<P> The average of the true solar day during the course of an entire year is the mean solar day, which contains 86,400 mean solar seconds . Currently, each of these seconds is slightly longer than an SI second because Earth's mean solar day is now slightly longer than it was during the 19th century due to tidal friction . The average length of the mean solar day since the introduction of the leap second in 1972 has been about 0 to 2 ms longer than 86,400 SI seconds . Random fluctuations due to core - mantle coupling have an amplitude of about 5 ms . The mean solar second between 1750 and 1892 was chosen in 1895 by Simon Newcomb as the independent unit of time in his Tables of the Sun . These tables were used to calculate the world's ephemerides between 1900 and 1983, so this second became known as the ephemeris second . In 1967 the SI second was made equal to the ephemeris second . </P> <P> The apparent solar time is a measure of the Earth's rotation and the difference between it and the mean solar time is known as the equation of time . </P> <P> Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098 903 691 seconds of mean solar time (UT1) (23 56 4.098 903 691, 0.997 269 663 237 16 mean solar days). Earth's rotation period relative to the precessing or moving mean vernal equinox, named sidereal day, is 86,164.090 530 832 88 seconds of mean solar time (UT1) (23 56 4.090 530 832 88, 0.997 269 566 329 08 mean solar days). Thus the sidereal day is shorter than the stellar day by about 8.4 ms . </P> <P> Both the stellar day and the sidereal day are shorter than the mean solar day by about 3 minutes 56 seconds . The mean solar day in SI seconds is available from the IERS for the periods 1623--2005 and 1962--2005 . </P>

Earth's magnetic axis does not with its rotational axis