Each of these seconds is slightly longer than an SI second because Earth's solar day is now slightly longer than it was during the 19th century, due to ephemerides between 1900 and 1983, so this second became known as the ephemeris second.
12.
In 1967 / 8 the length of the SI second was redefined to be 9, 192, 631, 770 cycles of the caesium resonance, equal to the previous measurement result for the ephemeris second ( see Ephemeris time-redefinition of the second ).
13.
The ephemeris second is the duration of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, cause the laws of motion to accurately predict the observed positions of solar system bodies.
14.
The atomic second, often called the SI second, was intended to agree with the ephemeris second based on Newcomb's work, which in turn makes it agree with the mean solar second of the mid-19th century . ( McCarthy & Seidelman, 2009, pp . 81 & ndash; 2, 191 & ndash; 7)
15.
Inspection of the formulae above shows that the ( ideally constant ) unit of ephemeris time such as the ephemeris second has been for the whole of the twentieth century very slightly shorter than the corresponding ( but not precisely constant ) unit of mean solar time ( which besides its irregular fluctuations tends gradually to increase ), consistently also with the modern results of Morrison and Stephenson ( see article & Delta; T ).
16.
The ephemeris second, based on a fraction of the tropical year derived from Simon Newcomb's expression for the mean solar motion, became a standard in 1960, but in 1967, at the 13th Conf�rence G�n�rale des Poids et Mesures, the second was redefined in terms of a value for the ephemeris second that had been precisely measured by Essen in collaboration with William Markowitz of the United States Naval Observatory in terms of the frequency of a chosen line from the spectrum of caesium.
17.
The ephemeris second, based on a fraction of the tropical year derived from Simon Newcomb's expression for the mean solar motion, became a standard in 1960, but in 1967, at the 13th Conf�rence G�n�rale des Poids et Mesures, the second was redefined in terms of a value for the ephemeris second that had been precisely measured by Essen in collaboration with William Markowitz of the United States Naval Observatory in terms of the frequency of a chosen line from the spectrum of caesium.
18.
The product of Simon Newcomb's J1900.0 mean tropical year of ephemeris seconds and a speed of light of produced a light-year of ( rounded to the seven significant digits in the speed of light ) found in several modern sources was probably derived from an old source such as C . W . Allen's 1973 " Astrophysical Quantities " reference work, which was updated in 2000, including the IAU ( 1976 ) value cited above ( truncated to 10 significant digits ).
