Earth's rotation
Earth's rotation period relative to the fixed stars, called its "stellar day" by the International Earth Rotation and Reference Systems Service (IERS), is of mean solar time (UT1), or Earth's rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is of mean solar time (UT1) . Thus the sidereal day is shorter than the stellar day by about 8.4 ms.