- Date: before 12th century
- b : a fit or opportune time : opportunity
- c : an ecclesiastical anniversary or festival; also : its season —usually used in combination <Eastertide>
- 2 a (1) : the alternate rising and falling of the surface of the ocean and of water bodies (as gulfs and bays) connected with the ocean that occurs usually twice a day and is the result of differing gravitational forces exerted at different parts of the earth by another body (as the moon or sun) (2) : a less marked rising and falling of an inland body of water (3) : a periodic movement in the earth's crust caused by the same forces that produce ocean tides (4) : a periodic distortion on one celestial body caused by the gravitational attraction of another (5) : one of the periodic movements of the atmosphere resembling those of the ocean and produced by gravitation or diurnal temperature changes
- b : flood tide 1
- b : a surging movement of a group <a tide of opportunists>
- 4 a : a flowing stream : current
- b : the waters of the ocean
- c : the overflow of a flooding stream
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. The tides occur with a period of approximately 12.5 hours and with an amplitude that is influenced by the alignment of the sun and moon and the shape of the near-shore bottom.
Most coastal areas experience two daily high (and two low) tides. One of these high tides is at the point on the earth which is closest to the moon. The other high tide is at the opposite point on the earth. This is because at the point right "under" the Moon (the sub-lunar point), the water is at its closest to the Moon, so it experiences stronger gravity and is raised. On the opposite side of the Earth (the antipodal point), the water is at its farthest from the moon, so it is pulled less; at this point the Earth moves more toward the Moon than the water does—causing that water to "rise" (relative to the Earth) as well. In between the sub-lunar and antipodal points, the force on the water is diagonal or transverse to the sub-lunar/antipodal axis (and always towards that axis), resulting in low tide.
The sun also exerts a (less powerful) gravitational attraction on the earth which results in a secondary tidal effect. When the earth, moon and sun are approximately aligned these two tidal effects reinforce one another (resulting in higher highs and lower lows). This alignment occurs approximately twice a month (around the full and new moon). These recurring, extreme tides are termed spring tides. The opposite, most moderate tides are termed neap tides.
While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.
Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the solid part of the Earth is affected by tides.