<P> Both the stellar day and the sidereal day are shorter than the mean solar day by about 3 minutes 56 seconds . The mean solar day in SI seconds is available from the IERS for the periods 1623--2005 and 1962--2005 . </P> <P> Recently (1999--2010) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between 0.25 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 (see Fluctuations in the length of day). </P> <P> The angular speed of Earth's rotation in inertial space is (7.2921150 ± 0.0000001) × 10 radians per SI second (mean solar second). Multiplying by (180 ° / π radians) × (86,400 seconds / mean solar day) yields 360.9856 ° / mean solar day, indicating that Earth rotates more than 360 ° relative to the fixed stars in one solar day . Earth's movement along its nearly circular orbit while it is rotating once around its axis requires that Earth rotate slightly more than once relative to the fixed stars before the mean Sun can pass overhead again, even though it rotates only once (360 °) relative to the mean Sun . Multiplying the value in rad / s by Earth's equatorial radius of 6,378,137 m (WGS84 ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of 465.1 m / s (1,526 ft / s), or 1,674.4 km / h (1,040.4 mph). Some sources state that Earth's equatorial speed is slightly less, or 1,669.8 km / h . This is obtained by dividing Earth's equatorial circumference by 24 hours . However, the use of only one circumference unwittingly implies only one rotation in inertial space, so the corresponding time unit must be a sidereal hour . This is confirmed by multiplying by the number of sidereal days in one mean solar day, 1.002 737 909 350 795, which yields the equatorial speed in mean solar hours given above of 1,674.4 km / h . </P> <P> The tangential speed of Earth's rotation at a point on Earth can be approximated by multiplying the speed at the equator by the cosine of the latitude . For example, the Kennedy Space Center is located at latitude 28.59 ° N, which yields a speed of: cos 28.59 ° × 1,674.4 km / h (1,040.4 mph; 465.1 m / s) = 1,470.23 km / h (913.56 mph; 408.40 m / s) </P>

The motion of the earth around the sun is called