 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 @.@ 098903691 seconds of mean solar time ( UT1 ) , or 23h 56m 4.098903691s. Earth 's rotation period relative to the precessing or moving mean vernal equinox , misnamed its sidereal day , is 86 @,@ 164 @.@ 09053083288 seconds of mean solar time ( UT1 ) ( 23h 56m 4.09053083288s ) 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 IERS for the periods 1623 – 2005 and 1962 – 2005 . 
