon moves further away (about 4 cm yr ), Ephemeris Time became the SI standard 1/31556925.9747 the length of the tropical year of 1900. In 1967, the second was based on the number of periods of vibration radiation emitted by a specific wavelength of Cs. 

Unit of time n. The fundamental invariable unit of time is the ephemeris second, which is defined as 1/31,556,925.9747 of the tropical year for 1900 January 0 12 ephemeris time. The ephemeris day is 86,400 ephemeris seconds. 

The ephemeris second (unit of time) is exactly 1/31 556 925.974 7 of the tropical year of 1900, January, 0 days, and 12 hours ephemeris time. 

Time second s (proper fraction of the ephemeris year of 1900, matched closely by the cesium 133 clock)... 

The unit of time, the second, is abbreviated by small s (and not sec). For all measurement purposes it is the ephemeris second, the properly fked fraction of the mean solar year 1900. The SI unit is chosen to match this time-interval closely using a cesium 133 clock. This definition makes the SI time independent of the astronomical time. The cesium 133 clock is capable of an accuracy of one part in 10. An ordinary stopwatch, not even a quartz watch, is usually sufficient for thermal analysis experiments. 

The ephemeris second served as the SI second from 1956 to 1967. The ephemeris second was a fraction of the tropical year, or the interval between the annual vernal equinoxes, which occm on or about March 21. The tropical year was defined as 31,556,925.9747 ephemeris sec. Determining the pieeise instant of the equinox is difficult, and this limited the uncertainty of Ephemeris Time (ET) to 50 msee over a 9-year interval. ET was used mainly by astronomers and was replaced by Terrestial Time (TT) in 1984, equal to International Atonrie Time (TAI) + 32.184 see. The uncertainly of TT is 10 psec. 

---