The rotation of the earth
The earth rotates on its axis once every 24 hours. As the earth rotates, any given point on the earths surface will pass thorough the tidal bulges created by the gravitational pull of the moon (Fig 5.3).
In the UK, every 24 hours or so we pass through an area of high water, followed by an area of low water, then a 2nd area of high water and finally the 2nd area of low water. This means we have two high tides and two low tides per day . This is called a semi-diurnal tidal cycle. Basically, in the UK we have a low tide followed by a high tide every 6 hours or so.
The moon is partly responsible for generating the tides in a 29.5 day cycle as it orbits the earth but as the earth revolves we experience semi-diurnal tides as we pass through the tidal bulges produced by the moon.