Earth's rotation
Earth's rotation period relative to the International Celestial Reference Frame, 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 mean vernal equinox, named "sidereal day", is of mean solar time (UT1) , ). Thus, the sidereal day is shorter than the stellar day by about .