Editing Tide (section)

=== Principal lunar semi-diurnal constituent ===
[[File:Global surface elevation of M2 ocean tide.webm|thumb|upright=1.7|Global surface elevation of M2 ocean tide (NASA)<ref name=NASA2016>{{Cite web |url=https://svs.gsfc.nasa.gov/4541 |title=Ocean Tides and Magnetic Fields |website=NASA Visualization Studio |publisher=[[NASA]] |date=30 December 2016 |access-date=20 November 2020 |archive-date=27 November 2020 |archive-url=https://web.archive.org/web/20201127195922/https://svs.gsfc.nasa.gov/4541 |url-status=live }}</ref>]]

In most locations, the largest constituent is the ''principal lunar semi-diurnal'', also known as the ''M2 tidal constituent'' or ''M<sub>2</sub> tidal 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. Simple [[tide clock]]s track this constituent. The lunar day is longer than the Earth day because the Moon orbits in the same direction the Earth spins.

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. During this time, it has passed overhead ([[culmination]]) once and underfoot once (at an [[hour angle]] of 00:00 and 12:00 respectively), so in many places the period of strongest tidal forcing is the above-mentioned, about 12 hours and 25 minutes. The moment of highest tide is not necessarily when the Moon is nearest to [[zenith]] or [[nadir]], but the period of the forcing still determines the time between high tides.

Because the gravitational field created by the Moon weakens with distance from the Moon, it exerts a slightly stronger than average force on the side of the Earth facing the Moon, and a slightly weaker force on the opposite side. The Moon thus tends to "stretch" the Earth slightly along the line connecting the two bodies. The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally (see [[equilibrium tide]]).

As the Earth rotates, the magnitude and direction of the tidal force at any particular point on the Earth's surface change constantly; although the ocean never reaches equilibrium—there is never time for the fluid to "catch up" to the state it would eventually reach if the tidal force were constant—the changing tidal force nonetheless causes rhythmic changes in sea surface height.

[[File:Tide type.svg|thumb|Types of tides (See ''Timing'' (below) for coastal map)|alt=Three graphs. The first shows the twice-daily rising and falling tide pattern with nearly regular high and low elevations. The second shows the much more variable high and low tides that form a "mixed tide". The third shows the day-long period of a diurnal tide.]]

When there are two high tides each day with different heights (and two low tides also of different heights), the pattern is called a ''mixed semi-diurnal tide''.<ref name=noaa7>{{cite web |publisher=[[National Oceanic and Atmospheric Administration|U.S. National Oceanic and Atmospheric Administration]] (NOAA) National Ocean Service (Education section) |url=http://oceanservice.noaa.gov/education/kits/tides/tides07_cycles.html |title=Types and causes of tidal cycles |archive-url=https://web.archive.org/web/20120201145550/http://oceanservice.noaa.gov/education/kits/tides/tides07_cycles.html |archive-date=February 1, 2012}}</ref>