Tides |
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The tide in the sea rises and falls on an approximate sine curve every 12.5 hours. The effect is due to gravity - the roughly equal fields of the Sun and the Moon (the Moon is smaller but closer). The oscillation is irregular (below); more pronounced at times when the Sun, Earth and Moon are lined up (spring tides). The lowest tides, at intemediate times, are called neap tides.  Smaller bodies of water also have tides but, except for the Great Lakes in the US, they are too small to be reported. |
| Tidal friction in the crust and mantle is slowing the rotation of the Earth by a millisecond or so per day per century. The day is getting longer. Tidal forces are large enough to break up a spinning body close to a planet etc. The critical distance is called the Roche limit. |