The Moon orbits the Earth in the same direction as the Earth rotates on its axis, so it takes slightly more than a day-about 24 hours and 50 minutes-for the Moon to return to the same location in the sky. This is analogous to the minute hand on a watch crossing the hour hand at 12:00 and then again at about 1:05½ (not at 1:00). The lunar day is longer than the Earth day because the Moon orbits in the same direction the Earth spins.
Simple tide clocks track this constituent. Its period is about 12 hours and 25.2 minutes, exactly half a tidal lunar day, which is the average time separating one lunar zenith from the next, and thus is the time required for the Earth to rotate once relative to the Moon. In most locations, the largest constituent is the principal lunar semi-diurnal, also known as the M2 tidal constituent or M 2 tidal constituent. Global surface elevation of M2 ocean tide (NASA) The daily inequality is not consistent and is generally small when the Moon is over the Equator. Similarly, the two low waters each day are the higher low water and the lower low water. The two high waters on a given day are typically not the same height (the daily inequality) these are the higher high water and the lower high water in tide tables. Tides are commonly semi-diurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). Slack water usually occurs near high water and low water, but there are locations where the moments of slack tide differ significantly from those of high and low water. The tide then reverses direction and is said to be turning. The moment that the tidal current ceases is called slack water or slack tide. Oscillating currents produced by tides are known as tidal streams or tidal currents.
The water stops rising, reaching a local maximum called high tide.The water stops falling, reaching a local minimum called low tide.Tide changes proceed via the two main stages: Types of tides (See Timing (below) for coastal map) For example, the shape of the solid part of the Earth is affected slightly by Earth tide, though this is not as easily seen as the water tidal movements. 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. While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to change from thermal expansion, wind, and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts. These data are compared to the reference (or datum) level usually called mean sea level. Gauges ignore variations caused by waves with periods shorter than minutes. To make accurate records, tide gauges at fixed stations measure water level over time. Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. A "mixed tide"-two uneven magnitude tides a day-is a third regular category. Other locations have a diurnal tide-one high and low tide each day. Many shorelines experience semi-diurnal tides-two nearly equal high and low tides each day. They are however only predictions, the actual time and height of the tide is affected by wind and atmospheric pressure. The predictions are influenced by many factors including the alignment of the Sun and Moon, the phase and amplitude of the tide (pattern of tides in the deep ocean), the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing). Tide tables can be used for any given locale to find the predicted times and amplitude (or " tidal range"). Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide coming in, video stops about 1 + 1⁄ 2 hours before high tide
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