ephemeris second in 1967. Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is seconds of mean solar time (UT1) Earth's rotation period relative to the precessing or moving mean vernal equinox, its sidereal day, is seconds of mean solar time (UT1) Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005. Recently (1999–2005) the average annual length of the mean solar day in excess of 86400 SI seconds has varied between 0.3 ms and 1 ms, which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds. Rotation period of selected objects * See Solar rotation for more detail. See also Apparent retrograde motion List of slow rotators (minor planets) List of fast rotators (minor planets) Retrograde motion Rotational speed Synodic day References External links Note, the rotation periods for Mercury and Earth in this work may be inaccurate. Kinematic properties Period Time in astronomy