What causes variations in Earth's orientation?
There are various factors which cause the orientation of the Earth to change with time. Polar motion is caused, in part, by large scale movements of water and changes in the atmospheric angular momentum. For example, the yearly melting of snow and ice in northern Spring contributes to the annual term of polar motion. It is also thought that large earthquakes and the embayment of water by dams and reservoirs might affect polar motion, but this has yet to be quantitatively demonstrated.
The secular variation of the rotational speed seen by the apparently linear increase in the length of the day is due chiefly to tidal friction. The Moon raises tides in the ocean diminishing the speed of rotation. This effect causes a slowing of the Earth's rotational speed resulting in a lengthening of the day by about 0.0015 to 0.0020 seconds per day per century.
The irregular changes in speed appear to be the result of random accelerations, but may be correlated with physical processes occurring on or within the Earth. These cause the length of the day to vary by as much as 0.001 to 0.002 seconds. Irregular changes consist of "decade fluctuations" with characteristic periods of five to fifteen years as well as variations which occur at shorter time scales. The decade fluctuations are related apparently to processes occurring deep within the Earth. The higher frequency variations with periods less than two years are now known to be related largely to the changes in the total angular momentum of the atmosphere.
Periodic variations are associated with periodically repeatable physical processes affecting the Earth. Tides raised in the solid Earth by the Moon and the Sun produce variations in the length of the day with a total amplitude on the order of 0.001 seconds and with individual periods of 18.6 years, 1 year, 1/2 year, 27.55 days, 13.66 days and others. A standard model including 62 periodic components, can be employed to correct the observations for tidal effects.
The rotational speed of the Earth remains essentially unpredictable in nature due to incompletely understood variations. Because of this, astronomical observations continue to be made regularly with increasing accuracy, and the resulting data are the subject of continuing research in the field.