<P> Earth's rotation period relative to the Sun (solar noon to solar noon) is its true solar day or apparent solar day . It depends on the Earth's orbital motion and is thus affected by changes in the eccentricity and inclination of Earth's orbit . Both vary over thousands of years, so the annual variation of the true solar day also varies . Generally, it is longer than the mean solar day during two periods of the year and shorter during another two . The true solar day tends to be longer near perihelion when the Sun apparently moves along the ecliptic through a greater angle than usual, taking about 10 seconds longer to do so . Conversely, it is about 10 seconds shorter near aphelion . It is about 20 seconds longer near a solstice when the projection of the Sun's apparent motion along the ecliptic onto the celestial equator causes the Sun to move through a greater angle than usual . Conversely, near an equinox the projection onto the equator is shorter by about 20 seconds . Currently, the perihelion and solstice effects combine to lengthen the true solar day near 22 December by 30 mean solar seconds, but the solstice effect is partially cancelled by the aphelion effect near 19 June when it is only 13 seconds longer . The effects of the equinoxes shorten it near 26 March and 16 September by 18 seconds and 21 seconds, respectively . </P> <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 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>

What is the speed of the rotation of the earth