 Earth 's rotation period relative to the fixed stars , called its stellar day by the International Earth Rotation and Reference Systems Service ( <unk> ) , is 86 @,@ 164 @.@ <unk> seconds of mean solar time ( UT1 ) , or 23h <unk> <unk> Earth 's rotation period relative to the precessing or moving mean vernal equinox , misnamed its sidereal day , is 86 @,@ 164 @.@ <unk> seconds of mean solar time ( UT1 ) ( 23h <unk> <unk> ) as of 1982 . 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 <unk> for the periods 1623 – 2005 and 1962 – 2005 . 
