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:''For related longer-term variations, see ΔT.'' The length of the day, which has increased over the long term of Earth's history due to tidal effects, is also subject to change on a shorter scale of time. Exact measurements of time by atomic clocks and satellite laser ranging have revealed that the length of day (LOD) is subject to a number of different fluctuations. These tiny fluctuations have periods which range from a few weeks to a few years. They are attributed to interactions between the dynamic atmosphere and Earth itself. The International Earth Rotation and Reference Systems Service monitors the changes. By custom and practice, a mean solar day is held to contain 86,400 seconds, although this is not strictly true when the term second is used in accordance with the SI system of units. ==Introduction== In the absence of external torques, the total angular momentum of Earth as a whole system must be constant. Internal torques are due to relative movements and mass redistribution of Earth's core, mantle, crust, oceans, atmosphere, and cryosphere. In order to keep the total angular momentum constant, a change of the angular momentum in one region must necessarily be balanced by angular momentum changes in the other regions. Crustal movements (such as continental drift) or polar cap melting are slow secular events. The characteristic coupling time between core and mantle has been estimated to be on the order of ten years, and the so-called 'decade fluctuations' of Earth's rotation rate are thought to result from fluctuations within the core, transferred to the mantle. The length of day (LOD) varies significantly even for time scales from a few years down to weeks (Figure), and the observed fluctuations in the LOD - after eliminating the effects of external torques - are a direct consequence of the action of internal torques. These short term fluctuations are very probably generated by the interaction between the solid Earth and the atmosphere. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Day length fluctuations」の詳細全文を読む スポンサード リンク
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