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The Earth's rotation is the rotation of the solid earth around its own axis, which is called Earth's axis or rotation axis. The earth rotates towards the east.

Contents 1 Foucault pendulum 2 Rotation period 3 References 4 External links

One remarkable piece of evidence for Earth's rotation was built by physicist Jean Foucault in 1851. By attaching an iron sphere to a very long wire, Foucault constructed a pendulum that was 20 stories high. Physicists of the time knew that once a pendulum is set in motion, its direction of swing would not change. Foucault, however, observed that the direction of swing of his pendulum seemed to change. Each hour it shifted about 11 degrees in a clockwise direction. After eight hours the pendulum was swinging at a right angle to its starting direction. Because the pendulum itself could not have changed its direction of swing, Foucault concluded that the shift he saw was caused by Earth's turning beneath his pendulum. The Foucault pendulum is now a famous demonstration of Earth's rotation.

The average rotation period of the Earth is called a "sidereal day". Its value is

 23 h 56 m 4.091 s

and describes the rotation with respect to the cosmic background of the stars. In contrast to this, the rotation with respect to the Sun is 24 hours, differing because the Earth revolves around the sun once per year. Per day the difference is
86 400 s / 365.25 = 236 seconds = 3 m 56 s (as can be seen above).

This must not be confused with the length of day (abbrev. LOD), which the IERS defines as the difference between the astronomically determined duration of the day (relative to the Sun) and 86,400 SI seconds.^[1]

Over millions of years, the rotation is significantly slowed by gravitational interactions with the Moon: see tidal acceleration.

1. ^ Universal Time (UT1) and Length of Day (LOD)

• Weather Makes Earth Wobble
• Earth rotation by Jean O. Dickey
• IERS site
• AAM Scientific Background Information (1.36 MB pdf file)
• Historical Eclipses and Earth’s rotation by F. Richard Stephenson (242KB pdf file)