Astronomical observations, mainly based on eclipses of the Sun, suggest a deceleration of the Earth’s rotation by about 41 seconds of arc per century. Fossil coral growth rings indicate rotation rates corresponding to about 400 days/year some 400 million years ago. Rotation is affected by periodic and irregular events. Most rapid irregular changes are meteorological or oceanic in origin and depend on the mechanical coupling between the atmosphere, oceans, and solid Earth, but they also affect the Earth’s moment of inertia. The major long-term change is due to the slowing of the Earth’s rotation by the tidal drag of the Moon and, to a much lesser extent, by the Sun and planets, most of it attributed to the M₂ ocean tide. Irregular fluctuations of largely unknown origin also occur, possibly associated with electromagnetic coupling between the Earth’s core and mantle. See also chandler wobble.