Editing Tide (section)

=== Example calculation ===
[[File:Tide.Bridgeport.50h.svg|thumb|Tides at [[Bridgeport, Connecticut]], U.S. during a 50-hour period.|alt=Graph with a single line rising and falling between 4 peaks around 3 and four valleys around −3]]
[[File:Tide.Bridgeport.30d.svg|thumb|Tides at Bridgeport, Connecticut, U.S. during a 30-day period.|alt=Graph with a single line showing tidal peaks and valleys gradually cycling between higher highs and lower highs over a 14-day period]]
[[File:Tide.Bridgeport.400d.svg|thumb|Tides at Bridgeport, Connecticut, U.S. during a 400-day period.|alt=Graph showing with a single line showing only a minimal annual tidal fluctuation]]
[[File:Tide.NZ.November.png|thumb|Tidal patterns in Cook Strait. The south part (Nelson) has two spring tides per month, versus only one on the north side (Wellington and Napier).|alt=Graph showing 6 lines with two lines for each of three cities. Nelson has two monthly spring tides, while Napier and Wellington each have one.]]

Because the Moon is moving in its orbit around the Earth and in the same sense as the Earth's rotation, a point on the Earth must rotate slightly further to catch up so that the time between semi-diurnal tides is not twelve but 12.4206 hours—a bit over twenty-five minutes extra. The two peaks are not equal. The two high tides a day alternate in maximum heights: lower high (just under three feet), higher high (just over three feet), and again lower high. Likewise for the low tides.

When the Earth, Moon, and Sun are in line (Sun–Earth–Moon, or Sun–Moon–Earth) the two main influences combine to produce spring tides; when the two forces are opposing each other as when the angle Moon–Earth–Sun is close to ninety degrees, neap tides result. As the Moon moves around its orbit it changes from north of the Equator to south of the Equator. The alternation in high tide heights becomes smaller, until they are the same (at the lunar equinox, the Moon is above the Equator), then redevelop but with the other polarity, waxing to a maximum difference and then waning again.